Advances in Food Research Volume 29

ADVANCES IN FOOD RESEARCH VOLUME 29

Contributors to This Volume C. M. Christensen J. I. Gray B. S. Luh F, MacRitchie A. M. Pearson K. Potthast L. T6th Zhang Wang

ADVANCES IN FOOD RESEARCH VOLUME 29

Edited by C. 0 . CHICHESTER The Nutrition Foundation Washington, D . C . and Vniversiiy of Rhode Island Kingston, Rhode Island

E. M. MRAK University of California Davis, California

B . S . SCHWEIGERT University of California Davis, California

Editorial Board F. CLYDESDALE E. M. FOSTER S. GOLDBLITH J. HAWTHORNE J. F. KEFFORD S . LEPKOVSKY

H. MITSUDA D. REYMOND E. SELTZER V . G . SGARBIERI W. M. URBAIN

1984

ACADEMIC PRESS, INC. (Harcourt Brace Jovanovich, Publishers)

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CONTENTS

CONTRIBUTORS TO VOLUME29

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vii

Cured Meat Flavor J . . Gray and A . M . Pearson

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................ I1. Curing Ingredients and Their Functions 111. Methods of Meat Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Smoking of Cured Meats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................. V . Role of Curing Agents in Flavor . . . . .......... VI . Antioxidant Role of Nitrite in Cured Meats VII . Alternatives to Nitrite in Cured Meats VIII . Chemical Compounds in Cured Meat Flavor . . . . . . . . . . . . . . . . . . . . . . . . . . . IX . Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ....................................................

2 3 16 21 24 33 46 62 70 72

Chemical Aspects of the Smoking of Meat and Meat Products L . T6th and K . Potthast

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Physical Properties of Smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Smoking Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Production of Smoke Condensates and Smoke Preparations . . . . . . . . . . . . . . . V . Chemical Composition of Smoke and Smoked Products . . . . . . . . . . . . . . . . . ................. VI . Effects of Smoking ............................. .. ... . . . . . . . . . . . . VII . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

87 89

90 102 104 135 150 150

Food Texture Perception C . M . Christensen I . Introduction ................................... I1 . Physiology of Texture Perception . . . . . . . . . . . . . . . . . 111. Texture Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Future Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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159 162 175 194 195

V

CONTENTS

v1

Baking Quality of Wheat Flours F . MacRitchie

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....................... I1. Evaluation of Baking Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Fractionation and Reconstitution Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Dough Properties and Mixing Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V . Role of Flour Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Theories of Baking Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

202 204 216 230 241 262 269 212

Kiwifruit B . S . Luh and Zhang Wang I. I1. I11. IV . V.

INDEX

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production . . . . . . . . . ................................... Growth and Ripening ogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

219 280 288 294 298 307

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311

CONTRIBUTORS TO VOLUME 29 Numbers in parentheses indicate the pages on which the authors' contributions begin.

C. M. Christensen, Monell Chemical Senses Center, and School of Dental Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104 ( 159) J . I . Gray, Department of Food Science and Human Nutrition, Michigan State University, East Lansing, Michigan 48824 ( 1 )

B . S. Luh, Department of Food Science and Technology, University of California, Davis, California 95616 (279) F. MacRitchie, CSIRO Wheat Research Unit, North Ryde, New South Wales, Australia (201) A. M. Pearson, Department of Food Science and Human Nutrition, Michigan State University, East Lansing, Michigan 48824 ( 1 )

K. Potthast, Institute of Chemistry and Physics, Federal Center for Meat Research, 8650 Kulmbach, Federal Republic of Germany (87) L. T6th,* Institute of Chemistry and Physics, Federal Center for Meat Research, 8650 Kulmbach, Federal Republic of Germany (87) Zhang Wang,2 Department of Food Science and Technology, University of California, Davis, California 95616 (279)

'Present address: Institute of Food Chemistry, J . W. Goethe University, 8000 Frankfurt am Main, Federal Republic of Germany. *Present address: Wuxi Institute of Light Industry, Wuxi, Jiangsu 214004, China.

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ADVANCES IN FOOD RESEARCH

VOLUME 29

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ADVANCES IN FOOD RESEARCH. VOL. 29

CURED MEAT FLAVOR

J . I . GRAY AND A . M . PEARSON Department of Food Science and Human Nutrition Michigan State University East Lansing. Michigan

............ I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Curing Ingredients and Their Functions . . . . . . . . . . . .. . . . . . . . . . . . A . Role of Sodium Chloride ...................... B . Role of Sugar and/or Other Sweetening Agents . . . . . . . . . . . . . . . . . . . C . Role of Nitrite and/or Nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D . Role of Sodium Ascorbate and/or Isoascorbates . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . E . Role of Phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 111. Methods of Meat Curing ..................... ............... A . Dry Salt Cure ................................ B . Conventional Dry Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... C . Pickle Curing ................................ ... ............ D . Artery Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . E . Stitch Pumping ...................... ...................... . . . . . . . . . . . . . . . . . ............. G . Wiltshire Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H . Curing Sausages and Other Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Smoking of Cured Meats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Color Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... ............ B . Flavor Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... ............ C . Antioxidant Activity . . . . . . . . . . . . . . . . . . . . . . . . . . D . Prevention of Microbial Growth . . . . . . . . . . . . . . . . . . . . V . Role of Curing Agents in Flavor . . . . . . . . . . . . . . . . . . . . . . . A . Nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Sodium Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... ............ C . Other Curing Ingredients ....................... D . Evaluation of Cured Meat Flavor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Antioxidant Role of Nitrite in Cured Meats . . . . . . . . . . . . . . . . . . . . . . . . . . ... ............ A . Composition of Meat Lipids . . . . . . . . . ............ B . Oxidation of Meat Lipids . . . . . . . . . . . . . . . . . . . . . . ...... C . Mechanisms of Lipid Oxidation . . . . . . . . . . . D . Catalysts of Lipid Oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E . Mechanism of WOF Development in Meat. . . . . . . . . . . . . . . . . . . .

2 3 3 6 7 14 15 16

17 17 18 18 19 20 20 21 21 22 22 23 23 24 24 30 32 33 33 33 35 36 37 39 1

Copyright 0 1984 by Academic Press. Inc . All rights of reproduction in any form reserved. ISBN 0-12-016429-9

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J . I . GRAY A N D A . M . PEARSON

F. Inhibition of WOF in Meats.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alternatives to Nitrite in Cured Meats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Antibotulinal Activity of Nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Nitrite Substitutes . . . . . . . . . . . . . . . . . . . . . . . C. N-Nitrosamine Formation in Cured Meats.. . . . . . . . . . . . . . . . . . . . . . . D. Reduction of N-Nitrosamine Formation by Blocking Agents. . . . . . . . . E. Combinations of Antibotulinal Agents and Nitrite . . . . ....... VIII. Chemical Compounds in Cured Meat Flavor.. . . . . . . . . . . . . . . . . . . . . . . . . A . General Aspects of Meat Flavor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Volatile Compounds in Cured Meats.. . . . . . . . . . . . . . . . . . . . . . . . . . . IX. Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII.

41 46 46 48 57 58 61 62 62 64 70 72

I. INTRODUCTION The origin of meat curing undoubtedly dates back to primitive times and represents one of man’s first successful attempts at food preservation. Kramlich et al. (1973) in discussing the history of meat curing stated that salting of fish as a means of preservation traces back to earlier than 3500 BC. Homer on writing The Odyssey in about 900 BC describes sausages prepared by adding salt to blood and fat (American Meat Institute, 1953). In fact, the word sausage is derived from the Latin word salsus, which translates as “salted,” or more literally as “preserved meat.” According to Jensen (1954), by the fifth century BC production of salted or cured meat products had become commonplace, with the Romans having small meat shops under strict sanitary control. Originally, meat curing was utilized almost exclusively as a means of preservation during times of a plentiful supply for use during periods of scarcity. The early meatpacking industry in the United States was centered in Cincinnati, Ohio, and was largely based on curing of pork products and rendering of lard, according to Vaughan (1945). The cured pork was packed into barrels and shipped to either the southern states or the large population centers on the east coast, whereas the lard was mainly exported. The crews of the early sailing ships relied heavily upon salted meats, mainly pork, and hardtack for their staple diet, which frequently resulted in scurvy, beri-beri, and other nutritional deficiencies (Begg and Begg, 1970). These cured products were extremely variable in quality and often were too salty and lacked uniformity of cure (Kramlich et a l . , 1973). The successful development of reliable refrigeration and its almost universal availability in the United States have changed the reason for meat curing from one of preservation to primarily that of adding variety to the diet (Kramlich et al., 1973). The latter is closely related to the desirable nature of cured meat flavor, which today is recognized as being a primary reason for meat curing. Most cured meat products on the market at present are mildly cured, and with a few exceptions, must be held under refrigeration. It is often difficult, however,

CURED MEAT FLAVOR

3

to separate the concept of preservation from that of cured meat flavor since consumers still perceive preservation and the characteristic flavor of cured meats as being inexorably related. Thus, this article covers the nature of the curing process, the role of the curing ingredients in flavor development, the nature of the components contributing to cured meat flavor, some alternatives to nitrite and/or nitrate, and finally the consideration of research needs in regard to cured meat flavor.

II. CURING INGREDIENTS AND THEIR FUNCTIONS The curing process is achieved by adding a number of curing agents to the meat, each ingredient having unique characteristics and playing an important role in the process as summarized by the American Meat Institute (1944, 1953). The major ingredients include salt, sugar, nitrite and/or nitrate, sodium ascorbate, and often phosphates. Each of these components are discussed from the standpoint of their function and their contributions to cured meat flavor. A.

ROLE OF SODIUM CHLORIDE

Jensen (1954) concluded that salt is the chief bacteriostatic substance used in meat curing. In order to properly understand the function of concentration of NaCl in cured meat products, he further suggested using the ratio of salt to moisture or the brine concentration in the finished products as the best measure of the inhibitory effect of salt on microbial growth. This can be calculated as follows: Brine concentration (%) =

NaCl in product (%) x loo NaCl (%) in product + H,O in product (%)

On reviewing the role of different ingredients used in meat curing, Kramlich et al. (1973) stated that salt is basic to all meat curing mixtures and is the only necessary ingredient. It acts by dehydration and alteration of the osmotic pressure which inhibits microbial growth and thereby limits spoilage by bacteria. However, use of salt alone produces a harsh, dry, salty product that lacks palatability. In addition, salt alone results in oxidation of the pigment myoglobin, producing an undesirable dark-colored lean that is objectionable to consumers (Moulton and Lewis, 1940; Greene and Price, 1975). The Committee on Food Stability (1982) reported that color changes related to oxidative deterioration of restructured meats, which seems to be associated with the flaking operation, are accelerated in the presence of added salt. On summarizing the effects of salt levels on bacterial growth, Jensen (1954)

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J . I. GRAY A N D A. M . PEARSON

concluded that anaerobic bacteria are completely inhibited by 5% NaC1, whereas this salt level has little effect upon aerobes, facultative anaerobes, or micrococci. At 10% salt, however, growth of most bacteria is inhibited, although some salttolerant species will grow in media containing up to 15% salt. In brines containing large pieces of animal tissues, on the other hand, growth of bacteria proceeds more rapidly, with most of the growth appearing at the meat-brine interface. The effects of salt on different types of bacteria were confirmed in a review by Ingram and Kitchell (1967), who concluded that the effect of salt in meat preservation is primarily due to the inhibition of certain microorganisms that cause putrefaction. In curing of comminuted meats, Jensen (1954) concluded that the ratio of salt to moisture accounts for the relatively low tissue salt concentrations that can effectively inhibit the growth of most spoilage microorganisms. For example, Moulton ( 1926) calculated that a composite beef muscle sample containing 71.0% moisture and 18.7% protein would have a water-to-protein ratio of 3.8, while lean pork with 64.5% water and 18.9% protein would have a water-toprotein ratio of 3.4. Thus, the amount of salt needed to prevent microbial growth would be less for the pork product. Rothbart et al. (1982) recently demonstrated that reducing salt levels by 50% in frankfurters resulted in unsatisfactory textural qualities. Although salt appears to play an important role in the texture of comminuted meat products, the interaction is not understood. It is possible that the solubilization of the myofibrillar proteins, which is dependent on extraction by salt (Kramlich et al., 1973), is incomplete and on cooking fails to give the characteristic texture. Nevertheless, Rothbart et al. (1982) found that products prepared using the reduced salt level produced an emulsion with good stability. Wierbicki et al. (1976) showed that addition of 1 and 3% salt to ground fresh ham decreased the shrinkage from 34 to 14%, respectively. This demonstrates that salt can influence shrinkage, with shrinkage decreasing between 1 and 3% salt, reaching a minimum between 3 and 6% and then increasing again as the level of salt is raised from 6 to 10%.The same authors also studied the effects of combining salt and phosphates in the same cure, which will be discussed in the section on the role of phosphates (Section 11,E). In general, their results showed that phosphates further decreased shrinkage when used in combination with NaCl for curing. Although salt is known to influence the flavor of foods, a recent study by Greenfield ef al. (1983) suggests that discretionary salt levels used by consumers are strongly influenced by the mode of presentation, i.e., especially by the size of the hole in the salt shaker. There is little evidence, however, that salt levels in cured products preferred by consumers and the levels provided for processing are the same. In fact, few experiments have been carried out to determine optimal

CURED MEAT FLAVOR

5

salt levels from the standpoint of either acceptability or maximization of the functional properties. The optimal level of salt in curing of hams from the standpoint of consumer acceptability appears to be around 2.5% (Pearson et al., 1962a), although the influence of salt levels on keeping quality and other functional properties has not been fully established. It seems likely that the levels needed may be different in various products, from the standpoint of both functional properties and effects on flavor. Although Price and Greene (1978) and Froehlich ef ul. (1983) indicated that it is the major contributor to cured meat flavor, salt has also been shown by Chang and Watts (1949) to be a prooxidant in meat systems. Ellis et al. (1968) reported that high proportions of lean increased autoxidation and the ratio of monocarbonyls to peroxides in cured pork. This caused them to suggest that NaCl may activate some component in the lean, which accelerates autoxidation in pork adipose tissue. In contrast, Chang and Watts (1949) observed that NaCl had no greater accelerating effect on oxidation in the presence of hemoglobin or a muscle extract than it did in their absence. It may be that ferrous iron (Fe2+) is the catalyst for lipid oxidation in cured meat as has been demonstrated by Igene et al. (1979) for the oxidative reaction involved in warmed-over flavor (WOF) development in cooked meat. The release of Fe2+ from the tissue myoglobin and/or hemoglobin could serve as the active catalyst for lipid oxidation, with its activity depending on some factor which causes the breakdown of the porphyrin ring as was recently demonstrated by Chen (1982). This could also be responsible for the conflicting reports indicating that salt serves as both a prooxidant and antioxidant in meat systems, which were summarized by Pearson et al. (1977). Aside from the effects of salt on oxidative rancidity in meat systems, its role in the development of cured meat flavor is not clear. The results of Price and Green (1978) and Froehlich et al. (1983) suggested that salt concentrations are closely related to flavor development in cured meats. This would indicate that cured meat flavor is strictly concentration related and is entirely a result of the taste of salt per se, which is a well-known taste phenomenon (Amerine et a l ., 1965). It is possible, however, that the desirable flavor contribution of salt is due to the interaction of salt with the lean and/or fatty tissues to produce desirable flavor components. There is some support for the latter viewpoint, since Eakes ef al. (1975) have shown that the flavor of aged country-style hams improves during the aging process. Blumer ( 1954), Craig et ul. ( 1964), Kelly ( 1965), and McCain et al. (1968) demonstrated that development of the aged flavor in country-style hams is accompanied by not only an increase in salt concentration but also increases in the levels of free amino acids, free fatty acids, peroxides, and watersoluble organic acids. It may well be that the flavor of cured meat is due to a combination of compounds rather than any simple one or two component system, as has been suggested by Herz and Chang (1970) for the flavor of fresh meat.

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I. 1. GRAY AND A . M. PEARSON

B.

ROLE OF SUGAR AND/OR OTHER SWEETENING AGENTS

Although meat can be cured with salt alone, according to Kramlich et al. (1973) sugar and/or corn syrup solids are added that counteract the harsh hardening effects of salt by preventing the excessive removal of moisture and by directly moderating the flavor. These authors also indicated that sugar is an effective preservative and will retard bacterial growth, but Moulton and Lewis (1940) concluded that the small amount of sugar used in sweet pickle formulas is not sufficient to prevent the growth of bacteria. The latter authors also indicated that sugar serves the primary function of imparting flavor to cured meat and may serve as food for the desirable flavor-producing bacteria. Kramlich et al. (1973) suggested that the sugars present in the cure may also contribute to the browning reaction and the production of browning products that develop during smoking and cooking to impart the characteristic flavor to cured meats. Pearson and Tauber (1984) pointed out that it is a common practice to add small amounts of dextrose to fermented sausages in order to accelerate the fermentation process. The added reducing sugar provides a readily fermentable food source for the bacteria and results in rapid acid production, which gives a sharp tang and adds to the flavor of cured fermented meat products. Any unfermented dextrose could also increase the Maillard reaction during cooking as has been shown by Pearson et al. (1962b, 1966) for fresh pork, in which the level of reducing sugars has been shown to markedly influence the amount of browning products. Moulton and Lewis (1940) concluded that sugar also creates reducing conditions during the curing process. This, of course, may help in explaining why cured meat does not normally have oxidized flavors. Although the reducing conditions may play an important function in development of cured meat color and in stabilizing the porphyrin ring as suggested by Chen (1982), the effects seem to be indirect and are probably due to the stabilization of the Fez+ rather than to the reducing conditions per se. Greenwood et al. (1940) studied the role of sugars in meat curing and concluded that sugar improves the color of cured meat by establishing reducing conditions that favor development of the desirable meat pigments. They further stated that sugar tends to prevent oxidation of the meat pigments thereby blocking formation of undesirable derivatives during the curing process. On the other hand, sugar provides food for the desirable microorganisms, which then produce acid and provide the pH range and the accompanying reducing conditions that favor formation of the desirable meat pigments. The desirable and undesirable meat pigments are described by Jensen and Urbain (1936) and by Urbain and Jensen (1940) in considerable detail, and sugar appears to aid in development of the desirable ones. Reducing conditions also play an important role in the reduc-

CURED MEAT FLAVOR

7

tion of nitrate or nitrite to nitrous acid, which is the active substance that reacts with the meat pigments, as explained by the American Meat Institute (1953). The reduction of either potassium or sodium nitrate to their corresponding nitrite salt is brought about by nitrate-reducing bacteria on the meat and equipment. This is the first step necessary in cures containing nitrates but is eliminated by the use of nitrites. The reducing conditions then aid in converting the nitrite to nitric oxide, which reacts with the muscle pigment myoglobin, to produce the bright pink pigments characteristic of cured meat color. Although these conditions are probably favored by sugar, it is not an absolute requirement. Studies by Pearson et al. (1962a) and by Marquardt et al. (1963) demonstrated that consumers prefer hams that contain sugar in the cure over those cured with only salt and nitrite. Marquardt et al. (1963) found that 73% of a large scale consumer panel preferred hams containing 2% sugar over similar hams cured with 0, 1, and 3% sugar. Maximum panel scores were found to occur at 2.5% salt in combination with 1.1% sugar by Pearson et al. (1962a). Although the levels of salt and sugar required to give maximum acceptability do not appear to be exact and reflect some consumer variability, there is little doubt that both salt and sugar contribute to the flavor of cured hams, and most likely to the flavor of other cured meat products. Although little research has been conducted on corn syrup solids and other sweetening agents, they are frequently substituted for sucrose according to Kramlich et al. (1973). Corn syrup solids (corn syrup after removal of most of the water) contain a mixture of dextrose, maltose, higher sugars, dextrins, and polysaccharides derived from the breakdown of starch. Both corn syrup and corn syrup solids are less soluble than sucrose and also less sweet to the taste but are used because they cost considerably less (Kramlich et al., 1973). The presence of dextrose may constitute an added advantage for corn syrup solids in products where either fermentation or the Maillard reaction play important roles. Further research on both corn syrup and corn syrup solids in meat systems as replacements for sucrose are needed, especially in regard to their possible role in flavor development. However, their widespread use by the sausage industry indicates that they give satisfactory results. C.

ROLE OF NITRITE AND/OR NITRATE

Nitrate probably was first used in meat curing as an accidental contaminant, according to Lawrie (1966), and was observed to stabilize cured meat color. Although sodium nitrate and potassium nitrate (often known as saltpeter and Chile saltpeter, respectively) have been used in curing meats for many years, nitrite was not allowed until 1925 after studies by Lewis and Vose (1926) and by Ken- et al. (1926) demonstrated that sodium nitrite could be substituted for

8

J . I . GRAY AND A . M . PEARSON

sodium nitrate. Moulton and Lewis (1940) demonstrated that nitrites greatly speed up color fixation and that much smaller quantities are required than for nitrates. Although nitrite was first used for color fixation, it has been demonstrated to have several important functions in addition, which were summarized by Kramlich et al. (1973) as follows: ( 1 ) to stabilize color, ( 2 ) to contribute to the characteristic flavor of cured meat, (3) to inhibit the growth of a number of food poisoning and spoilage bacteria, especially of Clostridium botulinurn, and (4) to retard development of rancidity. Even though color stabilization was the original purpose for adding nitrite to meat curing mixtures (Moulton and Lewis, 1940), its effects on flavor and bacterial growth appear to be even more important (Kramlich et al., 1973). Each of these purposes will be discussed later in greater detail. Chambers et al. (1982) concluded that nitrite improved the texture of poultry frankfurters, which is in agreement with earlier research by Eakes et al. (1975) indicating that nitrite imparted greater firmness to country-style cured hams. On the other hand, Randall and Voisey (1977) reported that nitrite had no effect on the texture of canned chopped hams or frankfurters. However, it was pointed out that the former workers measured texture by sensory analysis while the latter researchers used a mechanical compression test, which probably is responsible for the variance in results. Nevertheless, the mechanism by which nitrite effects emulsion firmness is not known.

I.

Meat Pigments and Color Stabilization

There are a number of meat pigments in muscle, including myoglobin, residual hemoglobin, the cytochromes, the flavin-containing enzymes, catalase, and others (Kramlich et al., 1973). However, Lawrie (1966) indicated that myoglobin is the most abundant pigment present in meat, and its status is largely responsible for meat color, both desirable and undesirable. Both myoglobin and hemoglobin are heme proteins, consisting of globin (a protein) complexed to heme, which consists of an iron nucleus attached to a porphyrin ring as summarized by Giffee et al. (1960). Figure 1 shows the structure of myoglobin, illustrating how the heme and globin are complexed together and the way the iron nucleus binds with nitrogen in each of the four pyrrole rings. Myoglobin differs from hemoglobin in that it has a molecular weight of 16,000-17,000 as compared to 64,000 for hemoglobin. Myoglobin has only one heme group per molecule while hemoglobin contains four heme groups in each molecule. Although hemoglobin has a molecular weight four times that of myoglobin, it has the same oxygen binding equivalence since it has four heme groups per molecule (Giffee et al., 1960). On the other hand, myoglobin has a greater affinity for oxygen, which facilitates the transfer of oxygen

9

CURED MEAT FLAVOR GLOBIN

FH2OH2 FH2 COOH

F"2

7%

COOH

FIG. 1. Diagram of the myoglobin molecule showing the manner in which the heme and globin complex together with the iron nucleus of the porphyrin ring binding to the nitrogens of the four pyrrole rings. From Kramlich et al. (1973). Courtesy AVI, Westport, CT.

from the blood pigment (hemoglobin) to the muscle pigment (myoglobin) in the living animal as explained by Kramlich et al. (1973). Kramlich et al. (1973) reviewed the factors influencing the levels of hemoglobin and myoglobin in various types of tissues, which include (1) the amount of muscular activity, (2) the blood supply, (3) the oxygen availability, (4) the age of the animal, and (5) the species of animal. Although hemoglobin is the predominant pigment in the living animal, after slaughtering by bleeding myoglobin becomes the major tissue pigment. Myoglobin accounts for only 10% of the total iron in the live animal, but after bleeding may account for as much as 95% of the iron in beef skeletal muscle (Moulton and Lewis, 1940). Kramlich et al. (1973) explained the color changes that occur both in living tissue and in meat systems with and without curing ingredients. Figure 2 shows a schematic diagram in which the changes in the heme molecule-the valence of the iron and the radicals attached to it-may vary to produce different colored

GLOBIN

GLOBIN

I

I

OH METMYOGLOBIN ( M Mb) BROWNISH-RED TO BLACK

GLOBIN

02

MYOGLOBIN (Mb) PURPLE - RED

OXYMYOGLOBIN (MbO,) BRIGHT CHERRY RED

FIG. 2. Diagram showing the effects on the valence and color of attachment of different radicals to the iron nucleus of rnyoglobin. From Kramlich et a!. (1973). Courtesy AVI, Westport, CT.

10

J . I . GRAY AND A . M. PEARSON

complexes. Thus, myoglobin may be in the reduced form and have a purple-red color, may be in the oxidized form which is brownish red to black in color, or else may exist in the bright red form known as oxymyoglobin. Although the three forms of myoglobin are interconvertible, changes from metmyoglobin to each of the other forms are slower and require more favorable conditions. Figure 3 shows the absorption spectra of myoglobin, oxymyoglobin, and metmyoglobin from fresh meat. Oxymyoglobin is characterized by sharp peaks at 535-545 nm in the green portion of the spectrum and at 575-585 nm in the blue portion of the spectrum, which gives it a bright red color as explained by Kramlich et al. (1973). Myoglobin gives a broad diffuse peak at 555 nm, and thus has a dull red color. On the other hand, metmyoglobin gives a peak at 505 nm in the green portion of the spectrum and a weak peak at 627 nm in the red spectrum, which results in a dull brown color. Nitrosomyoglobin of cured meat and carboxymyoglobin, which is due to carbon monoxide combining with myoglobin, are characterized by peaks in the green and blue portion of the spectrum, which gives them a bright red to pink color.

2 . Role of Nitrite andlor Nitrate in Cured Meat Color Kramlich et al. (1973) reviewed the function of nitrite in developing cured meat color. As indicated, nitrite and nitrate are used to counteract the undesirable effects of salt upon meat color by producing stable meat pigments. Nitrite, however, requires one less step in the stabilization of color, since nitrate must first be reduced to nitrite as shown below:

NOMb

Nitrate

nimte reducing organisms

Nitrite

favorable conditions, absence of light and a&

b

Nitrite NO

+ HzO

NO + Mb

favorable conditions

NOMMb

NOMMb

favorable conditions

NOMb

+ heat + smoke

NO-Hemochromogen

where Mb = myoglobin, NOMMb = nitric oxide metmyoglobin, and NOHemochromogen = nitrosohemochromogen (a stable pink pigment). Because nitrite reacts quicker and less is required for color stabilizadon, it is widely used in place of nitrate. In fact, USDA meat inspection regulations only allow nitrate in a few products, such as dry cured hams and Lebanon bologna. As

CURED MEAT FLAVOR

11

FIG. 3. Absorption spectra of myoglobin, oxymyoglobin, and metmyoglobin that show the wavelengths for maximum absorption and the primary color spectra. From Giffee et al. (1960). Courtesy W. H. Freeman Company, San Francisco, CA.

shown above, nitric oxide is the active compound that combines with myoglobin to form nitric oxide metmyoglobin, which is then reduced to nitric oxide myoglobin. Under the influence of heat and/or smoke, the stable pink pigment nitrosohemochromogen is then formed. This is the typical cured meat pigment that imparts the desirable color to cured meats (Kramlich et al., 1973). Figure 4 shows a scheme relating the various meat pigments to valence and color changes as explained by Kramlich et al. (1973). This diagram shows both the desirable and undesirable meat pigments and illustrates the conditions promoting each reaction. Conditions favoring those reactions that promote the formation of the cured meat pigments nitric oxide-myoglobin and nitrosohemochrome should be created during meat curing. 3. Relationship of Nitrite andlor Nitrate to the Sensory Properties of Cured Meats Brooks et al. (1940) reported that “the characteristic cured flavor of bacon is due primarily to the action of nitrite on the flesh, and a satisfactory bacon can be made using only sodium chloride and sodium nitrite.” This was later verified by Cho and Brazler (1970), who showed that pork cured with sodium nitrite had

12

J . 1. GRAY AND A , M. PEARSON

Free oxidized porphyrins (Brown, yellow or colorless) (Free of protein)

-

FIG. 4. Diagram showing the changes in the heme pigments with respect to valence, color, and conditions necessary for the conversion. From American Meat Institute (1944). Courtesy American Meat Institute, Arlington, VA.

more cured flavor than similar meat cured with salt but without nitrite. These authors concluded that smoke did not mask the flavor from nitrite curing, which was different from that attributable to salt in the curing pickle. Wasserman and Talley (1972) obtained similar results for frankfurters. These conclusions contrast those of Price and Greene (1978) and of Froelich et al. (1983), who have reported that cured meat flavor is more closely related to salt concentration than to that of nitrite. The latter studies, however, failed to examine the effects of salt and nitrite on cured flavor independently. Thus, they erroneously provided a bias against nitrite, which is required only at extremely low levels for the development of cured meat flavor. The American Meat Institute (1945) stated that nitrate is changed to nitrite by the action of microorganisms, and it is probable that these organisms have an important bearing on the characteristic flavor produced. Nitrite was first allowed as a replacement for nitrate for meat curing in 1925 and appears to have essentially the same effects upon cured meat flavor (Jensen, 1954).

CURED MEAT FLAVOR

13

The nature of flavor development effected by adding nitrite to the cure will be discussed later in this article. It is not clear if the effects of nitrite are due to prevention of oxidation, as indicated by a number of investigators (Sat0 and Hegarty, 1971; Bailey and Swain, 1973; Sat0 et al., 1973; Love and Pearson, 1976; Wilson et al., 1976; Fooladi et al., 1979; Igene et al., 1979; Chen, 1982), or if nitrite interacts with the tissue components to produce the characteristic flavor associated with cured meats (Brooks et al., 1940; Cross and Ziegler, 1965; Cho and Bratzler, 1970). 4 . Role of Nitrite andlor Nitrate in the Inhibition of Food Poisoning Microorganisms

Although nitrate and/or nitrite was/ were originally added to meat cures for color fixation, even small amounts exert a marked inhibitory effect upon bacteria (Grindley et al., 1929). Richardson (1907) wrote, “It has been persistently reported that the sole object in using saltpeter in the curing of meats is for the purpose of maintaining a red color. This is only one of several objects gained in using the substance. That it is an essential curing agent and that a large percentage of sour meats (in the trade sense) would result if its use were discontinued, it cannot be successfully controverted. While it may be admitted that saltpeter is not an antiseptic in the sense ordinarily understood by this term, it appears to protect in some way the nitrogenous tissue against bacterial invasions. ” Then indicating an understanding of the contribution of nitrates to the human dietary, he continued, “As much as the equivalent of from one to two grams of saltpeter could be consumed by a person eating fresh vegetables. Inasmuch as a fresh vegetable diet is entirely harmless and as no case of injury from saltpeter in cured meats is on record, saltpeter in the quantities used in cured meats must be classed as a harmless substance.” Jensen (1954) stated that nitrate is especially effective in inhibiting the activity of certain bacteria that are not effected by strong sodium chloride solutions alone. Moran (1928) showed that sodium nitrate exerted a strong inhibitory effect upon the growth and proteolytic activity of Clostridium sporogenes, with a 1% concentration in the range of 5% sodium chloride completely preventing all activity. Tanner and Evans (1934) showed that nitrate inhibited growth and toxin formation by a number by C . botulinum strains. Although nitrate may exert some bacteriostatic action on meats as pointed out by Grindley et al. (1929), Jensen (1954) concluded that nitrate is not toxic to microorganisms in nonacid solutions unless it is first reduced to nitrite. Tarr (1941a,b, 1944) demonstrated that nitrites play an important role in retarding the growth of many kinds of bacteria, including C . botulinum and C . sporogenes as well as a number of other kinds of bacteria associated with meat spoilage. He also reported that these bacteria were susceptible to nitrite inhibition only at pH

14

J . I . GRAY AND A. M . PEARSON

values below 7.0. Brooks et al. (1940) have shown that heating of canned meats destroys much of the nitrite present in the unheated raw products. Gibbons ( 1940) demonstrated that smoking of Wiltshire bacon resulted in a concomitant reduction in the level of residual nitrite. Further discussion on the role of nitrite in controlling microbial growth will be covered later.

D.

ROLE OF SODIUM ASCORBATE AND/OR ISOASCORBATES

I.

Effects on Color

Ascorbic acid, or vitamin C, is an essential nutrient but is not naturally present in appreciable amounts in meat products (Moulton and Lewis, 1940). Ascorbic acid and isoascorbic or erythorbic acid as their respective salts are commonly used as curing adjuncts (Wilson, 1960). Originally these compounds were primarily used to improve meat color, where their action appears to be through reduction of metmyoglobin to myoglobin, and in accelerating production of nitric oxide from nitrite according to Wilson (1960). Both of these mechanisms would aid in development and stabilization of meat color. Mills et al. (1958) noted that frankfurters made with sodium ascorbate or isoascorbate had a more desirable internal color than those made without these curing adjuncts. They also found that frankfurters which did not contain ascorbate were less uniform in color and that their color faded more rapidly in a lighted display case than those cured with ascorbates. Wilson (1960) pointed out that ascorbates/isoascorbates help to delay fading of cured meats under retail display lighting and suggested that this is probably the result of their action in maintaining reducing conditions at the exposed meat surface and the conversion of a higher proportion of the meat pigments to the stable nitrosohemochromogen during the curing process.

2 . Role in Blocking N-Nitrosamine Formation Perhaps even more important than the effects of ascorbates/isoascorbateson meat color, however, is their role in blocking the development of N-nitrosamine formation in cured meats. Herring (1973) first reported that addition of high levels of ascorbate (1000-2000 mg/kg) in bacon cures blocked formation of Nnitrosopyrrolidine during the pan frying of bacon. Then Greenberg (1974) summarized results demonstrating that ascorbates not only reduced or prevented the development of N-nitrosopyrrolidine in bacon during frying but also reduced or prevented N-nitrosamine formation in other cured meats. This ultimately led to the recommendation that all meat cures should contain ascorbates/erythorbates at 550 mg/kg. The importance of ascorbates/erythorbates in meat cures is discussed in greater detail later in this article.

CURED MEAT FLAVOR

15

3 . Effects on Flavor and Odor Ascorbates were reported by Watts and Lehmann (1952) to not only improve color formation in cured pork but also greatly improve flavor and odor. Although Tims and Watts (1958) showed that low levels of ascorbic acid alone (100 mg/kg) catalyze the development of oxidation in cooked meat, they demonstrated that ascorbic acid in combination with phosphates synergistically protected against rancidity. Sat0 and Hegarty (1971) verified the enhanced antioxidant activity of ascorbic acid and phosphate and theorized that ascorbic acid provided reducing conditions which prevented oxidation. These same workers demonstrated that Fez+ is the active catalyst of oxidation in cooked meats and suggested that the reducing conditions favored by the presence of ascorbate enhances antioxidant activity. Chang and Watts (1949) also showed that ascorbic acid and phosphates exert an important synergism in preventing oxidation in cured meats. Thus, the use of ascorbates alone or in combination with phosphates offers a logical explanation for the infrequency of rancidity in cured meat products. It seems likely that the relatively high levels of ascorbates now being used in cured meats to block Nnitrosamine formation may further enhance and help the prevention of oxidation. The mechanism of this action is discussed later. E.

ROLE OF PHOSPHATES

Phosphates have effects on water binding and on color and flavor. The mechanisms by which these actions take place in cured meats are discussed in this section.

1 . Efsects on Water Binding Watts (1950) demonstrated that polyphosphates improve water binding in cured meat products, which has led to their use in nearly all cured meat products. Swift and Ellis (1956) found that phosphates in sausages also increased water binding and explained their results in terms of pH, ionic strength, and effects on the meat system. Rust and Olson (1973) stated that phosphates reduce the degree of purge or cookout during canning and cooking of cured meat products and have explained this effect as the result of the higher pH increasing the space around the proteins so they can bind more water. According to Kramlich et al. (1973), the effects of phosphates in binding more water is twofold: (1) it raises the pH, which increases the capacity of the muscle to bind water; and (2) the higher pH causes unfolding of the muscle proteins, thereby making more sites on the proteins available to bind water.

16

J . I . GRAY AND A . M . PEARSON

Tripolyphosphates have been the most widely used phosphates, and in combination with other alkaline compounds they have been shown to act synergistically to increase the yields of hams and other cured meat products according to Kramlich et al. (1973). These same authors also pointed out that only alkaline phosphates are effective in improving brine retention and improving finished yields in cured meat products. The effects on improving yields become greater as the temperature of processing is increased (Rust and Olson, 1973).

2.

tnfluence on Color and Flavor

Ando (1974) reported that several phosphates, including monosodium and monopotassium orthophosphates, sodium hexametaphosphate, and disodium pyrophosphate, in combination with sorbic acid enhance cured meat color formation. Although there are few other reports on the effects of phosphates on meat color, this view seems reasonable because of their antioxidative action. There is good evidence that phosphates are effective antioxidants, which was reported by Watts (1950) and later confirmed by several other investigators (Watts, 1954; Tims and Watts, 1958; Sat0 and Hegarty, 1971). The reduction in oxidation is probably related to the complexing of trace heavy metal contaminants in the curing salts (Rust and Olson, 1973). It could also be due to the binding of the ferrous iron by the phosphates, since Igene et al. (1979) have shown that the free ferrous iron is the active oxidant in cooked meats. Chen (1982) has demonstrated that the porphyrin ring in cured meats is stabilized by nitrite so that free ferrous iron is not available to catalyze lipid oxidation after heating. It also appears likely that phosphates could scavenge any free ferrous iron and thus prevent oxidation in cured meats.

Ill.

METHODS OF MEAT CURING

Kramlich et al. (1973), in discussing meat curing, stated that there are three main methods of curing: (1) dry curing, ( 2 ) brine curing, and ( 3 ) combinations of the former two methods. As pointed out by these authors, the dry cure is applied without any added water, as the cure is solubilized by the original moisture present in the meat products. On the other hand, the brine cure uses water to solubilize the curing ingredients and covers the meat with a pickle or brine until the cure has thoroughly penetrated the meat products. The third or combination curing method may start out as a brine and end up in a dry cure or vice versa. Kramlich et al. (1973) stated that this results in a variety of combination curing methods, which are too variable and numerous to discuss in detail. The following discussion uses the classification utilized by Kramlich et al.

CURED MEAT FXAVOR

17

(1973), which is as follows: (1) dry salt curing, (2) conventional dry curing, (3) conventional brine or pickle curing, (4) artery pumping, (5) stitch pumping, (6) hot or thermal cures, (7) Wiltshire curing, and (8) curing of sausages and other products. Each of these methods of curing will be briefly discussed. A.

DRY SALT CURE

According to the American Meat Institute (1944) dry salt curing is accomplished by curing with salt alone or with salt plus nitrite. Only a small percentage of all meat cured is dry salt cured, usually as clear plates, heavy bellies, jowls, and heavy ribs, which are generally used to season other foods during cooking (Kramlich et al., 1973). This procedure is being used less frequently because of the space requirements and poor color of the lean, which forms metmyoglobin as shown in Fig. 2 . Kramlich et al. (1973) concluded that dry salt has the following advantages: (1) it is safe, i.e., there is little likelihood of spoilage, and ( 2 ) it is easy. However, they indicated that it also has some disadvantages: (1) it is too salty, (2) it has poor color, unless nitrite is added, and (3) the market for these highly salted products is limited.

B.

CONVENTIONAL DRY CURING

In contrast to dry salt curing, in which only salt and nitrite or nitrate are added, in conventional dry curing sugar or other sweetening agents are also used in the cure (American Meat Institute, 1944). Although dry curing is still utilized for production of country-style hams and the highest quality bacon, the amount of meat cured by this method has declined until it is used to produce only a small proportion of all cured meat (Kramlich et al., 1973). Its primary use today is for dry rubbing of brine-cured products for further holding while curing takes place, which strictly speaking is a combination method of curing as discussed by Kramlich et al. (1973). The advantages of the conventional dry curing method listed by Kramlich et al. (1973) were that (1) a relatively high-priced product is produced, and ( 2 ) cuts cured by this process are less perishable because of their dryness and firmness. These same authors concluded that the major disadvantages of the conventional dry curing procedure are (1) the high cost due to poor space utilization and the amount of labor involved, (2) the high inventory of products as a result of the slowness of curing, and (3) the harsh salty flavor of the final products. Although the high salt content of dry-cured meat is usually a disadvantage, it is desirable in country-style hams where it also makes them shelf stable at room temperature (Kramlich et al., 1973).

18

I. I . GRAY AND A . M. PEARSON

C. PICKLE CURING The pickle curing process utilizes the same curing ingredients as the conventional dry cure, namely, salt, nitrite or nitrate, and sugar, except that these ingredients are in solution and form a brine or a pickle (American Meat Institute, 1944). Since the brine contains sugar it is sometimes referred to as the sweet pickle cure (Kramlich et al., 1973). The strength of the brine is expressed as a salometer reading, which is essentially a measure of its density (American Meat Institute, 1944). The meat is submerged in the brine, but usually is overhauled at least once during curing. It is removed and placed back into the brine in reverse order to give a uniform rate of curing (Kramlich et al., 1973). Brine curing per se is primarily utilized for production of small cured meat items, such as corned beef and tongues (Rust and Olson, 1973). Although this method may still be used on a limited scale by small operators, it has been largely replaced by artery or stitch pumping (Kramlich et al., 1973). Kramlich et al. (1973) pointed out that the most common use of brine curing, however, is a cover pickle for meat that has been artery or stitch pumped. It may also in some cases by used for finishing the curing of dry-cured products. Pickle curing results in more uniform penetration of the cure, gives a less salty product, and requires less labor than dry curing (American Meat Institute, 1944). However, it has the disadvantage of (1) poor utilization of space, and (2) slow turnover of meat inventories (Kramlich et al., 1973).

D. ARTERY PUMPING Kramlich et al. (1973) stated that artery pumping was developed by a New Zealand mortician who decided that the principles used for embalming of the dead could be applied to meat curing. Thus, the method makes use of pumping a pickle into the meat cuts through the arterial system. The method relies on utilization of the intact arterial system, which in effect limits its use largely to the curing of hams. The ingredients include salt, sugar, and nitrite made into a brine as in the case for conventional pickle curing (Kramlich et al., 1973). The brine usually also contains an alkaline phosphate in order to increase yields (Rust and Olson, 1973). Sometimes the hams are pumped and then placed in a cover brine for 1224 hr in order to improve the uniformity of curing. However, it is more common to merely hold pumped hams overnight before beginning the smoking process, although some processors smoke the hams within a few hours of pumping (Rust and Olson, 1973). Artery pumping is extremely rapid since the brine is distributed throughout as pointed out by Kramlich et al. (1973). This results in reduction of inventories

CURED MEAT FLAVOR

19

through a more rapid turnover. Another advantage of artery pumping is the relatively high yield, which is usually further improved by adding alkaline phosphates to the brine. Kramlich et al. (1973) listed the following disadvantages to artery pumping: (1) It is limited to cuts with intact arteries, which generally include hams and in some cases picnics. (2) Special care is required in cutting to dissect out and leave the artery intact so that it can be utilized to convey the brine to the tissues. (3) Cuts prepared by this procedure normally require refrigeration since they are perishable. The latter also applies to all other mild curing procedures, which really means that only the highly salty dry-cured products are nonperishable and can be stored at room temperatures. Rust and Olson (1973) have emphasized the problem of care in cutting by stating that 25-50% of all purchased hams have damaged arteries, with the problem being more serious in those held for 4-5 days before pumping. Such damage may be due to drying out of the arteries or due to freezing. Regardless of the cause of damage, it limits the usefulness of the artery pumping method.

E. STITCH PUMPING Stitch pumping uses a brine or pickle of the same or similar composition to that used for artery pumping; thus, it contains a combination of salt, nitrite, sugar, and usually alkaline phosphate in an aqueous solution (Kramlich et al., 1973). The brine is pumped into the cuts through a needle or series of needles having multiple openings (Rust and Olson, 1973). The single needle method is generally used for intact cuts having uneven surfaces, such as hams, picnics, and shoulders (Kramlich et al., 1973). The operator commonly makes about three to five stitches (injections) per cut of meat to bring the weight of pickle added to the desired level. The multi-needle injector is used for even surfaced products, such as bellies, as explained by Kramlich et al. (1973). The needles are arranged so that all areas of the cut receive brine in their proximity, with pumping to give the desired level of brine in the product. Stitch pumping has the following advantages: (1) rapid curing, (2) reduction of meat inventories, (3) no special care is required during cutting as is the case for artery pumping, and (4) it requires less skill during injection than does artery pumping (Kramlich et al., 1973). However, stitch pumping gives a wetter product and requires longer for diffusion of cure throughout the tissues than is the case for artery pumping. Stitch pumping also has the advantage of increasing the yields in comparison to the dry curing methods. Meat cured by stitch pumping is usually held for a period of time to allow for penetration of the cure into all the tissues before smoking (Kramlich et al., 1973). Sometimes the cuts are either placed in brine or dry-rubbed following stitch pumping and held for a holding period, which improves the uniformity of

20

J . I. GRAY AND A . M. PEARSON

cure penetration and color development (Kramlich et al., 1973). Although most bacon produced is now cured by multiple needle pumping, the quality is not as good as that cured by dry curing (Kramlich et al., 1973).

F.

THERMAL OR HOT CURES

Several research groups have experimented with hot cures, which greatly accelerate the rate of curing as explained by Kramlich et al. (1973). These hot cures can be applied either as hot pickles or as hot dry cures. Hot pickle cures can be applied by either artery or stitch pumping, with greater acceleration of the curing process being achieved by placing the cuts in a hot cover pickle. The same ingredients are used in the hot pickle cure as are included in other pickle curing processes. Hot dry-curing is not well adapted to large cuts but has been primarily used for producing bacon as described by Kramlich et al. (1973). The best known of these procedures utilizes a machine with multiple needles similar to that used for stitch-pumping of bacon, except the needles do not contain brine and are used solely for making perforations in the tissues. After the bellies have been perforated by the multiple needle device, a hot dry cure mixture is applied at a level of 3-5 lb of cure/100 Ib of bellies and cured at 9-10°C for 3-5 days. The hot dry curing process is claimed to have the following advantages: (1) rapid curing with a reduction in inventory, (2) greater penetration of smoke flavor because the smoke is claimed to enter by way of the perforations, (3) increased yields over dry curing, although lower than injection-cured bellies, (4) absence of pickle pockets, and (5) improved flavor. The disadvantages include problems in heating and in applying the cure, as well as the reduction in yields compared to those obtained for injection curing. Furthermore, the method is only adaptable to relatively thin and uniformly shaped products, such as bellies. G.

WILTSHIRE CURING

Wiltshire bacon is commonly produced for the British market. The procedure for production was described by Kramlich et al. (1973). Wilson et al. (1981) pointed out that the trimmed sides are injected with a brine composed of salt, sodium nitrite, and sodium nitrate followed by immersion in a brine containing the same curing ingredients. After curing the bacon is removed and held for several days in order for maturation to occur. All operations should be at a temperature of 3-5°C. The final concentration of curing ingredients in Wiltshire sides should fall within these ranges: salt, 2.5-5.0%; nitrite, 50-200 mg/kg; and nitrate, about 200 mglkg (Wilson et al., 1981). Taylor et al. (1982) described a modified procedure for producing Wiltshire

CURED MEAT FLAVOR

21

bacon using hot (unchilled) sides, in which the brine is pumped into the sides within 5 hr of slaughter. Dry salting by pressure blasting was used to replace the immersion step. This modification in the curing procedure produced acceptable Wiltshire bacon within 5 days of slaughter, which was about 1 day earlier than that for normal Wiltshire curing. The authors stated that this method could be used by small processors that lack the necessary specialized facilities for immersion curing. The use of salt blasting, which has been described by Taylor et al. (1980), also represents a recent modification in curing of Wiltshire bacon. The procedure uses the same curing ingredients as the traditional Wiltshire cure, i.e., salt, sodium nitrite, and sodium nitrate. However, the ingredients are not dissolved in water to make a brine, but instead are pressure blasted into the uncured sides. This causes deep penetration of the cure and accelerates curing and color development. Since there is no added water, the process of maturation can be reduced. H.

CURING SAUSAGES AND OTHER PRODUCTS

Sausages and other restructured meat products are cured by adding salt, nitrite and/or nitrate, phosphates, and sugar or other sweetening agents (Kramlich et al., 1973). The functions of the various curing agents are the same as discussed earlier. The curing agents, however, are mixed into the products during processing, and thus usually exert their influence quite early during processing. The fermented products rely upon conversion of the sugars to acid by lactic acid-producing bacteria as outlined by Pearson and Tauber (1984). Thus, dextrose is commonly added to the cure to provide a sufficient quantity of substrate so that the pH drop is adequate to give the products a tangy taste. Spices and their oleoresins or essential oils are also added to many of the sausages, with the kind and amount depending on the type of sausage. Because of the variability in both the curing ingredients and spices added to sausages, their flavor differs greatly, which is also influenced by the differences in the composition of the meat block.

IV. SMOKING OF CURED MEATS Smoking is covered by T6th and Potthast elsewhere in this volume, so it will only be briefly covered in this review. Smoking has also been discussed in considerable detail by Wilson (1961), Kramlich et al. (1973), and Daun (1979). Thus, the present discussion focuses on the effects of the smoking of meats on their flavor and its relationship to cured meat flavor. Toth and Potthast in their article review the desirable effects of wood smoke upon processed meats. These desirable effects include the influence of the smok-

22

J . 1. GRAY AND A. M. PEARSON

ing process on color, flavor, antioxidative activity, and prevention of microbial growth. All of these effects are interrelated and can influence the flavor of cured meats. Thus, each of them will be briefly reviewed in regard to their relationship to the flavor of cured meats.

A.

COLOR EFFECTS

Both smoking and cooking are involved in development of the color of smoked meat products (Kramlich et al., 1973). Furthermore, as indicated in Fig. 4, the development of cured meat color depends on application of heat to produce the desirable stable meat pigment nitrosohemochrome. In addition, smoking enhances the development of the browning or the Maillard reaction. This involves the reaction of the free amino groups of the meat proteins or other nitrogenous compounds with the carbonyl groups of the reducing sugars present to produce brown-colored compounds on the surface of the meat as explained by Kramlich et al. (1973). Wood smoke also contains an appreciable amount of carbonyls, and these groups probably interact directly with the free amino groups of the meat to produce the brown color on the surface of smoked meats. Since browning products are known to contribute to the color of cooked meats, it is not unreasonable to assume these components play a role in cured meat color. Phenols also contribute to color development in smoked meats (Daun, 1979). Maximum color development is directly related to the smoke concentration, the temperature, and the moisture content of the products, with 6-10% surface moisture producing an ideal balance between absorption and penetration of the smoke by the meat (Daun, 1979). Some of the browning products were shown by Sat0 et al. (1973) to be inhibitors of oxidized flavors and thus could serve an important function in cured meat flavor. However, Cho and Bratzler (1970) concluded that smoke flavor was independent of the cured meat flavor imparted by nitrite. Nevertheless, the relative role of smoke in the flavor of cured meats has not been clearly elucidated. B.

FLAVOR EFFECTS

The characteristic flavor associated with smoked meats is primarily due to the contribution of the phenols according to Wilson (1961) and Daun (1979). Over 20 different phenols, which are located in the vapor phase of the smoke, contribute to smoke flavor and aroma. The most important ones include guaiacol, 4methylguaiacol, 2,6-dimethoxyphenol, and syringol; the first three compounds contribute most of the flavor and the latter is the primary contributor to aroma. According to Wistreich (1977), vanillic acid imparts a sweet mellow note to the aroma and flavor of wood smoke.

CURED MEAT FLAVOR

23

The phenols not only contribute to the flavor and aroma of smoke but also impart antioxidant properties to smoked meat products (Daun, 1979). The antioxidant properties may be important in imparting and maintaining desirable flavors in cured meats as pointed out by Daun (1979). The work of Cho and Bratzler ( 1970), however, suggests that the antioxidant effects obtained from wood smoke and those coming from nitrite curing are not the same since the flavors appeared to be distinctly different and separable. Nevertheless, both smoke and nitrite appear to contribute antioxidant effects to cured meat, which have at least some common effects on cured meat flavor. C.

ANTIOXIDANT ACTIVITY

As already pointed out, it is extremely difficult to sort out flavor and antioxidant effects of cured meat flavor because it is not clear whether cured meat flavor is due to the antioxidant effects of curing and smoking, or if the curing ingredients impart a flavor that is characteristic of cured meat. Smoke does, however, exert a strong antioxidant action on smoked meats, which is associated with the presence of phenols (Wilson, 1960; Daun, 1979). Pearson and Tauber (1984) stated that most of the antioxidant activity of wood smoke is due to presence of the phenols with high boiling points, especially 2,6-dimethoxyphenol, 2,6-dimethoxy-4-methylphenol, and 2,6-dimethoxy-4-ethylphenol.On the other hand, phenols with low boiling points have only weak antioxidant activity. These same authors also pointed out that smoke produced by smoldering also possesses strong antioxidant activity. Since both smoke (Wilson, 1963; Daun, 1979) and nitrite (Bailey and Swain, 1973; Fooladi et al., 1979) possess strong antioxidant activity, it is possible that this constitutes their major contribution to cured meat flavor. Except for the work of Cho and Bratzler (1970), who reported that smoke did not contribute to cured meat flavor, there have been few attempts to study the influence of smoking on cured meat flavor. D.

PREVENTION OF MICROBIAL GROWTH

The bactericidal action of smoke is due to the combined effects of heating, drying, and the chemical constituents present in wood smoke (Pearson and Tauber, 1984). The phenols, especially the high boiling phenols, have strong bacteriostatic activity. Thus, the phenols by adding the inhibition of bacterial growth may have a desirable influence on the flavor of cured meats, in addition to contributing a desirable flavor over and above this effect, as discussed earlier. Tdth and Potthast also discuss the effects of smoke on microbial growth in their article in this volume.

24

I . I . GRAY AND A . M. PEARSON

V.

ROLE OF CURING AGENTS IN FLAVOR A.

NITRITE

1. Bacon

The contribution of nitrite to cured meat flavor is well established. Brooks et

al. (1940) demonstrated that the characteristic flavor of Wiltshire bacon was due to the action of nitrite on the meat. They suggested that as little as 10 mg of nitrite per kilogram of lean meat was sufficient to produce satisfactory flavor and color. Similar results were reported by Macara (1939). The first in-depth organoleptic evaluation of cured bacon flavor was reported by Mottram and Rhodes (1974) who studied the effect of nitrite concentration on the flavor of bacon made by the Wiltshire process. Pork middles were pumped to a 7-8% increase in weight with brines containing sodium chloride (20% w/v) and sodium nitrite at levels between 0 and 2000 mg/liter. Two separate sets of sensory data showed significant differences ( p < 0.01) between treatments for bacon flavor and pork flavor. Panelists also found a bacon flavor in salted pork that increased almost linearly with nitrite concentration to a level of approximately 1000 mg/kg. These results were confirmed in a subsequent series of experiments by MacDougall et al. (1975), who included an additional treatment of 2400 mg of nitrite in 1 liter of brine. Three separate sets of sensory evalutions each showed significant differences between treatments for bacon flavor and pork flavor. The panelists found a bacon flavor in the nitrite-free product, and this increased with nitrite concentration to a level in excess of 1500 mgiliter (Fig. 5). Panel scores for bacon flavor were linearly related to the logarithm of the nitrite concentration in the brines (Fig. 6). Herring (1973) studied the effects of nitrite on flavor acceptance scores and the development of off-flavors in bacon treated with various levels of nitrite (0- 170 mgikg). Vacuum-packaged bacon prepared without nitrite had the lowest flavor acceptance scores and one of the fastest rates of off-flavor development on storage for 6 weeks. Acceptance scores for these samples declined much more rapidly than those of bacon cured with nitrite. Bacon with an acceptable flavor can be prepared without nitrite (National Academy of Sciences, 1981). Kimoto et al. (1976a) evaluated the effects of sodium nitrite, sodium chloride, and smoke on bacon flavor, using a multiple paired comparison test. Differences in bacon flavor were due to the presence of sodium chloride ( p < 0.001) in bacon, with a lesser but significant effect due to nitrite ( p < 0.01). The effect of smoke was not significant. This group also studied the contribution of lean and adipose tissues to the flavor of bacon (Kimoto et al., 1976b). Fried lean and adipose tissues separated from strips of

25

CURED MEAT FLAVOR

6

a 4

E

g

W

8/a

a m

2

0

~~

0

1000 2000 3000 Nitrite in brine (rng/liter)

FIG. 5 . The relationship between nitrite concentration in curing brines and panel assessment of bacon flavor. (0)Each point is the mean of nine tasters and two replicates; (0) each point is the mean of nine tasters and four replicates; (A) each point is the mean of eight tasters and five replicates. From MacDougall et a/. (1975). Courtesy Society of Chemical Industry.

6 -

A

0

200 500 1000 2000 3000 Nitrite in brine (rng/liter)

FIG. 6. The relationship between log nitrite concentration and panel assessment of bacon flavor. For explanation of symbols, see Fig. 5 . From MacDougall et a/. (1975). Courtesy Society of Chemical Industry.

26

J . I . GRAY AND A . M. PEARSON

bacon before and after frying were evaluated by panelists who reported that bacon flavor was significantly ( p < 0.01) more pronounced in the adipose than in the lean tissues. Consumer response to bacon cured with and without nitrite was reported by Wasserman and Kimoto (1977) and Wasserman et al. (1977). Pork bellies were cured with salt, sugar, sodium tripolyphosphate, ascorbate, and either 0 or 120 mg of sodium nitrite/kg. Average ratings of 6.1 on a 7-point hedonic scale for the conventionally cured bacon and 6.0 for the bacon treated without nitrite were recorded. Consumers were also asked to indicate their preference between the bacon samples cured by the two procedures. Statistical analyses ( p < 0.05) indicated that 2 1- to 30-year-old men who consume bacon less than once a month prefer bacon prepared without nitrite. There was no significant preference in all other categories. These investigators concluded that an acceptable bacon could be prepared without nitrite. Further confirmation of these trends was provided by Huhtanen et al. (1981b) in studies with untrained panelists who demonstrated no preference differences between nitrite-free and nitrite-cured bacon. These results were supported by Williams and Greene (1979) who found no differences between the amounts of nitrite-free and nitrite-cured bacon left on plates by students at breakfast. In another related study, Paquette et al. (1980) varied the amount of sodium nitrite in bacon samples from 0 to 120 mg/kg. Samples containing nitrite had a significantly more desirable flavor than did nitrite-free samples. However, there was no significant difference in desirability among samples with nitrite at different concentrations (40, 80, 120 mg/kg). Bacon formulated with 0.26% potassium sorbate in combination with 40 and 80 mg nitrite/kg was not significantly different from bacon prepared with 120 mg nitrite/kg from the standpoint of color and organoleptic qualities. These studies suggest that nitrite contributes to bacon flavor, but only low levels (-40 mg/kg) are necessary.

2 . Ham and Ham-Like Products Early studies by Barnett et al. (1964) indicated that panelists showed equal preference for hams pumped with pickling brines containing from 100 to 1500 mg nitrite/liter. However, hams prepared with nitrite levels as high as 4500 mg/liter had an undesirable bitter flavor. Later studies by Cho and Bratzler (1970) revealed that pork loins cured with 300 mg nitriteikg possessed a more intense cured meat flavor than similar samples cured without nitrite ( p < 0.05 or 0.01). This trend persisted even when the hams were smoked or when sodium chloride was omitted from the brine. Similar results were obtained by Swain (1972) who investigated the effects of nitrite and smoke on the sensory attributes of ham. Conclusions from this study were (1) there was no significant ( p < 0.05) difference in flavor of non-nitrite-

CURED MEAT FLAVOR

27

and nitrite-treated ham; (2) smoked hams had better flavor and greater overall desirability than nonsmoked hams for both the non-nitrite and nitrite treatments; (3) nitrite-treated smoke and unsmoked hams had better cured flavor and were more desirable than non-nitrite-treated hams; (4) non-nitrite-treated hams were less stable to autoxidation than nitrite-treated hams during refrigerated storage for 10 days; and (5) smoking retarded autoxidation of cured hams during storage at 7.2"C for 10 days. The effects of both nitrate and nitrite on various properties of cured hams were studied by Kemp et al. (1974), who reported significantly higher ( p < 0.01) flavor scores for hams cured with nitrite than for hams treated with only salt and sugar. These results are similar to those of Brown et al. (1974) who also reported higher flavor scores for cured hams ( p < 0.05). Panelists in this study were unable to detect differences in cured meat flavor between samples containing 91 and 182 mg nitrite/kg ( p < 0.05). Eakes et al. (1975), using an experienced panel comprising two or three members, evaluated various country-style hams which had been aged for 60, 90. and 100 days. Unlike previous findings, there was no significant effect on aged flavor due to the presence of nitrite in the hams. It was also reported that cured flavor scores were higher for samples containing only nitrate, or a combination of nitrate and nitrite, as compared to hams cured with nitrite alone (Eakes and Blumer, 1975). Country-style ham has a very distinctive aged flavor that is probably influenced by increases in salt, free amino acids, and free fatty acids as well as nitrate that is gradually reduced to nitrite (Eakes and Blumer, 1975). It was also concluded from this study that levels of nitrate and nitrite can be significantly reduced without adverse effects on flavor and color of country-style hams. Kemp et al. (1975) studied the effects of curing ingredients, holding times, and temperatures on the sensory properties of dry-cured sliced ham and reported that flavor was affected by both curing treatment and storage conditions. Flavor scores were significantly higher when nitrite or nitrate was used compared to controls containing only salt and sugar ( p < 0.05). In a later investigation, Kemp and Fox (1977) examined the effects of two levels of curing ingredients (a curing mixture made up of 84.9% salt, 14.2% sugar, and 0.9% sodium nitrite was applied at the rate of 5 and 7% of uncured weight) and two aging periods (4 and 8 weeks) on the palatability of boneless dry-cured bacon. There were no significant differences in flavor attributed to levels of cure or aging period. Data from this study indicated that boneless hams can be successfully cured using salt, sugar, and nitrite and with less ingredients than normally used for bone-in hams. Curing is faster and the aging time is shorter than those for intact hams (Kemp and Fox, 1977). MacDonald et al. (1980a) investigated the effect of various levels of nitrite on development of cured meat flavor in pickle-cured hams. Sensory evaluation

28

J . 1. GRAY A N D A . M . PEARSON

studies revealed that nitrite levels as low as 50 mgikg produced a significant ( p < 0.05) cured meat flavor when compared with samples containing only sodium chloride. The low level of nitrite was also as effective as higher levels in retarding development of off-flavors during aerobic storage of hams for several days at 4°C (Fig. 7). Ham samples treated with 0.02% butylated hydroxytoluene (BHT) or 1000 mg citric acid/kg ham also exhibited reduced off-flavor formation, although these compounds were not as effective as 50 mg nitrite/kg in producing a typical cured ham flavor. Simple linear regression and simple linear correlation analyses of transformed panel scores for various sensory attributes (Table I) indicated that the intensity of typical ham aroma and flavor was directly related to a decrease in the amount of off-odor and off-flavor present in the product. Furthermore, overall palatability was closely associated with a typical ham aroma and flavor, as well as with a decrease in off-flavor and off-odor. Price and Greene ( 1 978) showed that overall flavor desirability ratings were significantly correlated with rancid flavor scores ( r = 0.89, p < 0.02). These workers reported a high, but not statistically significant, correlation ( r = 0.81) between cured meat flavor scores and scores for overall flavor desirability. It was concluded that cured flavor did not appear to be related directly to a lack of “OXidized” flavor but that a “desirable” flavor appeared to be associated with a more intense cured flavor and a less rancid flavor. In a recent study, Froehlich et al. (1983) cxamined the effect of nitrite and salt on color, flavor, and overall acceptability of ham. Semitendinosus pork muscles were cured with four levels of nitrite (0, 50, 100, and 150 mg/kg) and three levels of salt (0, 1, and 2%) and the products evaluated by a trained sensory 100

-

..

m C 0

--

Salt + N a N 0 2 (500 mg/kgja Salt + NaN02 (200 rng/kg)

w.4

Salt + N a N 0 2 (50 rng/kg)b

H Salt + BHTC

&---A

Salt +citric acidC

k-+ SaltC

-

0 0

4 8 12 Storage time a t 4°C (days)

16

FIG. 7. Effect of curing agents, antioxidants, and metal chelating compounds on mean transformed panel scores for typical ham odor during storage of pork at 4°C. Mean values for treatments bearing different superscripts were significantly different (p < 0.05). From MacDonald et a / . (1980a). Courtesy Institute of Food Technologists, Chicago.

29

CURED MEAT FLAVOR

TABLE I SIMPLE LINEAR REGRESSION AND SIMPLE LINEAR CORRELATION ANALYSES BETWEEN TRANSFORMED PANEL SCORESO Sensory attributes X

Y

Off-odor Off-odor Off-odor Off-odor Off-flavor Off-flavor Off-flavor Ham odor Ham odor Ham flavor

Off-flavor Ham odor Ham flavor Overall palatability Ham odor Ham flavor Overall palatability Ham flavor Overall palatability Overall palatability

a

Regression equation y = -1.24

y

=

y = y y y y

= = = =

y = JJ

=

y =

Correlation coefficient ( x ) ( p < 0.001)

+ 0.96~

93.32 92.61 107.18 87.31 86.25 101.28 -5.23 + -2.47 + 5.41 +

0.80~ 0.84~ 0.95~ 0.75~ 0.79~ 0.92~ 1.05~ 1.16~ 1.05~

0.97 -0.94 -0.92 -0.95 -0.87 -0.84 -0.90 0.97 0.98 0.97

From MacDonald ef af. (1980a) with permission

panel. Both nitrite and salt were found to contribute to the “typical” characteristics of cured hams. The main contribution of nitrite was the development of pink color, which was considered desirable by the panelists. However, increasing salt at the levels tested had a greater effect on the intensity of cured meat flavor than did increasing the level of nitrite. Salt was also shown to affect the perception of the sweetness component as well as saltiness in the cured meat flavor. The authors considered these components to be important in characterizing cured meat flavor and to warrant further investigation. An untrained (consumer) panel found no significant difference between the 50 mg/kg and 150 mg/kg nitrite levels with the 2% salt sample, which suggests that from the standpoint of sensory acceptability, greatly reduced levels of nitrite are possible.

3 . Flavor of Comminuted Meat Products Comminuted products, such as frankfurters, are prepared by addition of salt and nitrite to meat emulsions and usually contain significant amounts of spices and seasonings. Often, these products are also smoked. Wasserman and Talley (1972) evaluated the flavor of frankfurters prepared with and without nitrite using triangle difference tests. A significant number of judges were able to distinguish between the two treatments ( p < 0.001). Using a scaling test where panelists rated the flavor from 0 (no frankfurter flavor) to 10 (excellent frankfurter flavor), nitrite was shown to produce a highly significant difference in flavor. On the other hand, the flavor of smoked frankfurters was judged to be similar irrespective of whether or not sodium nitrite was used in their manufacture.

30

J . I . GRAY AND A . M . PEARSON

Simon et al. (1973) studied the influence of nitrite on the flavor of half-pork, half-beef frankfurters and reported no significant difference (p < 0.05) between 39 and 78 mg, and 78 and 156 mg nitrite/kg. However, when all-beef frankfurters were tested, panel scores were initially the same for all treatments, and gradually decreased during storage at 4°C for 4 weeks. Waldman et al. (1974) also studied the effect of nitrite on the sensory properties of preblended sausage meats and frankfurters. Panel ratings based on a 9-point hedonic scale showed a clear advantage of adding nitrite to meats premixed with salt. The beneficial effect of sodium nitrite in preserving the quality of meats preblended with salt found in this study appears to differ from the findings of other investigators, even taking into account that experimental conditions were at variance. Ellis et al. (1968) reported that nitrite in combination with sodium chloride caused a greater oxidative effect in frozen pork mixtures than sodium chloride alone. Comments by the taste panel members indicated that frankfurters prepared from meats premixed only with salt became objectionably rancid while this did not occur when nitrite was added at preblending. Sebranek et al. (1977) reported significant improvement ( p < 0.01) in the flavor, color, and overall acceptability of frankfurters as the nitrite concentration was increased from 0 to 156 mg/kg. Hadden et al. (1975), using triangle taste tests, evaluated the contribution of various levels of nitrite to the flavor of comminuted pork. Panelists were able to distinguish samples cured with nitrite (20, 156, and 200 mg/kg) from those prepared without added nitrite. However, panelists significantly preferred samples cured with 156 or 200 mg nitrite/kg over those cured with only 20 mg/kg. From the sensory data obtained in this study, it appears that sodium nitrite added to cooked, canned comminuted pork plays a vital role in developing and maintaining cured pork flavor.

B.

SODIUM CHLORIDE

The majority of investigators have concluded that sodium nitrite is needed to produce the typical flavor associated with cured meat products including bacon (MacDougall et al., 1975), ham (Barnett et al., 1964), smoked and nonsmoked frankfurters (Wasserman and Talley, 1972), cooked pork emulsions (Hadden et al., 1975), thuringer sausage (Dethmers et al., 1975), and corned beef briskets (Schults et al., 1977). Other research, however, has indicated that salt is the most important factor in cured meat flavor (Mottram and Rhodes, 1974; Greene and Price, 1975; Kimoto et al., 1976a,b). Consequently, several studies have been carried out to examine factors that might explain the apparently conflicting data. Price and Greene (1978) studied the effect of various curing agents on cured meat flavor and overall flavor desirability in samples of ground pork. Their results indicated that sodium chloride was the major contributor to cured meat flavor as all samples containing sodium chloride had significantly more cured

CURED MEAT FLAVOR

31

meat flavor than samples without it ( p < 0.01). Nitrite alone produced very little cured meat flavor when used at a level of 200 mg/kg. These authors concluded that sodium nitrite produces a more desirable and, probably, a stronger “cured” flavor in cured meat products. However, data from their study clearly demonstrate that curing without nitrite still results in a product that tastes “cured” as long as sodium chloride is included in the formulation. Price and Greene (1978) also investigated several factors which may influence the perception of flavor characteristics in cured meat. Panelists’ judgments were affected by score sheets, sample appearance, and the copresence of off-flavors. Factors that did not significantly influence panel scores included the type of product, the internal processing temperature, and the amount of fat per sample. Similar results were reported by Froehlich et al. (1983) who investigated the effect of nitrite and salt on the color, flavor, and overall acceptability of ham. It was concluded that both nitrite and salt had a significant effect on the intensity of cured meat flavor. However, they concluded that salt appeared to be of greater importance than nitrite in developing cured meat flavor. The intensity of cured meat flavor increased significantly over all three salt levels ( 0 , 1, and 2%). These authors speculated that the difference in the importance of nitrite in developing cured meat flavor as reported by others (Dethmers et al., 1975; Hadden et al., 1975; MacDonald et al., 1980a) may be due to differences in the way the sensory testing was carried out. Salt was shown to affect the perception of the sweetness component as well as saltiness in the cured meat flavor. Price and Greene (1978) also emphasized the need for greater control over sensory procedures when attempting to answer the question of whether or not sodium nitrite is needed for cured meat flavor. They pointed out that in several published cured meat studies, panelists were asked to indicate how well they liked a meat product via a hedonic scale: a particular attribute (e.g., cured meat flavor) is identified and panelists are asked to classify samples on the basis of the magnitude of the identified attribute or asked to scale its magnitude in a variety of samples. Results were often interpreted as meaning that nitrite was necessary for cured flavor development rather than for an acceptable flavor. In their studies with ground pork cured with and without nitrite, Price and Greene (1978) used triangle tests to evaluate the influence of questions asked panelists on the ability of the experimenter to interpret data. Their data indicate that consumer (untrained) panel members identified significantly the odd sample in both triangle tests. More of the experienced (trained) laboratory panelists identified the odd sample in both tests than did not (Table 11). These tests do not tell whether a sample tasted more like cured meat, but, as reported by the authors, they do suggest that a difference in flavor probably existed and also point out which flavors appeared to be preferred. Thus, while meat prepared with sodium nitrite and salt may be preferred over meat cured with only salt, sodium nitrite may not be exclusive in its ability to produce an acceptable cured flavor. The methods used in studies on the effect of nitrite on cured meat flavor have

32

J . I . GRAY AND A, M . PEARSON

TABLE I1 TRIANGLE TEST FOR ASSESSING REACTION TO GROUND PORK CURED WITH AND WITHOUT NITRITE (EXPERIENCED AND CONSUMER PANELISTSP Experienced laboratory panel Test no.

Samples compared NaCl NaCl NaCl NaCl

a

+ NaNOZ + NaNOi

+ antioxidants

No. of observers

Correct identification

12

7

9

5

Preference 6*

I 2 3

Consumer panel No. of observers

Correct identification

35

25***

30

17**

Preference 20* * 5 6 11

Data from Price and Greene (1978) with permission. * p 5 0.05.

** p *** p

5 5

0.01. 0.001.

been discussed in two recent reports by the National Academy of Sciences (NAS) (1981, 1982). It was also concluded that, because the flavor contribution of nitrite depends on the nature of the meat product, comparisons using these methods must be made for each product if they are to be useful.

C. OTHER CURING INGREDIENTS The effects of other curing ingredients on the sensory properties of cured meats have not recieved as much attention as those of sodium nitrite and salt. The role of phosphates in cured meats is well documented in the literature and has been discussed earlier. However, one recent study showed that sodium acid pyrophosphate (SAPP) caused a slight improvement in the flavor of frankfurters by enhancing beef, salt, smoke, and seasoning flavors and by diminishing fat flavor and mouthful (Hargett et al., 1980). The presence of SAPP resulted in intensified sour flavor and decreased aroma and flavor of pork. Similarly, there have been only a few reports pertaining to the involvement of ascorbic acid in cured meat flavor. Brown et al. (1974) studied the characteristics of cured ham as influenced by levels of sodium nitrite and sodium ascorbate. Their results indicated that residual nitrite in the hams was influenced by the level of ascorbate used and by the length of the storage period. Results of visual observations indicated that color stability and color uniformity were superior in ascorbate-treated hams during the 16-day storage period. However, no effect on overall flavor was reported. Sebranek et al. (1977) investigated the influence of sodium erythorbate on color development, flavor, and overall acceptability of frankfurters cured with reduced levels of sodium nitrite. Frankfurters were cured

CURED MEAT FLAVOR

33

with various levels of sodium nitrite and sodium erythorbate and evaluated by a consumer taste panel. Color, flavor, and overall acceptability decreased with decreasing nitrite concentrations. Flavor and acceptability of frankfurters were increased by the use of erythorbate, but only at low (52 mg/kg) concentrations of sodium nitrite. Smoking also plays an overriding role in the appreciation of the flavor of cured meats (MacDougall et a l . , 1975). For example, Wasserman and Talley (1972) demonstrated in a triangle test that the panel could differentiate products prepared with and without nitrite, but in paired tests in which the flavor of frankfurters was scored, the smoked nitrite-free sample was rated as highly as the nitritecontaining sample. Kimoto et al. (1976a) on the other hand reported that the effect of smoke on bacon flavor was not significant. These authors concluded that differences in bacon flavor were due to the presence of sodium chloride in bacon with a lesser but significant effect due to nitrite.

D.

EVALUATION OF CURED MEAT FLAVOR

Any comparison of sensory evaluations on the wide variety of cured meats and meat products must consider the differences in processing conditions and domestic usage (MacDougall et al., 1975). Thus, the contribution of nitrite to cured meat flavor must be assessed product by product. Bacon with an acceptable flavor can be prepared without nitrite, whereas nitrite does make a significant contribution to flavor in hams cured in pickling solution (NAS, 1981). Other product ingredients (e.g., spices) or processes (e.g., smoking) may also be important contributors to flavor, particularly in frankfurters. It has been speculated that flavor characteristics of cured meat products may not be attributable to a specific chemical component or components of cured meat, since the olfactory system can “synthesize” or fuse individual aromas into one that is different from that of the individual components (NAS, 1981). Thus, it reasonable to assume that cured flavor is a composite sensation derived from the contributions of many individual odoriferous compounds.

VI. ANTIOXIDANT ROLE OF NITRITE IN CURED MEATS A.

COMPOSITION OF MEAT LIPIDS

Lipids found in meat can be classified as depot, or intermuscular, and as intramuscular, or tissue, lipids (Watts, 1962; Love and Pearson, 1971; Pearson et al., 1977). The depot or intermuscular lipids are largely localized as subcutaneous deposits although appreciable amounts may be present in the thoracic and abdominal cavities and between the muscles as intermuscular deposits. Tri-

34

J. 1. GRAY AND A . M. PEARSON

glycerides are the principal components of adipose tissue. Intramuscular or tissue lipids are integrated into and widely distributed throughout muscle tissues (Kono and Colowick, 1961). The intracellular lipids exist in close association with proteins and contain a large percentage of the phospholipids (Watts, 1962). According to Dugan ( 197I), the essential membrane-bound lipids (phospholipids) constitute only 0.5-1.0% of the total tissue, whereas Wilson et al. (1976) have reported that the phospholipid content varies from a low of 0.5% for beef red muscle to a high of 1.6% for chicken red muscle (Table 111). Campbell and Turkki (1967) reported phospholipid content of beef muscle to be 0.560.61%; the value for pork muscle was 0.68%. Though the phospholipid content of meat is relatively small, the susceptibility of the phospholipids to oxidation makes them important in determining meat quality. The lability of the phospholipids is due to their high unsaturated fatty acid content (Giam and Dugan, 1965; O’Keefe et al., 1968). For example, Hornstein et al. (1961) reported that 19% of the fatty acids in beef phospholipids have four or more double bonds, while only 0.1% of the triglyceride fatty acids from beef show this degree of unsaturation. These authors identified only arachidonic acid in the phospholipid fraction of beef and pork; however, in a later study by the same group (Hornstein er al., 1967) they reported three or four polyunsaturated fatty acids (PUFAs) in the phospholipid fraction of beef muscle. This is in contrast to the analyses reported by Body and Shorland (1974) for sheep muscle in which they observed a wide range of C,, and C,, PUFAs and approximately 7% 2,3-methylene fatty TABLE 111 MEAN LIPID LEVELS AND STANDARD DEVIATIONS FOR RED AND WHITE MUSCLES FROM MUTTON, BEEF, PORK, CHICKEN, AND TURKEYO,b Total lipid (% tissue)
Mn2 > Cu > Ni2+ > Fe3 . Evidently, there are conflicting results that should be clarified. The prooxidant activity of low levels of contaminating iron in muscle foods is well established (Moskovits and Kielsmeier, 1960; MacLean and Castell, 1964). Only recently has the nonheme iron endogenous to muscle been attributed a major role as the principal catalyst of oxidative rancidity. Sat0 and Hegarty (1971) showed that nonheme iron accelerated the oxidation of lipids in waterextracted cooked meat. These results were later confirmed by Sat0 et al. (1973) and Love and Pearson (1974), who found that Fe2+ was an effective catalyst of lipid oxidation in meat. +

+

+

+

+

E.

MECHANISM OF WOF DEVELOPMENT IN MEAT

Several recent studies with model meat systems have provided further evidence as to the role of myoglobin and nonheme iron in the development of WOF (Love and Pearson, 1974; Igene and Pearson, 1979; Igene et al., 1979). In addition, these studies determined the respective roles of triglycerides and phospholipids as substrates for oxidation in these systems (Igene and Pearson, 1979). Love and Pearson (1974) described a model meat system in which bovine muscle was ground and extracted with distilled deionized water at 4°C until it was devoid of color, indicating the removal of all meat pigments, i.e., myoglobin and hemoglobin. The remaining extracted muscle residue was then used as a model system to which purified myoglobin, ferrous ions, and femc ions were added to determine their role in WOF. Metmyoglobin had no effect upon autoxidation, whereas Fe2+ increased oxidation of the model system with the extent being directly related to the concentration (Love and Pearson, 1974). These results agree with those of Sat0 and Hegarty (1971), who showed that both Fez+ and ascorbate catalyzed development of WOF. Thus, it has been clearly shown that myoglobin is not directly responsible for development of WOF, although this concept has been widely accepted (Tappel, 1962; Wills, 1966; Liu and Watts, 1970). Igene et al. (1979) using a similar model meat system reported that removal of the heme pigments or addition of nitrite significantly inhibited lipid oxidation in cooked meat, which supports the earlier concept that myoglobin is involved in WOF. Igene et al. (1979) further demonstrated that the level of free Fe2+ greatly

40

J . I . GRAY AND A . M . PEARSON

increased during cooking, and accelerated lipid oxidation in cooked meat (Table IV). This indicates that myoglobin serves as a source of Fe2+, being readily broken down during the cooking process and catalyzing autoxidation. It was also observed that treatment with hydrogen peroxide destroys even more of the heme pigments than heating, resulting in still greater oxidation of the meat system (Igene et al., 1979). These results clearly indicate that intact myoglobin is not the catalyst of lipid oxidation in cooked meat. However, cooking destroys part of the myglobin, releasing Fez which then catalyzes the development of WOF. Pearson and Gray (1983) speculated that the susceptibility of ground meat to oxidation may be due to the release of Fez during the grinding of meat. Phospholipids have been shown to be the lipid component most rapidly oxidized in cooked meat (Younathan and Watts, 1960), in lipid fractions isolated from beef and pork (Hornstein et al., 1961), in freeze-dried beef (El-Gharbawi and Dugan, 1965), and in cod muscle (Roubal, 1967). Wilson et al. (1976) and Igene and Pearson (1979) provided further evidence that phospholipids are the major contributors to WOF in cooked meats. Igene and Pearson (1979) presented information on the relative effects of adding back various phospholipids [phosphatidylcholine (PC), phosphatidylethanolamine (PE)] and total blood serum phospholipids (TP) to the model meat system described previously. The addition of both PE and TP increased the amount of oxidation as measured by the 2thiobarbituric acid (TBA) method (Tarladgis et al., 1960), with the effect of PE being the greatest. Sensory scores also indicated that oxidation of PE, TP, and PC contributed to WOF, with PE again having the greatest effect. +

+

TABLE IV ROLE OF HEME AND NONHEME IRON ON DEVELOPMENT OF TBA NUMBERS IN COOKED BEEF(!!' 0H C"H-R

CWH-R

c=c-c

Ethylamine

"0

I

H

I

+CH,OHCHO - H,O

/" ,H

c-c-c\

C-NH-

II

CH

I

C-OH

II/H'OH\'

I

H

',H

Polymerization

Polymerization

t [R = CH,CH,OHJ

FIG. 8. Reaction course as supposed in color formation by the reaction of glycolaldehyde and .:thylamine. From Ruiter (1979).

2.

Flavoring of Foods

Curing smoke with good flavoring properties may be obtained from different kinds of wood (Spanyar et al., 1960a; Tilgner, 1958b). Some types of wood such as beech, oak, maple, birch, spruce, and pine are said to provide smoke of preferred quality, while lime, alder, aspen, and fir are less suitable for smoke production. However, we believe that every kind of wood may be used successfully in smoking if the proper generation parameters are maintained, in particular generation temperature. Tilgner (196 1) recommends smoke generation at tem-

140

L. TOTH AND K . POTTHAST

peratures ranging from 200-6OO0C, where wood materials are just in a glowing state. According to Miler (1962a) and Tilgner and Daun (1970), temperatures of pyrolysis should not exceed 350-450°C and subsequent oxidation should be done at about 200°C. Smoke generation at higher temperatures provides an unpleasant bitter and/or tarry smoke (Reuter, 1966). The composition of smoke varies. As Tilgner (1970) states, production of a uniform smoke over a longer period of time is nearly impossible, independent of the kind of smoke generation, if a one- or a two-step procedure is used. More recent investigation (Potthast, 1979) shows that there is a good possibility of obtaining smoke with a permanently unchanging composition when smoke is generated with the same type of wood of the same humidity at constant temperatures. The best generation temperature is between 650 and 700"C, and the water content of wood should be about 50%. By using wood with a high water content, high density smoke is obtained. The humidity works as a temperature-controlling mechanism since the heat of evaporation required for steam development cools the smoldering materials. Between 650 and 700°C, phenols and PAH are in the most desirable proportions (Fig. 9). At these temperatures it is possible to smoke products to quite a dark color without producing a high degree of contamination with benzo[a]pyrene or other carcinogenic PAH (Potthast, 1982a). The composition of curing smoke and its flavoring properties may be affected by different treatments. Foster and Simpson (1961) achieved well-flavored meat products from an electrostatically treated smoke. The smoke color was also described as good. About 40% of the smoke constituents are lost by electrostatic treatment as Table XI1 shows. The highest loss of flavoring smoke compounds is due to the loss of phenols, which accounts for 64%. Ninety-eight percent of the PAH have been trapped. Obviously, the more volatile compounds are responsible for a good flavor development, and therefore the removal of tar products is desirable. Thus, the simplest method to produce smoke with good flavoring

'

I LOO0

700"

T , 1Mx) OC

FIG. 9. Temperature dependency of benzo[a]pyrene and phenol contents in curing smoke. The value u for the phenol content corresponds to more than 1000 times the value of benzo[a]pyrene.

141

CHEMICAL ASPECTS OF MEAT SMOKING

TABLE XI1 CHEMICAL COMPOSITION OF ELECTROSTATICALLY TREATED OR UNTREATED CURING SMOKE" Smoke condensate

Untreated (mg/100 g)

Treated (mg/100 g)

Loss (%)

Acids Phenols Carbonyls

211.0 1.5 341.0

150.0 2.7 192.0

44.7

Total

565.5

344.7

X 39.1

Benzo[a]pyrene a

38.5 pg/liter

0.8 p,g/liter

29.0 64.0

98.0

From Rusz (1976) with permission.

properties is to separate the generator and the smokehouse, that is, to cleanse the smoke of high-boiling compounds by condensation in the connecting pipe (T6th and Blaas, 1972a). To evaluate the flavoring constituents, smoke is generally divided into several fractions. The most common fractionation method is steam distillation. By using this procedure, Husaini and Cooper (1957) and Krylowa et al. (1963) were able to demonstrate that acids, aldehydes, and phenols have special flavoring properties. Miler and co-workers (1965) reported the favorable contribution to flavor of a phenol extract obtained from smoke during the treatment of sausages. Bratzler et al. (1969) observed a close correlation between smoke flavor and phenol content of smoked products. This finding has been confirmed by Tilgner et al. (1962~)and by Daun (1969, 1972), who separated phenol fractions by preparative column chromatography prior to sensory evaluation. Fiddler and co-workers (1966) separated a good-smelling smoke fraction by extracting smoke condensate in water with ether. This extract consisted mainly of phenols. As main compounds guaiacol and syringol were identified. Similar results have been achieved by T6th (1981), who used ethyl acetate in the extraction procedure. T6th's investigations showed that other compounds were involved in smoke flavoring as well. This is in agreement with the findings of Daun (1972) and Fujimaki et al. (1974), who found a certain phenolic chemical or uncharacteristic smoky taste. To overcome these disadvantages of pure phenol fractions, Miler (1970) recommended mixing several smoke fractions in liquid smoke production on the basis of flavoring considerations. After separation of flavoring constituents from liquid smoke by liquid-liquid partition and determination by gas chromatography, Fujimaki et al. (1974) came to the following conclusions. Apart from phenols, smoke flavor is due mainly to carbonyls and

142

L. TOTH AND K. POTTHAST

lactones with higher boiling points. Some alkylated 1,2-~yclopentadionesand 2butenolide derivatives show a sweet, burnt, or caramel-like smell and should be important for smoke flavor. The smell of burning is mainly caused by 2butenolides. Furfurol, 5-methylfurfurol, acetylfurane, and acetophenone provide a sweet, pleasant smelling, flowery flavor. They soften the strong smoky flavor of the phenols. The numerous derivatives of cyclopentanones have a bitter taste and smell like grass. Their influence on the smoke flavor is not significant. Single phenols have been described as follows: Phenols, cresols, 2,3-, and 2,4-xylenols

Characteristic, biting

2,6-, 3,4-, 3,5-Xylenols, 2- and 3-ethyl-5-methylphenols, and 2,3,5-trimethylphenoIs

Cresolic

Guaiacol

Sweetly smoky. a little bit hot

4-Methyl-, 4-ethyl-, 4-propyl-, and 4-propenylsyringol

Mild, strong, smell of burning

Pyrocatechol and its derivatives

Sweet, with a burning taste

4-Methyl-, 4-ethyl-, and 4-vinylguaiacol

Sweet, smoky

4-Allylguaiacol

Wood-like

Syringol

Smoky

Olsen (1976), as well as Barylko-Pikielna and co-workers (1976), demonstrated the flavoring properties of several phenol fractions distilled from a liquid smoke (after Miler, 1970) by mixing them with milk. The first fraction (60-90°C) consisting of phenol, cresols, guaiacol, methyl-, and ethylguaiacol produced a hot and bitter taste. The second fraction (91-132°C) containing cis- and transisoeugenol, syringol, and methylsyringol had a pure and mild smoke flavor. The third fraction (133-200°C) provided an acid, chemical taste of poor flavor. From these investigations it may be concluded that the phenols of medium volatility are most desirable in curing smoke, as was stated by Daun (1972). Wasserman (1966) and Sochtig (1979) dealt with the question of whether single phenols or mixtures of single phenols are sufficient for smoke flavor development, Wasserman used a taste panel to identify guaiacol, methylguaiacol, and syringol in meat products. Of the panelists, 67% classified the products as smoke-like, the rest as phenolic. A real smoke flavor was not found. In the Sochtig experiments, 12 panelists tested single phenols in solutions of differing concentrations. The results are summarized in Table XIII. All of the tested phenols showed smoke-like characteristics together with other sensory attributes.

143

CHEMICAL ASPECTS OF MEAT SMOKING

TABLE XI11 SENSORY EVALUATION OF SMOKE COMPOUNDS AS DERIVED FROM MODEL MIXTURESa ~

~~

Compound

~

Optimum concentration (mg/100 ml H20)

Guaiacol

3.15

2,6-Dimethoxyphenol

7.50

4-Methylguaiacol

1.90

Isoeugenol

9.80

Cyclotene

1.90

0-Cresol

7.50

Dimethylphenol

0.90

a

~

~

Smell

~~

Taste

Phenolic, hot spicy, flavor of smoked ham, sweet, dry Smoky, spicy, aromatic, Phenolic, smoky, like like smoked ham, pheburning, charcoaled nolic, hot, sweet wood, whiskey-like, dry, hot Sweet, vanilla-like, caraSweet , vanilla-like, fruity, like cinnamon mel-like, aromatic, smoky, pleasant hot pleasant smoke flavor, burning Sweet-fruity, mild smoke Sweet, fruity, vanillalike, phenolic flavor, dry, hot Like Maggi,b like Like Maggi,b smoked ham, spicy, aromatic, smoked ham, sweet, like clove hot Phenolic, sweet fruity, Sweet, hot, unpleasant aromatic, caramel-like, smoky, burning like smoked ham Phenolic, hot, a little bit Phenolic, like ink, aromatic, sweet carbonized, sweet, dry Phenolic, smoky, aromatic, hot, sweet

From Siichtig (1979) with permission. Common flavor-enhancing protein hydrolysate in Germany.

Cyclotene, tested at the same time, ought to enhance meat flavor by its “Maggi”-like taste (Maggi is a flavor protein hydrolysate common in Germany). A mixture of the sensorilly most favored compounds syringol, guaiacol, 4-methylguaiacol, and cyclotene in a concentration was judged only weak in smoke flavor but sweeter, fruity, and generally aromatic. Obviously, a mixture of the several components produces a flavor different than single compounds normally do. There is no doubt that phenols are mainly responsible for smoke flavor, but nonetheless a great number of nonphenolic compounds participate in smoke flavor development. Tilgner (1977) estimates the number of smoke flavoring compounds to be about 500. If so, one may not expect to produce an artificial mixture comparable to a natural smoke flavor by merely combining a few compounds that play a predominant flavor role in smoke treatments. The typical flavor of smoke-treated products is not fully explained by a deposi-

144

L. TOTH AND K . POTTHAST

tion of smoke constituents on the surface. A great number of smoke flavor compounds are composed of reactional groups that may react with food constituents. These reaction products provide a taste different from that of the original reaction components. It is most probable that carbonyl compounds from smoke and aliphatic as well as aromatic carbonyls are mainly involved in these reactions. Whether some compounds, e.g., phenols with reactional carbonyl groups, disappear at all, as Lustre and Issenberg (1970) found, or only are reduced (Toth, 1980a, 1982) is of less importance. The extent of the reactions may be remarkable, as has been shown in the investigations of Knowles and co-workers (1975a) (Table XIV). 3. Preservation Effect

Smoke is known to have an antioxidative and an antimicrobial effect (Kersken, 1973), both of which are influenced mainly by phenolic compounds. TABLE XIV COMPOSITION OF PHENOL EXTRACTS FROM LIQUID SMOKE AND LIQUID SMOKE-TREATED BACON" Percentage in the phenol extract Compounds Furfurylalcohol Cycloten Guaiacol Phenol 0-Cresol 4-methylguaiacol m-,p-Cresol

4-Ethylguaiacol 4-Propylguaiacol 4-Allylguaiacol 4-Vinylguaiacol cis-Isoeugenol trans-Isoeugenol Syringol 4-Methylsyringol 4-Ethylsyringol 4-Propylsyringol 4-Allylsynngol 4-Propenylsyringol 0

b

Liquid smokeh

1.o 2.3 6.0 2.6 1.6 10.5

Baconh

2.1 0.3 11.4 3.5 1.3 13.6

0.1 0.2 5.2 4.0 0.2 10.1

0.1 0.1 2.9 2.9 0.5 8.6

2.1

2.6

3.8

4. I

5.1 3.6 4.3 1.3 1.7 11.0 8.5 14.5 3.1

4.1 2.3 2.9

1.o 0.6 0.8

0.9 0.4 1.2

-

-

-

2.2 6.8 11.9 12.5 3.9

0.5 1.6 15.5 13.9 2.9 __ 14.5 0.5

0.4 I .9 16.7 17.6 3.4

1

.o

5.0 5.0

-

3.8 3.8

From Knowles et al. (1975a) with permission. Two separate experiments were conducted for each.

-

16.7 0.1

CHEMICAL ASPECTS OF MEAT SMOKING

145

TABLE XV PHENOLIC COMPOUNDS IN CURING SMOKE WITH PROVED ANTIOXIDATIVE PROPERTIESa Phenols

Phenol aldehydes

Phenol acids

~~

Pyrocatechol Hydrochinone Guaiacol Eugenol Isoeugenol a

Vanillin Salicylaldehyde

2-Hydroxybenzoic acid 4-Hydroxybenzoic acid

From Seher (1967).

During the storage of foods the spoilage of fat by rancidity is the limiting factor if microbial spoilage is prevented by cooling, refrigeration, or drying. Fat oxidation, known as the peroxidation process, increases with decreasing water activity. Hydroperoxides are present, which decompose to aldehydes and ketones, and are responsible for the rancid taste. The peroxidation process may be prevented by antioxidants, all of which have a phenolic structure. Seher (1967) described antioxidative properties of some phenols present in smoke. Besides the tested phenols (Table XV), other phenols supposedly act as antioxidants in as much as they are able to turn into quinoid structures. Kurko (1959) investigated basic, acid, and neutral fractions of smoke for antioxidative properties in mixtures with pork fat. Only the neutral fraction that contained most of the phenols showed excellent antioxidative effects. The acid fraction had minor antioxidative properties. The basic fraction proved to be prooxidative. The neutral fraction was separated again by vacuum distillation to isolate the most effective compounds. Fractions with lower boiling points, with phenol, cresol, and guaiacol as main constituents, were less antioxidative than fractions of higher boiling points, containing mainly syringol and its derivatives. Pokorny and co-workers (1963) took the phenols with two unsubstituted hydroxy groups in the ortho or para position and phenols with several alkyl groups to be most effective. Tilgner and Daun (1970) compared the properties of smoke from smoldering sawdust and from a friction smoke generator and found that smoke from smoldering sawdust was more antioxidative. This observation may be explained by the findings of T6th (1980b) that friction smoke is less rich in dihydroxyphenols. Whether the effectiveness of phenols is enhanced by dicarboxylic acids (fumaric, succinic, etc.) has not yet been investigated. Smoke constituents exert a restraining effect on bacteria in pure cultures as well as in meat products (Kersken, 1973). However, there are factors other than smoke affecting the growing of microorganisms, such as lowering the water

146

L. TOTH AND K . PO'ITHAST

activity and the pH value. Heat treatment may be of additional influence (Lerche et al., 1957). Bacteria that do not form spores or bacteria in a germinating state are killed. Spores of bacteria and molds remain unaffected. Cocci in particular are most resistant. The antimicrobial effects of smoke occur mainly at the surface, but to a certain extent smoke can influence the interior of sausages, too. As most smoke constituents are enriched at the surface of the meat products, several authors explain germicidal effects by pH changes. Hess (1928), Kochanowski (1962), and Incze (1965) believe formaldehyde to be the most bactericidal compound. Kurko and Perova (1961) contradict this opinion since in their experiments only organic acid and phenol fractions of the smoke proved to be bactericidal. The fungicidal effect of smoke is produced by phenols and formaldehyde as well as by other compounds of the smoke (Kersken, 1973). This effect is of particular importance in the prevention of mycotoxic mold growth.

4.

Other Effects

During surface treatment of fish or meat products with smoke, formation of a secondary skin is observed. The reactions of carbonyls and proteins are mainly responsible for this formation (the tanning effect). 2 >N

+

;

RCH

>NCHRN,

/

+ H,O

This reaction is favored by high temperatures and dry smoking conditions or, if smoke was applied in a wet atmosphere, by subsequent reduction of the humidity. The extent of tanning shows a strong relation to the smoke composition and the climate of the smokehouse. This reaction also takes place with natural casings and artificial casings made of collagen, and is responsible for the resistance of a sausage in a natural casing to bursting when heated in a hot water bath. In general, this reaction imparts greater firmness, influences sausage consistency, and stabilizes microbial growth.

B.

UNDESIRABLE EFFECTS

Besides the above desirable effects, smoking or smoke flavoring of meat products may cause some disadvantages in the form of a reduction of essential nutrients and contamination with toxic compounds.

I.

Influence on Nutrients

The formation of color and flavor as well as the tanning effect are based on reactions of proteins with carbonyl compounds of the curing smoke. These

CHEMICAL ASPECTS OF MEAT SMOKING

147

reactions lead to a loss of amino groups. With curing smoke, only surface proteins are damaged. Mixing liquid smoke with foods distributes smoke compounds evenly, and a more extensive degradation of proteins is possible. Experiments with formaldehyde-treated collagen show that the reaction products are not digested in the stomach but are split by the enzymes of the intestine (Mohler, 1978). Proteins with free amino groups undergo changes very easily. Losses of lysine, an essential amino acid, may occur (Chen and Issenberg, 1972), and these increase with increasing smoke deposition on the foods. Despite remarkable losses of lysine from the surface layers of some heavily smoked meat products, a nutritional deficiency problem is not apparent. Whether other essential amino acids or vitamins are affected as well is not yet known.

2. Contamination with Carcinogenic Polycyclic Aromatic Compounds The first observations that smoke may cause cancer were made by Pott (1773, who described skin cancer in a chimney sweep. Since about 1940 some PAH have been perceived as being carcinogenic. One of the best known PAH is benzo[a]pyrene (B[a]p), which was demonstrated to be mutagenic and carcinogenic in numerous animal experiments. Benzo[a]pyrene and other PAH have been detected in foods of vegetable and animal origin. Vegetables are contaminated during growing by air pollution or by an uptake from the soil. Foods of animal origin such as milk products or meats are practically free of PAH, until they become contaminated during processing, in particular with smoke. In meat cookery, barbacuing is a secondary source of PAH contamination. Polycyclic hydrocarbon content may show a wide range of concentration as is shown for B[a]p (Table XVI). TABLE XVI BENZO[n]PYRENE CONTENT OF SMOKED AND BARBECUED MEAT PRODUCTS

Remarks Smoked sausages Ham, bacon, etc. Ham, bacon, etc. Ham, bacon, etc.

0-6.15 0.1-3.10 0.1-15.0

Barbecued sausages

0.1-86.0

Barbecued meat

0.657.0

1%300

Highest concentration in small-diameter sausages Fermented or cooked; smoked to a bright color Fermented or cooked; smoked to a dark color Fermented or cooked; smoked to a dark color; more or less soot may be found on the surface on country-type ham produced according to home-style procedures Depending on the fuel and fire condition; increasing with increasing fat content of the sausages Depending from the fuel and fire condition; increasing with increasing fat content of the meat

148

L. TOTH AND K . POTTHAST

Studies on the influence of smoking technology (Toth, 1972; Potthast, 1975, 1979, 1982a; Spanyar et al., 1960a; Tilgner, 1958a, 1977; Fritz, 1977) have revealed that the contamination of B[a]p shows a close relationship to smoke generation parameters and that very high smoke generation temperatures, in particular when soot-developing materials have been used, lead to B[a]p concentrations higher than 1 ppb. Although there is no proof that B[a]p causes cancer in humans as a result of eating smoked or barbecued meat products, a potential danger cannot be excluded. As Fritz and Soos (1980) pointed out, stomach and colon cancer are significantly higher in countries or areas where meat products with high B[a]p and other carcinogenic PAH are common. From the estimated 200 PAH present in smoke, some are mutagenic or carcinogenic and some have a cocarcinogenic or antagonistic effect. Carcinogenicity or mutagenicity has been proven for the compounds listed in Table XVII. The total possible carcinogenicity of PAH is estimated to be about 10-fold that of B[a]p alone. From a technological point of view smoked meat products need not contain more than 1 ppb (1 pg/kg) B[a]p. even if the meat products have been smoked to a very dark color and have a smoky flavor. For this reason the maximum B[a]p content in smoked meat products has been limited by regulation to 1 ppb in the Federal Republic of Germany.

3. Formaldehyde Constituents of smoke other than PAH have toxic effects. One such compound is formaldehyde, shown to be mutagenic by animal experiments. Since formaldehyde is degraded by enzymatic action in a relatively short time, foods may contain up to 50 mgikg without any health hazard (Mohier, 1978). Recent studies by the Chemical Institute of Industrial Toxicology and the Institute of Occupational Safety and Health revealed that formaldehyde vapor may induce nasal cancer. There is consequently a keen interest in keeping formaldehyde concentrations in the workshop as low as possible. The maximum concentration allowed in Germany is 1.2 mg/m3. This value is never reached under normal conditions. 4 . Phenolic Compounds with Mutagenic Activity

The phenols of curing smoke are of some interest with respect to toxicological properties. Carcinogenic or cocarcinogenic influences have been reported from phenols of tobacco smoke and tea (Gibe1 and Gummel, 1967; Kaiser and Bartene, 1966, 1967). Pool and Lin (1981, 1982) found mutagenic effects arising from some phenols detected within purified total phenol fractions of curing smoke (Pool and Lin, 1982). In addition, phenols may react with nitrites in cured

CHEMICAL ASPECTS OF MEAT SMOKING

149

TABLE XVII POLYCYCLIC AROMATIC HYDROCARBONS FROM CURING SMOKE WITH CARCINOGENIC OR MUTAGENIC PROPERTIES",' PAH

7,12-Dimethylbenz[a]anthracene 3-Methylcholanthrene Benzo[ alpyrene Dibenz[a,hlanthracene Dibenzo[a,ilpyrene Benzo[clphenanthrene Benzo[ylanthracene Indeno[ 1,2,3-c,d]pyrene Benzo[blfluoranthene Chrysene Benzo[klfluoranthene Picene Benzo[g.h, ilperylene 1-Methylpyrene Dibenz[a,c]anthracene Benzo[elpyrene Anthanthrene 2-Methylanthracene 9-Methylanthracene 2-Methylphenanthrene Pyrene Benzo[alfluorene Triphenylene Fluoranthene Perylene

Carcinogenic

Mutagenic

+ + + + + -

+ + + + + + + + + + + + + + +

-

+(+++) +(+++)

" From Fretheim (1976), Kaden et al. (1979), Umweltbundesamt (1979). - , Not carcinogenic (only sometimes considered); -t , uncertain; + , carcinogenic, mutagenic; + + , + + + , + + + + : increasing carcinogenicity; -, not determined. meat products, thereby forming nitro- and nitrosophenols (Knowles et al., 1975b). Nitrosophenols are potential catalysts in nitrosamine formation (Davies and McWeeny, 1977; Walker et al., 1979, 1982). Nitrosophenols themselves show a mutagenic effect (Gilbert et al., 1980). Whether nitro- and nitrosophenols are present in smoked meat products in an amount dangerous to human health is not yet known. If liquid smoke is mixed with nitrite containing brine, nitro- and nitrosophenols are formed. Therefore, an application of smoke-flavored brines to meat products may be undesirable.

150

L. TOTH AND K . POITHAST

VII. CONCLUSION In the last few decades curing smoke has been analyzed with increasing success. Now several hundred compounds have been detected, and most of them identified by gas chromatography or GC-MS, using pure substances for their identification. The molecular weights of more compounds are known. Several smoke fractions have been investigated for sensory, technological, and toxicological effects. Browning reactions, flavor development, and preservation have been studied. These studies demonstrate that phenols play a predominating role in flavoring foods but that a real smoke flavor is due to other additional cyclic and noncyclic compounds. From chemical analyses we learned that smoke composition varies with different smoke generation conditions. It is dependent on the type of wood (softwood or hardwood), the humidity of the wood, and the generation temperature. The temperature influence is most important. High generation temperatures are responsible for contamination of smoked foods with high amounts of PAH and low temperatures for the development of an unpleasant smoke flavor. Smoke has to be generated in the presence of small but adequate amounts of oxygen, as oxygen causes the temperature to rise. The generation temperatures should not exceed 650-700°C. This optimal generation temperature, at which the smoke composition shows the most favorable ratio of desired to undesired compounds, may be easily achieved by wetting the wood (50-60% H,O). With high water content of the wood, the smoke density increases, and the smoking procedure may be done in a shorter time. Smoke of higher humidity is easier to condense in water; thus a reduction of emissions seems to be possible. Coloring, flavoring, and preserving properties are excellent. Temperature and humidity control in curing smoke generation are the most important factors whether the smoke is used in food or liquid smoke production.

ACKNOWLEDGMENT We would like to thank R. McCormick for his help in reviewing and correcting this article

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151

Berger, B., Hemra, L., and Nilsson, R. 1975. The evaluation of some smoke condensates. Sitzungsber. Eur. Fleischforscherkongr., 21st, 1975 p. 184. Block, H. 1979. Untersuchungen an den hohermolekularen braunen Teerstoffen von RaucharomaPraparaten. Dissertation, D 83, No. 88/FB 13. Technische Universitat, Berlin. Boys, A,, Obiedziwski, M., and Olkiewicz, M. 1976. Die Identifikation von phenolischen Verbindungen in einem polnischen Raucherrauchextrakt. Congr. -Dok., Eur. Fleischforscherkongr., 2 2 d , 1976 Vol. 11, Part 8, pp. 1-7. Bratzler, L. J., Spooner, M. E., Weatherspoon, J. B., and Maxey, J. A. 1969. Smoke flavor as related to phenol, carbonyl and acid content of bologna. J. Food Sci. 34, 146. Chen, L., and Issenberg, P. 1972. Interactions of some wood smoke components with €-amino groups in proteins. J. Agric. Food Chem. 20, 1113. Daun, H. 1966. The influence of generation parameters upon the amount of the phenolic fraction on the sensoric quality of the dispersed phase of curing smoke. Ph.D. Dissertation, Politehnika Gdanska. Daun, H. 1969. Chemische und organoleptische Eigenschaften von durch Saulenchromatographie untersuchten Raucherrauchfraktionen. Tehnol. Mesa 10, 12. Daun, H. 1972. Sensory properties of phenolic compounds isolated from curing smoke as influenced by its generation parameters. Lebensm.-Wiss. Technol. 5, 102. Daun, H. 1979. Interaction of wood smoke components and foods. Food Technol. 33, 66. Davies, R., and McWeeny, D. J. 1977. Catalytic effect of nitrophenols on N-nitrosamine formation. Nature (London) 266, 657. Deng, J. C., and Lillard, D. A. 1973. The effect of curing agents, pH and temperature on the activity of porcine muscle cathepsins. J. Food Sci. 38, 299. Doerr, R. C., Wasserman, A. E., and Fiddler, W. 1966. Composition of hickory sawdust smoke. Low-boiling constituents. J . Agric. Food Chem. 14, 662. Draudt, H. N. 1963. The meat smoking process: A review. Food Technol. 17, 8 5 . Drawert, F., and Beck, B. 1974. Gaschromatographisch-massenspektroskopischeUntersuchungen von bestrahlten bzw. erhitzten Fetten, Olen und Modellsubstanzen. 111. Kohlenwasserstoffe im Raucherschinken. Z. Lebenm-Unrrrs. -Forsch. 156, 20. Drawert, F., Lessing, V., and Leupold, G. 1977. Uber die Abtrennung und gaschromatographische Bestimmung von niedermolekularen phenolischen Verbindungen. Chem. Mikrobiol., Technol. Lebensm. 5, 65. Dvorak, Z., and Vognarova, I. 1965. Available lysine in meat and meat products. J. Sci. Food Agric. 16, 305. Elmenhorst, H . , and Grimmer, G. 1968. Polycyclische Kohlenwasserstoffe aus Zigarettenrauchkondensat. Z. Krebsforsch. 71, 66. Fenner, R. A,, and Lephardt, J. 0. 1981. Examination of the thermal decomposition of Kraft pine lignin by Fourier transform infrared evolved gas analysis. J . Agric. Food Chem. 29, 846. Fessmann, G. 1965. Vorrichtung zum kontinuierlichen Herstellen eines Rauchermittels. Patentschrift 1,492,617 der BR Deutschland. Fiddler, W., Doerr, R. C., Wasserman, A. E., and Salay, J. M. 1966. Composition of hickory sawdust smoke. Furans and phenols. J . Agric. Food Chem. 14, 659. Fiddler, W., Parker, W. E., Wasserman, A. E., and Doerr, R. C. 1967. Thermal decomposition of ferulic acid. J . Agric. Food Chem. 15, 757. Fiddler, R., Doerr, R. C., and Wasserman, A. E. 1970. Composition of an ether-soluble fraction of a liquid smoke solution. J . Agric. Food Chem. 18, 310. Filipovic, J . , and TBth, L. 1971. Polycyclische Kohlenwasserstoffe in geraucherten jugoslawischen Fleischwaren. Fleischwirfschaft 51, 1323. Foster, W. W., and Simpson, T. H. 1961. Studies of the smoking process for foods. I. The importance of vapours. 11. The role of smoke particles. J. Sci. Food Agric. 12, 263 and 635.

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ADVANCES IN FOOD RESEARCH, VOL. 29

FOOD TEXTURE PERCEPTION C. M. CHRISTENSEN Monell Chemical Senses Center and School of Dental Medicine, University of Pennsylvania Philadelphia, Pennisylvania

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Complexity of Textural Sensations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Definition of Texture. . . . . . . . ............... Physiology of Texture Perception. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Anatomy of the Oral Cavity.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Oral Sensations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Oral Responses to Foods.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ill. Texture Perception. . . . . . ................................ A. Textural Dimensions . . . . . . . . . . . . . . . . . ...... ... B. Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Roughness . . . . . . . .................................... E. Fracturability . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ IV. Future Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . ................................. 11.

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159 160 162 162 163 164 170 175 176 179 I85 189 190 I94 195

INTRODUCTION

The importance of flavor to the acceptability of food has long been acknowledged. It is only more recently that texture has also been recognized to be of significant importance in consumers’ responses to foods. As a result, there is a growing need to understand how humans perceive the textural characteristics of foods. This includes delineating which sensory systems are used to perceive texture, defining the physical elements of food that reliably produce textural sensations, and searching for physical, physiological, and psychological variables that modify texture sensations. The purpose of this article is to describe what is presently understood of human abilities to perceive food texture and to highlight areas where further research is needed. IS9 Copyright Q I084 by Academic Pre\\ Inc All rights of reproductton ~n any form rmened ISBN 0-12~016429-9

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A. COMPLEXITY OF TEXTURAL SENSATIONS Of all the sensory stimulating properties of food, it is texture that produces the most complex sensory experience. The complexity is derived principally from two sources: perceptual and physical. Perceptual complexity is introduced because the identification and differentiation of most textural characteristics of food occur only after food is actively mainpulated or deformed. Thus, complexity occurs because the physical stimuli comprising food texture cannot be sensed directly but, rather, are perceived via sensations produced by the motor movements that manipulate and deform food. For example, the hardness of a food sample cannot be sensed merely by holding the sample in the mouth. Force must be applied by the teeth to deform or rupture the food before hardness can be determined. As the upper diagram in Fig. 1 illustrates, sensory stimuli usually impinge directly on the organism (0)to stimulate the appropriate receptors. Thus, the organism is mainly a passive recipient of sensory information as, for example, when perceiving the sweetness of a beverage. However, with texture perception, another step is addcd in the chain of events from stimulus to response, as illustrated in the lower portion of Fig. 1. The peripheral motor activities of the organism transform the original set of stimuli (S,) to a different but related set of stimuli (S,) that are then processed more centrally. In the case of texture perception, the organism transforms the original stimuli through the manipulation and deformation of food. Whether perception is active or passive, a number of events

0

Passive perception

Active perception

(

texture

)

FIG. I . Texture perception is at one end of a sensory continuum in which stimulation of the organism often does not occur passively ( S , ) (upper) but occurs when stimulation (S2) is produced after active manipulation or deformation of !he physical object (lower).

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occur internally between sensory stimulation at the periphery ( S , ) and the response (R) that the organism gives reflecting the quality and strength of the resultant sensations. These events are collectively termed “processing” and consist of preprogrammed (genetic) transformations of the peripheral sensory input as well as influences on the input from experience (learning) and immediate events such as physiological, motivational, and attentional variables. The other principal source of complexity derives from the physical system. Physical measures of the textural properties of foods often vary with the applied force and the rate and degree of deformation. For example, the viscosity of most food systems is dependent on the rate that the food is sheared. This relationship can be quite dramatic; for some food systems, a severalfold change in solution viscosity can be recorded over a shear rate range assumed to exist in the oral cavity. Therefore, one first must determine the physiologically relevant ranges of applied force, shear, or deformation before being able to relate the appropriate physical and sensory measures of texture. Unfortunately for the researcher, the rate and degree of manipulation or deformation exerted by individuals can vary considerably, and the variability is evident both between different individuals and among a single individual during an encounter with food. Once a physiological range is identified, one then must determine the rule of integration used to process the range of textural sensations. Perhaps, for example, individuals take a weighted average of the different sensations that occur along the relevant deformation or shear rate range. Many sensory integrating rules are possible, and, as a consequence, it is rarely possible to define with any certainty the direct (S,) or transformed (S,) stimuli used by the perceiving organism. More than one sensory system may code aspects of food texture, which also complicates the task of understanding texture perception. Before food is ingested, some textural judgments can be made visually and with the hands. In the mouth, texture is perceived by the tongue, teeth, and tissues lining the oral cavity as well as by kinesthetic information produced by movement of and feedback from the oral musculature. Sounds that accompany food manipulation and breakdown may also signal elements of food texture. Thus, in order to understand how texture is perceived, one must identify whether only one sensory mode dominates the perception, or if sensory information is integrated from several modalities. This becomes critically important in instances where it is possible for conflicting sensory information to arise from different sensory channels. There has been relatively little investigation of texture perception, and no doubt this is due, in part, to the difficulties outlined above. Research is also hindered by the difficulties in preparing physical stimuli for texture studies. The physical systems that provide stimuli for texture sensations are intricate, and it is oftentimes difficult to identify, isolate, and reproduce these critical physical elements for studies of texture perception. It is relatively easy to produce a series of solutions that vary in sweetness; one merely dissolves different quantities of

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sucrose in an aqueous base. However, it is not a simple task, for example, to produce a series of textural stimuli that vary solely in cohesiveness. As a consequence, many of the studies in the literature that obtain textural judgments do so without directly manipulating the underlying physical stimuli. Hence, the research literature is replete with texture studies; but few of them help us better understand the physical stimuli and the resultant sensations that together produce texture perception.

B.

DEFINITION OF TEXTURE

As texture has been difficult to measure, it also has been difficult to define. There appears to be no single generally accepted definition of texture. Some (e.g., Szczesniak, 1963) would define texture in terms of its combined physical and sensory properties, whereas others (e.g., Kramer, 1964) would define it only in terms of the sensations that are produced. It would seem less ambiguous and probably more valid to define texture in terms of both physical and sensory properties. A general definition of texture that includes physical and sensory properties was proposed by Szczesniak (1963); “the composite of the structural elements of food and the manner in which it registers with physiological senses” (p. 386). Another definition of texture offered by Szczesniak (1979) is “the manifestation of the structural elements of food in terms of appearance, feel and resistance to applied forces.” Jowitt (1974) defined texture as “the attribute of a substance resulting from a combination of physical properties and perceived by the senses of touch (including kinesthesis and mouthfeel), sight, and hearing. Physical properties may include size, shape, number, nature and conformation of constituent structural elements” (p. 353). This definition captures an important aspect of texture, namely that the physical and sensory elements of texture are secondorder properties derived from more basic elements of the food structure and sensory systems. The definition would be more complete if it also highlighted the dynamic aspects of texture; i.e., that most textural properties are derived from manipulation or deformation of the food structure by human or machine.

11.

PHYSIOLOGY OF TEXTURE PERCEPTION

Obviously a very significant portion of food texture perception is derived from sensations originating in the oral cavity. As such, an overview of the anatomy of the oral cavity and a description of the sensory receptors and sensations arising from the oral cavity are important for understanding texture perception. These elements are briefly described. Also described are the actions of chewing and salivation which are the principal oral responses to foods. These responses are

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also important for an understanding of texture perception because they produce significant alterations in the physical texture of food. A.

ANATOMY OF THE ORAL CAVITY

An illustration of the human oral cavity is depicted in Fig. 2. The labia (lips) are the most anterior structure of the oral cavity. The portion of the labia inside the oral cavity has a moist mucous lining, as do the majority of the tissues in the oral cavity. The mucous lining contains many minor salivary glands which consist of small aggregates of salivary secretory cells that empty saliva into the oral cavity. There are three pairs of major glands: parotid, submandibular, and submaxillary. The major glands are characterized by the presence of a large number of secretory cells interconnected by a series of ducts that empty saliva into the oral cavity via one or more major ducts. The parotid glands are located in the buccal (cheek) walls, from which saliva empties into the oral cavity via Stenson’s duct, located opposite the maxillary molars. The submaxillary and submandibular glands are found within tissues comprising the floor of the oral cavity and opposite the base of the inferior portion of the tongue. There are several ductal openings for these glands. The maxillae and the mandible are the bony portions of the skull that comprise the jaw. The maxillae form the upper jaw and are stationary during chewing. The mandible, or lower jaw, has opening, closing, lateral, and protrusive movements controlled by several muscle groups inserted on the mandible. The temperoman-

Uvula

- Hard palate

Circumvallate papillae

FIG. 2. A frontal view of the oral cavity with principal structures labeled.

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dibular joint joins the mandible to the cranium (skull) and serves to define the movements of the lower jaw. The alveolar process is the bony mass in which the maxillae and mandible teeth are embedded. It forms a ridge behind the maxillary anterior teeth termed the alveolar ridge. Posterior to the alveolar ridge on the roof of the mouth is the hard palate. The soft palate is even more posterior and begins roughly where the teeth end. The pharynx begins at the nasal cavity in the region of the throat. The musculature of the soft palate and the pharynx are of considerable importance for swallowing. The periodontium, composed of muscles and epithelial tissue, support the teeth. From the anterior midline to posterior areas, the teeth are broadly classified as incisors, canines, and molars; the surfaces of the teeth facing the lips are referred to as the labial surfaces and those facing the oral cavity are termed the lingual surfaces. The tongue is a muscular mass that occupies most of the area of the oral cavity. Its movements are exquisitely controlled for speech and for the manipulation of food. The dorsal tongue surface contains many small bump-like projections, termed papillae, of which there are four morphologically distinct types: filiform, fungiform, foliate, and circumvallate. The filiform papillae are the most numerous and the only papillae that contain no taste receptors. Though much less numerous, there are also papillae on the hard and soft palates and on the pharynx. It is useful to distinguish the tip from the blade or body of the tongue because, as described below, there are regional differences in sensory responsiveness.

B.

ORAL SENSATIONS

The oral cavity contains receptors serving the senses of taste and somesthesis (skin senses), and it is the latter sense that is most important for the perception of texture. With perhaps the exception of the fingers, the oral cavity contains a more dense innervation of nerve fibers and receptors serving somesthesis than other regions of the body (Mountcastle, 1974a). This density means that thresholds for the perception of somesthetic stimuli frequently are lower than on most other skin areas of the body, and the discrimination of the location and intensity of these stimuli are also generally more acute. Somesthetic receptors are found in all regions of the oral cavity, including the lips, tongue, teeth, and mucosa. As on the body surface, oral sensitivity shows regional variation. Using small punctate stimuli, oral regions have been found that are differentially sensitive to somesthetic stimulation, such as that produced by tactile or temperature probes. There are considered to be five sensory qualities associated with somesthesis: touch-pressure, pain, warmth, cold, and joint position. In addition to free nerve endings, there are a number of specialized and morphologically distinct neural and epithelial tissues that have evolved for detecting somesthetic stimuli. Several

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of these are illustrated in Fig. 3. There appears not to be a unique receptor type serving each quality of the somatic sense, although there are characteristic receptor-nerve fiber units associated with each quality. The fifth (V) cranial nerve, the trigeminal nerve, carries the majority of somesthetic information from the face and oral cavity to the central nervous system. Branches of the VII, IX, and X cranial nerves also convey some somatic sensations, particularly from posterior regions of the oral cavity.

FIG. 3. Section of glabrous skin cut transversely across papillary ridges. The section illustrates various types of capsulated and unencapsulated cutaneous sensory receptors and their relative location in the dermal layers. From Mountcastle, Vernon B. (1980). Sensory receptors and neural encoding: Introduction to secondary processes. In “Medical Physiology” (Vernon B. Mountcastle, ed.), 14th ed. Mosby, St. Louis, MO.

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The sensations associated with food texture perception are composed of subsets of the “primary” somatic qualities. However, as stated earlier, the mere presence of food in the oral cavity stimulates few texture-related sensations. Most sensations associated with food texture occur only when food is manipulated, deformed, or moved across the oral receptor sheath.

I.

Touch-Pressure

The sensory qualities of light touch-pressure and vibration are detected by several classes of rapidly and slowly adapting neural elements that respond to small deformations of the skin. The properties of these mechanoreceptors and their peripheral nerve fibers have been identified and studied mostly in non-oral regions of the skin, although it appears that the same basic relationships are found in the oral cavity (Grossman and Hattis, 1967). Rapidly adapting units signal the velocity with which the skin is deformed; they respond only to the application and removal of a deforming stimulus, and are associated with specialized epithilial laminar endings such as Pacinian and Meissner’s corpuscles. The Pacinian corpuscle has also been identified as being particularly sensitive to vibratory stimulation, with maximum sensitivity in the 250 Hz range (Verrillo and Gescheider, 1979). Slowly adapting neural units discharge for the duration of the deformation, and, hence, are well suited for signaling the location of stimulation; they are associated with Merkel-cell neurite complexes and Ruffinitype endings (Johansson, 1979). Many of the nerves conveying mechanoreceptive information are large, myelinated fibers; hence, the transmission of this information to the brain is rapid. There have been only a handful of studies exploring the mechanoreceptive sensitivity of the oral cavity. Using nylon filaments, thresholds for light touch were found to be lower on the tip of the tongue and hard palate than on the blade of the tongue; mean values in log force (0.1 mg) were 1.95, 2.07, and 2.53, respectively, for the tongue tip, hard palate, and tongue blade (Henkin and Banks, 1967). These areas also demonstrated the same relative sensitivity in a two-point discrimination task that determines the smallest distance required to detect the presence of two separate sources of punctate tactile stimuli: tongue tip (1.5 mm), hard palate (1.95 mm), and tongue blade (2.59 mm) (Henkin and Banks, 1967). In another study, Ringel (1970a) found that the lip and the tip of tongue had lower two-point thresholds than the soft palate and alveolar ridge and, further, that midline regions of these areas were more sensitive (had lower thresholds) than lateral regions. In the same two-point discrimination task, the fingertips were found to be less sensitive than the tongue (midline and lateral regions) but more sensitive than other oral regions. Verrillo (1966) examined the sensitivity of the tongue to vibratory stimuli. The dorsal region is reportedly devoid of Pacinian-type corpuscles, and, thus, it was

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predicted that the surface of the tongue would not demonstrate the same sensitivity to vibratory stimuli evidenced by regions of the skin containing Pacinian corpuscles. This prediction was confirmed. An earlier study (Sherrick, 1953) attributed the poor vibratory sensitivity of the lips and tongue to the reduced capacity of these softer, fleshy tissues to conduct vibrations. The comparative sensitivity of various regions of the oral cavity to the textural stimulus of roughness/smoothness was examined by Ringel (1970b). Swatches of emery cloth of various grit densities (higher densities should be perceived as smoother) were stroked with a constant force over various surfaces of the oral cavity. The midline, lower, and upper lips (at the mucocutaneous junction) were most sensitive to gradations in grit density. The fingertips are considered to be quite sensitive to differences in roughness/smoothness, and, in this study, discrimination by the tip of the tongue and fingertips were comparable. The tongue blade and alveolar surface were found to be the least sensitive oral regions. The periodontium has also been investigated for its sensitivity to touch-pressure. The teeth are reported to contain receptors sensitive only to painful stimuli (Anderson et al., 1970); thus, the apparent sensitivity of the teeth to applied forces is thought to be sensed chiefly by receptors located in the periodontium. Manly el al. (1952) measured thresholds for detecting forces applied along the long axis and lateral surfaces of various teeth. The thresholds for detecting forces applied to the biting surface (long axis) were found to be 1 g for anterior teeth and 10 g for posterior teeth. The anterior teeth were also more sensitive to forces applied laterally, although the threshold range was lower (0.5-2.0 8). Loewenstein and Rathkamp (1955) also found that the anterior teeth were more sensitive to applied forces and noted that subjects were better able to identify the locus of stimulation when anterior teeth were tested. Using the maxillary central incisor teeth, minimal detectable differences between two applied forces were found to be lowest in the 50-500 g range (Bowman and Nakfoor, 1968). The optimal force range for discrimination of two applied forces was found to vary among different teeth (Bonaguro et al., 1969). The absolute force values required for detection have been found to vary directly with the rate of applied force (van Steenberghe and de Vries, 1978); force detection thresholds were lower at higher rates of applied force. This relationship was paralleled by the degree of measured tooth movement, suggesting to the authors that the viscoelastic properties of the periodontal tissues were responsible for the relationship of threshold to rate of applied force. 2 . Pain

Intense tissue-damaging stimulation of thermal, mechanical, or chemical origin produces sensations of pain. Pain is complex because it is not a single, qualitative sensation. Melzack and Torgerson (197 1) studied the sensory experi-

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ence of pain by examining verbal descriptors associated with pain; a number of qualitatively different forms of pain were identified, as well as forms of pain that had strong temporal and spatial qualities. Pain is also associated with emotional and evaluative responses, and these emotions may contribute to the variation in perceived pain induced by psychological and social variables (Melzack, 1973). The peripheral and central nervous system pathways for pain differ from other sensory system pathways, permitting a number of physiological variables to modulate the experience of pain (Mountcastle, 1974b; Melzack, 1973). Painful sensations seem not to be associated with specialized receptor endings in the skin. It is assumed that free nerve endings associated with several types of smaller diameter myelinated and unmyelinated sensory nerves subserve different aspects of pain. The larger diameter, myelinated fibers appear to conduct shorter latency pricking pain, whereas the smaller diameter unmyelinated fibers conduct the longer latency burning, diffuse type of pain (Mountcastle, 1974b). This difference in the conduction time of the two types of pain is not so noticeable in the head region because of the proximity of these fibers to the brain (Dubner et al., 1978). Electrophysiological studies demonstrate that the majority of fibers detecting pain are polymodal; that is, they respond to noxious stimulation of varying origin. The oral and nasal regions of the face are more densely innemated with pain receptors compared to many other regions of the body (Dubner et a l . , 1978). Presumably all regions of the oral cavity respond to painful stimuli, but comprehensive investigations of possible regional differences in sensitivity are lacking. Investigations using punctate stimuli demonstrate that more pain-sensitive spots are found in posterior areas of the tongue and oral cavity (Schriever and Strughold, 1926; Hirsch and Schriever, 1929). Although there is some controversy, the tooth pulp and adjoining areas presumably respond only to painful stimuli, as described earlier, and not to mechanoreceptive and thermal stimuli in normal ranges (Anderson et al., 1970). 3.

Thermal Sensations

Warm and cold sensations are served by separate sensory systems, although the perception of temperature very likely reflects the integration of activity in both systems. The results of psychophysical and electrophysiological research to date suggest that cold receptors lie closer to the surface of the skin than do warm receptors (Hensel, 1973). A specialized receptor ending for cold stimuli has been identified, although other types of receptors may also be sensitive to cold stimuli; receptors for warm stimuli have not yet been identified with certainty. Through the use of small diameter temperature probes, very small regions of the skin (- 1 mm) have been identified that are differentially sensitive to either warm or cold stimulation. These studies have revealed that there are many more

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cold than warm spots and that the face, particularly the lips, contains more temperature-sensitive spots than other regions of the body (Hensel, 1973). Intraoral studies suggest that the anterior portion of the mouth contains more temperature spots than other oral regions (Yamada et al., 1952). Their density should not be interpreted to mean that these regions of skin are necessarily more sensitive to macroscopic thermal stimulation (Stevens and Green, 1978). A very prominent property of thermal sensation is spatial summation; the perceived intensity of thermal sensation grows as a function of the area of skin that is stimulated. Thus, the number of cold or warm spots is not necessarily indicative of the intensity of perceived hot or cold stimuli in a more natural setting where a greater area of the receptor sheath would be stimulated. Research on possible oral regional differences in sensitivity to macroscopic thermal stimuli remains to be done.

4. Joint Position Proprioception is the general term applied to the sensations associated with the perception of the position of parts of the body. The sense of movement (and associated position) is more specifically described as kinesthesis. In the mouth, the sensations of position and movement arise from receptors in the temperomandibular joints. Four types of receptors have been identified in the tissues of the joint: free nerve endings, Ruffini, Golgi tendon organs, and Vater-Pacinian corpuscles (Dubner er al., 1978). The free nerve endings are the most numerous type of receptor cell, but the function of this receptor ending or the other specialized receptor endings remains poorly characterized. Receptors in muscle tissue (e.g., Golgi tendon organs) may also contribute to kinesthetic perception and presumably are especially important in signaling the position of the tongue. Oral kinesthetic sensations are quite relevant to food texture perception because they provide feedback on the angular movement and position of the mandible. These sensations are likely to be critical for the estimation of food particle size and to the identification of the shape of food before and during mastication. In concert with mechanoreceptors in the periodontium, the joint receptors are also likely to contribute significantly to the estimation of food hardness. A number of investigators have examined threshold and discriminatory capabilities for the perception of the size (thickness) of objects held between the maxillary and mandibular teeth. The threshold for detection of foil held between either the molars or the incisors was found to be approximately 0.010 mm in one study (Siirila and Laine, 1963) and 0.016 mm in another (Caffesse et al., 1973). Manly et al. (1952) found that the threshold for detection of different disk thicknesses placed between the teeth was 0.18 mm when the disks were 5-6 mm thick. The threshold for size discrimination was found to be 2 mm when the thickness range was 9.6-28.8 mm (Ringel, 1967). Decrements in size discrimi-

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nation near absolute threshold levels (0.012-0.10 mm) have been noted when periodontal areas are anesthetized (Riis and Giddon, 1970), suggesting that periodontal mechanoreceptors may also contribute to the perception of the thickness of objects held between the teeth. The determination of object hardness in the oral cavity probably depends to varying degrees on the sensations arising from both the temperomandibular joints and the periodontium. Compared to individuals with normal dentition, denture wearers were equally able to discriminate the hardness of a series of relatively soft rubber discs but were significantly less able to discriminate a series of hard rubber discs (Manly et al., 1952). The soft series would be more easily compressed by the teeth, and so significant amounts of mandibular movement would be associated with determinations of hardness. Thus, the authors reasoned that kinesthetic sensations may be more important than periodontal tactile/pressure sensations for hardness judgments of a soft series. The reverse may be true when a hard series is judged because the objects would be compressed very little by the forces associated with biting, and so judgments of hardness would be more dependent on periodontal sensations that could be distorted by dentures. Oral stereognosis describes the ability to identify the shape of objects in the oral cavity. There is a small body of literature comparing non-oral and oral tactile abilities to identify shapes as well as comparing the oral stereognotic ability of normal individuals with those who are either speech impaired or have some form of oral motor impairment (see Bosma, 1967, 1970). From these studies it can be inferred that the tongue contributes very significantly to oral stereognotic ability (Mason, 1967), significantly more so than sensations arising from the lips or the hard palate (McDonald and Aungst, 1967). However, much remains to be done to characterize the strategies used to identify objects in the oral cavity.

C.

ORAL RESPONSES TO FOODS

Food in the oral cavity becomes a stimulus for a series of oral responses; most significantly for the responses of mastication and salivation. These responses alter the texture of the food stimulus to produce a constantly changing pattern of textural sensations from the moment food is introduced in the oral cavity until it is swallowed. Mastication deforms and shears the foodstuff by crushing and grinding actions, and sensory feedback from the actions of mastication supply essential information for oral texture assessment. Salivation wets the food bolus and causes it to become softened, disintegrate, and become diluted. The characteristics of mastication and salivation are described below, and the effects of the type of food bolus on modifying the expression of these oral responses are also discussed.

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Mastication

Mastication (chewing) is produced by the hinge-like movements of the mandible. Relative to the maxillae, the temperomandibular joints permit vertical, lateral, and protusive-retrusive movements of the mandible. A number of muscle groups participate in the organized concert of actions that produce the cyclic and repetitive pattern of chewing movements. A system of neural reflexes and a central neural pattern generator are thought to control the sequence of excitatory and inhibitory actions on the various muscle groups that produce the opening and closing jaw movements (see Dubner et a f . , 1978, for a review of this topic). Although the lower jaw movements are most obviously associated with the act of chewing, mastication of food also requires the complex coordination of other msucle groups, particularly those controlling the tongue, lips, and cheeks. These latter serve chiefly to repeatedly select and position portions of the food between the dentition for further breakdown. Although there is significant intrinsic control of mastication, a number of extrinsic variables alter the masticatory pattern. This is reflected both in large individual differences in chewing movements and in intraindividual accommodation of the masticatory pattern to different food textures. As described below, these variations are manifest in a number of parameters that define the masticatory act: the pattern of mandibular movements comprising a chewing stroke as well as the force, rate, and duration of these movements. The pattern of mandibular movements in a chewing cycle roughly traces a tear-shaped pattern as depicted in Fig. 4 (Ahlgren, 1966). The stroke begins with a nearly vertical jaw opening, a lateral shift of the mandible to the side with the food bolus, and, finally, jaw closing with lateral and vertical movements that restore the mandible to its central position (intercusp position). Masticatory patterns have been found to differ among individuals (children), but for a single individual, the pattern was characteristic (Ahlgren, 1966). Individual differences in the masticatory pattern were partially related to different patterns of dental occlusion. The shape of the masticatory pattern was also affected by the foodstuff being chewed (see Fig. 4). For example, when a carrot was chewed, the chewing stroke was more completely vertical, producing a chopping action; when chewing gum was used, the stroke had a wider lateral movement producing a grinding of the gum as the jaw closed at the end of the chewing stroke. After the initial chewing strokes, food is usually chewed on one side of the jaw (Ahlgren, 1966). The bolus may be shifted from side to side during chewing, but individuals usually favor one side. Chewing is concentrated more on molars, and only at the beginning of the chewing sequence do the incisors share significantly in the breakdown of food. As incisors are more sensitive to applied forces than more posterior teeth (see Section II,B, l), it is important to determine the relative

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right side

Opening phase (preparatory parti

Closing phase lrnasticatory stroke)

Sagittal plane

Chewing gum

Carrot cvcle 2

3

5

4

15

FIG. 4. The upper figure illustrates the pattern of mandibular movement for a typical chewing cycle in a frontal plane (IP, intercuspal position). The lower figure illustiates the effect of food type on changes in the pattern of an individual’s chewing cycle. From Ahlgren (1966).

importance for texture perception of sensations derived principally from the incisors during the first several bites and sensations derived principally from the molars during subsequent chewing. For individuals with full dentures, the chewing pattern is different; the food bolus makes more frequent contact with the incisors during the entire chewing sequence (Wictorin, 1972). Forces developed during chewing will vary both among individuals (Jenkins, 1966) and for different foods. Yurkstas and Curby (1953) found that lower peak and average chewing forces developed with softer foods. Among 24 common foods that were tested, the average force per stroke varied from 0.3 to 1.8 kg, and the maximum force measured was 12 kg when a hard French roll was chewed. Chewing rates are observed to vary both between individuals and during the sequence from the introduction of the food until it is swallowed (Bourne, 1977). In the latter case, the chewing rate is slower when the food bolus is first introduced in the oral cavity. Chewing rate is also affected by the type of food; one report found a higher chewing rate (- 100 strokedmin) for softer foods (Yurkstas

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and Curby, 1953), but two other reports using raw carrots ana either soft bread or gum found the opposite relationship between chewing rate and food hardness (Steiner et al., 1974; Ahlgren, 1966). These findings may not be contradictory because sticky or tough foods which also may be hard are reported to slow the chewing rate (Ahlgren, 1966); obviously, a set of physical texture measures is required to tighten up these loosely defined relationships. Slower chewing rates for certain foods appear to be attributable to a longer duration chewing stroke (Ahlgren, 1966; Steiner et al., 1974); for example, Ahlgren (1966) found that the opening portion, but not the closing portion, of the stroke was slower for chewing gum than for carrots. It should be noted that the speed of masticatory movements is not constant throughout the stroke; the mandible accelerates at the beginning of the opening and closing phases and decelerates at the turning points of the stroke. Not surprisingly, the number of chews occurring before swallowing also varies with the texture of food. Pierson and LeMagnen (1970) found that the average number of chews could vary from 0 for water and 11.5 for mashed potatoes to 103 for hard candy. Chewing breaks food into particles that can be swallowed, and the amount of chewing required to prepare food for swallowing obviously varies between foods. Individuals do not chew foods to the same particle size before swallowing, in part because of differences in masticatory efficiency. Masticatory efficiency can be impaired in individuals with full dentures, which means that these individuals fail to reduce food particle size to normal averages when tested with a constant number of chewing strokes. Yurkstas (1965) found that individuals with reduced masticatory efficiency did not compensate for this deficiency by increasing the number of chews before swallowing; rather they swallowed larger food particles. Among individuals with normal dentition, the process of reducing food particle size by mastication is not random; large particles are reduced in size more rapidly than smaller ones (Yurkstas, 1965). On the other hand, individuals with full dentures seem to be less selective and grind coarse and fine grained particles nonpreferentially (Kapur and Soman, 1964). 2.

Salivation

Saliva is one of a series of secretions that serves an alimentary function and is the first secretion directly stimulated by food. Saliva participates in digestive functions by contributing to food breakdown, by dissolving and releasing food taste and odor chemicals, and by lubricating the food bolus for swallowing. It also serves other functions in the oral cavity, chiefly the maintenance of oral tissues and dentition. Saliva is more than 99% water but contains a large number of organic and inorganic constituents. Proteins are the largest class of organic constituents, and the mucoproteins are a significant subgroup. The mucoproteins contribute to the

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viscosity and sliminess of saliva, thus providing the lubrication required for swallowing food. Enzymes are present in saliva, and many of them appear to perform antibacterial functions. Amylase is the only digestive enzyme present in saliva in significant quantities. The principal salivary electrolytes are sodium, chloride, potassium, calcium, bicarbonate, and inorganic phosphate. The concentrations of salivary sodium and chloride are less than the same ion concentrations in plasma, whereas the salivary concentrations of potassium and bicarbonate are higher. Saliva is secreted in the oral cavity principally by three pairs of major glands: parotid, submandibular, and sublingual. Their location in the oral cavity was described earlier (see Section 11,A). There are numerous minor glands spread over a wide area of the oral mucosal region, and they are identified by their location in the oral cavity: minor sublingual, lingual, labial, buccal, palatine, and glossopalatine. The relative contribution of each pair of major glands to total salivary volume depends on whether the glands are in a resting or stimulated state. The resting state is designated when there is no obvious source of salivary stimulation. Of the three major glands, the submandibular contributes 69%, the parotid 26%, and the sublingual glands 5% of the total major gland volume during resting flow (Schneyer and Levin, 1955). The parotid fraction increases disproportionately during stimulation and can contribute more saliva than the other major glands (Shannon, 1962). Dawes and Wood (1973) calculated that minor gland secretions account for approximately 10% of the total salivary volume. There are large individual differences in salivary flow rate. Exempting extreme values, salivary flow rate can vary more than eightfold among healthy individuals. Figure 5 illustrates the range in resting whole mouth salivary flow rates (saliva'of mixed origin collected from the oral cavity) typical of the adult

Resting Flow Rate ( g /rnin)

FIG 5 . The histogram depicts the wide range of resting whole mouth salivary flow rate$ observed in a healthy population of young adults ( n = 85).

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175

population. It is not presently known whether these large differences in characteristic flow rate among healthy individuals affect the ability of the oral cavity to prepare food for swallowing and, thereby, influence dietary choices. It is known that when substantial reductions in salivary flow are induced by pharmacological means, individuals display a decreased ability to perceive sour tastes presented in either a liquid or dry medium (Christensen ef al., 1983, 1984). Also, there are reported to be alterations in dietary patterns in individuals with chronic and severe dry mouth such as that, for example, which accompanies Sjogren’s syndrome (Henkin et a/., 1972). Salivary flow is increased by a variety of food-related stimuli in the oral cavity. Saliva can even be stimulated by the sight of certain of these stimuli; for example, the sight of a real lemon or even the picture of one (Christensen and Navazesh, 1984). Of taste stimuli tested, acids are the most powerful salivary stimulants (Chauncey et al., 1967). Higher concentrations of sodium chloride are relatively weak salivary stimulants (Pangborn and Chung, 1981), and other taste stimuli, for example, sucrose, are relatively ineffective salivary stimulants. In general, chemicals that produce sensations of oral irritation (trigeminal nerve stimulation) will also produce significant increases in salivary flow, for example, alcohol (Martin and Pangborn, 1971) and chili peppers (Rozin et al., 1981). Irritants that are sniffed may also increase salivary flow (Pangborn and Berggren, 1973), although this route is not nearly as effective as oral stimulation. Mastication is another powerful stimulant of salivary flow, and increasing the size or hardness of a bolus being chewed will increase salivary flow further (Kerr, 1961). Other trigeminal stimulants that increase salivary flow are cold or hot oral stimuli and drying of the oral mucosa (Kerr, 1961; Pangborn et al., 1970): Although the identities of the basic elements of food that will trigger salivary flow are known, there are virtually no data available on the degree to which different foods will stimulate salivary flow. It is technically difficult to measure flow when solid foods are used, although Pangborn and Lundgren (1977) measured salivary flow in response to crisp bread served in either powder or piece form and found that greater salivary flow was elicted by the powdered form of bread. A small amount of data is available on salivary flow in response to liquid foods; for example, to differently flavored broths (Pangborn and Chung, 1981) and to wine (Martin and Pangborn, 1971). Considerably more research is required in this area in order to measure and subsequently predict the impact of saliva on the perception of the flavor and texture of foods.

Ill. TEXTURE PERCEPTION The preceding sections have established a framework for understanding the goals of food texture perception research and a means for conducting, and

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interpreting the results of, research in this area. In particular, two concepts that were developed in earlier sections should be recalled. First, the perception of food texture (indeed, even the instrumental measurement of food texture) requires the active manipulation or deformation of food. This is significant because differences in the degree, rate, and force of food manipulation and deformation will result in different textural sensations both for physiological and physical reasons. Second, there are no physiological receptors for texture, per se. Textural sensations are an integration of sensations arising from tactile and kinesthetic receptors in dermal, muscle, and connective tissue. This is significant because it means that there is little physiological basis for textural “primaries” as there are for many other classes of sensations; thus, our concept of qualitatively different textures is a product of our grouping and classifying sets of sensations into separate perceptual categories based on a feeling of their natural divisions. The rcmaining sections of this article describe the principal findings from studies designed to measure food texture perception in humans. The review focuses on those studies that have aided in a better understanding of not only the nature of textural sensations but also the manner in which these sensations are perceived. The review begins with a description of the most common “working” set of textural qualities. A.

TEXTURAL DIMENSIONS

As there is little a priori basis for assembling and defining a set of mutually exclusive and exhaustive terms that encompass all sensory aspects of food texture, the construction of such a set has necessarily been a subjective exercise. There is no external referent to validate the constructed system, and this is true whether an expert in the field constructs a set of textural primaries based on experience (Szczesniak, 1963) or a set of textural categories in constructed based on the interpretation of dimensions in a factor analysis (Yoshikawa et al., 1970). The uninitiated in the texture field might reasonably assume that textural sensations may be classified based on the so-called objective measures of the physical properties of foods; e.g., rheological measures of foods. This is not possible for several reasons. First, the physical measures of texture do not mimic the sensory experience. This should be obvious from the previous section describing the physiology of texture perception; the sensory receptors do not perform the same functions as the transducers designed to obtain physical parameters associated with texture. Second, measures of the fundamental properties of food, e.g., Young’s modulus of elasticity or determinations of viscosity, do not encompass either singly or in combination all the measures that embody food texture (Bourne. 1975). Relatedly, many of the instruments constructed to measure food texture (Szczesniak, 1973) are empirical, i.e., they are constructed to measure and be correlated with perceptual conceptions of texture. Hence, the designation

177

FOOD TEXTURE PRECEPTION

commonly made that texture measures obtained from instruments are “objective” whereas those obtained from humans are “subjective” is a misleading expression for the differentiation of textural assessments made by machine or man. The most pervasive set of textural dimensions used to guide texture research and applications in academic and industrial settings was developed in 1963 by Alina Szczesniak. Table I presents a version of this food texture classification system. This scheme was developed chiefly for solid and semisolid foods, and more recently, a textural classification system was also proposed to encompass the range of textural sensations encountered with liquids (Table 11) (Szczesniak, 1979). Although modifications of the 1963 texture classification system have been proposed (e.g., Sherman, 1969), the original framework continues to dominate as a guide to classify food texture sensations. In this system (see Table I), TABLE I RELATIONS BETWEEN TEXTURAL PROPERTIES AND POPULAR NOMENCLATUREa

Mechanical characteristics Primdry parameters Hardness Cohesiveness

Secondary parameters

Fracturability Chewiness Gumminess

Viscosity Springiness Adhesiveness

Popular terms Soft + firm + hard Crumbly + crunchy + brittle Tender + chewy + tough Short + mealy + pasty + gummy Thin * viscous Plastic -+ elastic Sticky + tacky + gooey

Geometrical characteristics Class

Examples

Particle size and shape Particle shape and orientation

Gritty, grainy, coarse, etc. Fibrous, cellular, crystalline, etc.

Other characteristics Primary parameters Moisture content Fat content

Secondary parameters

Oiliness Greasiness

Popular terms Dry -+ moist + wet Oily Greasy

+ watery

~

~

From Szczesniak (1963, 1975) Repnnted from Journal of Food Scrence 28, 385-389 Copyright 0 by Institute of Food Technologists

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C . M. CHRISTENSEN

TABLE I1 CLASSIFICATION OF SENSORY MOUTHFEEL TERMSn ~

Category

Typical words

Viscosity-related terms Feel on soft tissue surfaces Carbonation-related terms Body-related terms Chemical effect Coating of oral cavity Resistance to tongue movement Afterfeel-mouth Afterfeel-physiological Temperature-related terms Wetness-related terms

Thin, thick, viscous Smooth, pulpy, creamy Bubbly, tingly, foamy Heavy, watery, light Astringent, burning, sharp Mouthcoating, clinging, fatty, oily Slimy. syrupy, pasty, sticky Clean, drying, lingering, cleansing Refreshing, warming, thirst quenching, filling Cold, hot Wet, dry

a

From Szczesniak (1979).

textural characteristics are divided into three categories: mechanical, geometrical, and “other. ” The mechanical characteristics represent the textures of a food perceived upon its deformation. The geometrical characteristics represent chiefly tactile and kinesthetic (but also visual) sensations derived from manipulation of food over or between receptor surfaces, such as the sensations arising from food particles between the teeth or between the tongue and other oral surfaces. The “other” characteristics also represent the feel of foods but are not geometric in character and include such sensations as wetness and oiliness. The mechanical characteristics are further divided into primary and secondary textural characteristics, the latter being composed of more than one primary textural characteristic. The usefulness of this classification scheme has been enhanced by the development of a sensory texture profiling method using the food textural properties developed by Szczesniak (Brandt et al., 1963; Civille and Szczesniak, 1973) and modeled after the flavor-profiling method developed at Arthur D. Little (Cairncross and Sjostrom, 1950). Panels are trained to similarly identify, label, and quantify texture sensations by familiarizing them with sets of food “standards” that represent examples of different qualities and levels of textural sensations. The analysis of textural characteristics using the profiling method is patterned after the natural sequence of food ingestion. Textural judgments are solicited after visual and non-oral examination of the food, after the food is initially manipulated in the oral cavity, including one or several bites, after further mastication, and, finally, after the food is swallowed. Because texture is a complex sensory experience, it is not surprising that trained texture panelists perform differently than untrained panelists. When trained and untrained panels

FOOD TEXTURE PRECEPTION

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judged the texture of a series of fish fillets, the texture judgments of the two groups were significantly and positively correlated (with the exception of judged moisture), but the untrained panel consistently perceived a narrower range of textural differences (Cardello et al., 1982). The same pattern of results was found even after consumer panelists received training in the use of the various texture terms at the beginning of the test session. The usefulness of the set of textural terms developed by Szczesniak has also been enhanced, particularly for the mechanical terms, by the development of instrumental measures that correlate closely with sensory measures obtained with the profiling technique. These measures have been developed with the General Foods Texturometer (Friedman et aE., 1963) and with the Instron Universal Testing Machine (Bourne, 1978). Although the textural classification system developed by Szczesniak is quite useful, it represents only the first step in a field of research still in its infancy. Substantial additional research is required to more completely identify foodrelated textural sensations and to determine their uniqueness or interrelatedness. At the same time there is an obvious need to develop a vocabulary of textural terms and accompanying definitions that can be consistently and unequivocally applied to texture sensations. B.

VISCOSITY

One of the most salient sensory characteristics of semisolid and liquid foods is its viscosity. When respondents generated a list of textural traits for beverages, reference to their thickness or thinness was the most frequently mentioned textural descriptor (Szczesniak, 1979). Reflecting its prominence as a textural sensation, and reflecting practical considerations, most research on food texture perception has focused on investigations of the perception of solution viscosity. Compared to other physical measures of food texture, it is relatively easy to measure and interpret the parameters associated with solution viscosity. It is also relatively easy to prepare model systems that reliably vary in viscosity-related parameters. Viscosity describes a solution's resistance to flow. For Newtonian fluids, shear rate ( i ,expressed as sec-') is exactly proportional to shear force (7, expressed as dyne/cm- *); thus the viscosity (q,usually expressed as centipoise) of Newtonian fluids is independent of shear rate since absolute viscosity is defined as q = TI+. Figure 6 illustrates the flow characteristics of Newtonian solutions. Simple sugar systems will exhibit Newtonian characteristics, as do certain oils, but Newtonian flow is uncommon among food systems. More frequently foods exhibit non-Newtonian flow characteristics, which means that viscosity is dependent on shear rate (i.e., the shear force is not proportional to the applied shear rate). Foods usually exhibit shear-thinning and are termed pseudo-

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C . M . CHRISTENSEN

f

m

Shear rate (sec-')

FIG. 6. An illustration of three types of flow exhibited by solutions.

plastic; as shear rate is increased, the apparent viscosity of the solution decreases (Fig. 6). The shear stress-shear rate relationships for a number of food systems are illustrated in Fig. 7. Food systems may also exhibit plastic flow, which means that a certain amount of shear stress must be applied before movement of the solution occurs. This threshold stress is termed the yield point (Fig. 6). Ketchup is a classic example of a food system exhibiting plastic behavior. Another characteristic of some food systems is time-dependent flow behavior; shear force is observed to decline (thixotropy) or increase (rheopectic flow) with a constantly applied shear rate. When the shearing ceases, these foods usually do not recover completely, so subsequent shear force-shear rate flow curves will be different. Liquid and semisolid foods usually show a very complex flow profile. The same food may exhibit Newtonian and non-Newtonian characteristics when prepared at different concentrations and over a range of shear stresses and rates. In studying viscosity perception, foods usually are chosen that have relatively simple flow relationships, or model systems are developed by using different thickeners. usually aqueous gums that provide unique and characteristic flow patterns. Though simple flow patterns aid the researcher in understanding the characteristics of human texture perception, complexity cannot be completely eliminated, and, thus, interpretation of results can be difficult. This is particularly true when oral assessments of perceived viscosity are studied. Solution flow characteristics can be altered by saliva mingling with solutions in the oral cavity, thus changing their composition and temperature. For example, saliva decreases the tendency of thin solutions to exhibit turbulent flow; a 5 % glucose solution exhibits turbulent flow at a shear rate of -400 sec - I , however, when mixed with saliva, turbulence develops at a shear rate approx-

Foon

181

TEXTURE PRECEPTION

imately 20 sec-l higher (Parkinson and Sherman, 1971). Turbulent flow increases the apparent viscosity of solutions, and Parkinson and Sherman (1971) found that the apparent viscosity of a 5% glucose solution increased 25% after salivary mixing. The implication of these findings is that saliva may influence the perceived viscosity of thin solutions when those judgments are made in the oral cavity, thus making it difficult to compare physical and perceived measures of viscosity. In the experimental setting, salivary effects can be minimized by using large volumes of test solution and by limiting the time subjects hold solutions in the oral cavity. However, salivary factors may play a significant role in the perception of solution viscosity in natural eating situations and therefore should be studied. Investigators have taken advantage of the different shear stress-shear rate functions of foods in an attempt to identify the range of shear rates and stresses that develop when human subjects evaluate solution viscosity. Wood (1968) used a Newtonian glucose solution and several pseudoplastic sauces and measured their shear stress-shear rate functions over a wide range. The flow curves for the pseudoplastic solutions crossed the glucose function at different points along the shear force-shear rate range; thus, a pseudoplastic solution could have both a higher and lower viscosity than the glucose solution depending on the shear rate. GLUCOSE SYRUP

105

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104

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EAMED TOMATO SOUP (CROSSE 5 BLACKWELL)

c m

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TOMATO SOUP(ST MICHAEL) TOMATO SOUPBAXTERS)

v) v)

LT w

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102

SHEAR RATE

103

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(SEC")

FIG. 7. The relationship between shear stress and shear rate for a range of food samples. The band cutting across the various shear stress-shear rate functions defines the region where oral and instrumental judgments of viscosity correspond. From Sharna and Sherman (1973b).

182

C . M. CHRISTENSEN

Subjects orally judged the viscosity of the sauces in relation to the perceived viscosity of the glucose solution. Based on subjects’ responses and the flow curves of the test solutions, it was concluded that measures of perceived solution viscosity were best correlated with viscosities recorded for solutions in the region near SO sec-l. Wood hypothesized that the shear stress that developed at a constant shear rate of approximately SO sec- was the oral sensory stimulus for viscosity. This hypothesis was then indirectly tested by using a different set of stimuli (a series of soups) and asking subjects to rate their viscosity. Wood obtained a good linear fit between log sensory and instrumental viscosity measures when solution viscosities were obtained at SO sec l . Shama and Sherman (1973b) employed the same strategy as Wood (1968) but used food stimuli that spanned a much wider viscosity range. By comparing flow curves and subjects’ oral judgments of solution viscosity obtained with a paired comparisons methodology, they identified a band of shear rates and stresses that produced solution viscosities that ranked similarly to subjects’ judgments of viscosity. The band is depicted in Fig. 7 crossing the flow curves for the various test food items. The authors hypothesized that for thin solutions approximately below 70 cp (centipoise) (roughly equivalent to the viscosity of a 60% sucrose solution), solution viscosities were discriminated by different solution shear rates developed at a constant shear stress. For more viscous solutions, viscosities were discriminated by different shear stresses developed at a constant shear rate (approximately 10 sec - l ) . Although it offers a working hypothesis, one cannot simply conclude that shearing stresses and rates operating in the oral cavity during determination of solution viscosity are the same as those for portions of the flow curve that best correlate with subjects’ assessments of solution viscosity. Both Wood (1968) and Shama and Sherman (1973b) found that solution viscosity was best predicted when either shear rate or shear stress was held constant. Obviously, subjects will be experiencing solutions at a range of shear rates and shear forces in the process of intake and swallowing. However, their data may suggest that one variable may be more important than another when subjects judge viscosity; for example, it may be that subjects use shear force more than shear rate to judge the viscosity of thicker solutions because a greater range of shear stresses may develop in the mouth but only a limited range of shear rates may develop. Christensen (1979, 1980) found that higher shear rates operate in the oral cavity than those suggested by Wood (1968) and Shama and Sherman (1973b). A series of sodium carboxymethylcellulose (CMC) solutions that varied in their degree of shear-thinning were prepared to have the same physical viscosity in the 10-40 sec I shear rate range, the same range proposed by Shama and Sherman (1973b) to be operating when subjects judged thickened solutions in the oral cavity. At higher shear rates, the solutions exhibited different degrees of shearthinning, and thus differed in apparent viscosity. Using magnitude estimation, subjects assigned numbers in proportion to the perceived viscosity of the solu~

~

183

FOOD TEXTURE PRECEPTION

C M C -M CMC - 1

CMC-H

0.0

1.0

2.0

3.0

Log Viscosity (centistokes)

FIG. 8. Relationship between log physical viscosity and perceived viscosity for a series of aqueous solutions thickened with different types of sodium carboxymethylcellulose (CMC). Judgments represent the geometric mean of normalized values from 10 subjects. From Christensen (1979).

tions. Instead of perceiving them to be equally viscous as Shama and Sherman would have predicted, subjects rated as less viscous the solutions exhibiting greater shear-thinning. Figure 8 illustrates one set of results; CMC-H solutions exhibited greater shear-thinning than either CMC-L or CMC-M solutions, particularly at higher viscosity levels, and subjects' responses mirrored these differences. As illustrated in Fig. 9, the thinning became evident at shear rates higher than 50 sec-I, suggesting that subjects used shear rates above 50 sec-

'

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=-===-=,.-

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184

C . M. CHRISTENSEN

to evaluate viscous solutions. Although subjects clearly were sensitive to differences in the Newtonian behavior of solutions, they did not report that thickness varied during the time a solution was in the oral cavity being subjected to a range of shear rates. This suggests that the sensory information is combined or averaged in some fashion. It is also not possible to directly compare instrumental and sensory values of viscosity because, although the measures are related, they have not been found to be equivalent. Stevens and Guiaro (1964) trained subjects to use magnitude estimation to obtain ratio judgments of the thickness of a series of Newtonian silicone oils. Subjects stirred the solutions with and without being blindfolded, or visually determined viscosity by shaking capped containers of the oil samples. Log-log plots of instrumental and sensory values of viscosity reveal that the slope of the function was approximately 0.4 regardless of the test condition. A similar value was obtained for oral assessments of solution viscosity when Newtonian solutions were thickened with sodium carboxymethylcellulose (Christensen, 1979). A slope of 1.0 would imply that instrumental and sensory measures of viscosity were identical; a 10-fold change in solution viscosity as measured in poise would be perceived as a 10-fold change in viscosity. The low slope values found for viscosity reveal that judgments of solution viscosity are quite compressed compared to physical measures; a 10-fold change in solution viscosity produces little more than a twofold change in perception. An understanding of the perception of viscosity by non-oral means is relevant to an understanding of perceived viscosity in food systems because sensory information about viscosity can be obtained before food reaches the oral cavity. Viscosity judgments can be made by visual observation, by feeling with the fingers, or by stirring. As described above, the slopes of power functions relating physical and sensory measures of viscosity are similar for non-oral and oral sensory methods. However, this finding does not necessarily imply that absolute or discrimination thresholds are the same regardless of the sensory modality used to assess viscosity. Measures of threshold sensitivity have yet to be obtained. Research by Shama and Sherman (1973a) suggests that subjects may use different strategies to determine solution viscosity when different non-oral methods are used. When subjects tilted solutions to visually determine viscosity, the correlation between subjective and instrumental measures of viscosity suggested that subjects used different shear rates developed at a constant shear stress to make their viscosity determinations. The opposite appeared to be true when subjects stirred the solutions. A comparison between visual and tactile perception of viscosity should yield interesting and informative differences in perception. With visual determinations, subjects presumably perceive viscosity by relating the rate of movement of the solution in response to a presumed shearing force which will often be gravitational force. With oral and non-oral tactile judgments, subjects may perceive

FOOD TEXTURE PRECEPTION

185

viscosity by using all three elements of the equation for viscosity: shearing force, shear rate, and the resistance of the solution to flow (viscosity). It is probable that mechanoreceptors (touch-pressure) detect the solution’s resistance to flow and the rate of flow over the receptor surface, and intramuscular receptors detect the force or effort applied to move the solution. Because most food systems are non-Newtonian, the very real possibility exists that individuals could be receiving mismatched sensory information about solution viscosity from different sensory systems. Even if different sensory strategies for determining solution viscosity (e.g., visual, non-oral stirring) are found to be comparably sensitive to viscosity, it is unlikely that shear rates developed in the handling of the solutions (e.g., pouring) would match the shear rates and stresses operating in the oral cavity. Thus, solution viscosities could actually be different for the visual and oral sensory systems. Which sensory system dominates, if any, in judgments of viscosity is presently not known and merits investigation. C.

HARDNESS

Another prominent textural characteristic of food is its hardness. A considerable literature exists on the theory, application, and pitfalls of the instrumental measurement of hardness in foods (e.g., Mohsenin, 1977) but there is almost no research on the perceptual basis of hardness. It is a frequently measured sensory response, but its sensory foundation is not well understood. Young’s modulus of elasticity, a fundamental property of solid material that reflects it rigidity, is a measure of hardness. It is the proportionality constant describing the linear relationship between applied stress and the resultant strain. Stress (6) is force per unit area and is usually produced by compression, but it can also be produced by shearing or tensile forces. Strain (E) is the ratio of the change in the size or shape of the stressed material to its original size or shape. The modulus is usually calculated with very small deformations because most materials will fail, i.e., the structure will fracture and break apart, at moderate or high deformations. Young’s modulus of elasticity is rarely applicable to food systems because a number of assumptions associated with its use cannot be met (Bourne, 1983). A valid measure of the modulus of elasticity requires that the test material be continuous and homogeneous, and of uniform shape and size. It is also difficult to apply Young’s modulus because foods generally exhibit viscoelastic properties rather than pure elastic behavior. A perfectly elastic material instantaneously develops strain when a force is applied and instantaneously returns to its original shape or size when the force is removed. With viscoelastic materials, the response to stress is not instantaneous. The response to stress is also not complete with viscoelastic materials because some of the energy associated with stress is dissipated, and this is termed stress relaxation. The relationship between stress

186

C . M. CHRISTENSEN

and strain deviates from linearity because a portion of the applied stress is absorbed in the restructure of the stressed material rather than in the development of strain. Stress relaxation is dependent on the rate and duration of applied force; the characteristic relaxation time of a material (for foods there can be more than one relaxation time) determines the degree to which temporal variables associated with the application of force (rate and duration) will influence stress-strain functions. In place of fundamental measures of hardness, empirical measures resembling stress-strain functions are obtained for foods. These measures are usually termed force-deformation (or compression) functions. There is difficulty in their use and interpretation because the measures are affected by the deformation rate, the size and geometry of the plunger compressing the material, and the test material geometry (size and shape) and composition. Most foods have a varied composition and structure; consequently the force-deformation functions can be very complex. Figure 10 illustrates force-compression curves for different foods and the effect of different compression rates on the relationship between force and compression. Rupture tests are another means used for measuring food hardness. In these tests, the deformation force required to rupture the test material is measured. 105

104 n ts) W

0

z

103

LL 0

1o2

10'

0 10 20 30 40 50 60 70 80 90

0 10 20 30 40 50 60 70 80 90

Compression ("/) FIG. 10. Force-compression curves for various foods using an Instron crosshead speed of 1 .O cm min-' (left) or 10.0 cm min-' (right). From Boyd and Sherman (1975).

FOOD TEXTURE PRECEPTION

187

Most foods will rupture if enough deformation or shearing occurs as is evidenced by the success of mastication in reducing the original food bolus to a number of smaller pieces suitable for swallowing. Like force-deformation measures, the force required for rupture will vary with deformation rate and duration and with the geometry, composition, and homogeneity of the test material. Sensory determinations of hardness are no less easy to characterize. Szczesniak (1963) defined perceived hardness as the force required to attain a specified amount of deformation. This is certainly a plausible means by which food hardness judgments are made, and, indeed, perceived hardness is well correlated with physical force-deformation measures (Szczesniak et al., 1963). However, this is not the only means possible for sensory determinations of hardness; the force required to rupture a food may also be an important element in judgments of hardness (Mohsenin, 1977). It seems possible that both signals or either sensory signal separately may be important in judgments of food hardness depending upon the characteristics of the food; i.e., whether a food ruptures and the degree of compression it will exhibit. It is unlikely that deformation will be an important cue for hardness if a particular food exhibits little deformation before rupturing. That subjects do apparently use different strategies to determine food hardness was described by Szczesniak and Bourne (1969). They allowed subjects to use non-oral means, chiefly their hands and a spoon, to determine the firmness of a group of foods ranging in hardness from semisolid puddings to solid foods such as apples and carrots. With solid foods, subjects chose either to flex, compress, or puncture the foods depending on the type of sample; for example, subjects flexed carrots, punctured apples and pears, and usually compressed foods of medium firmness. Even when a single general strategy is used by subjects to evaluate food hardness, there is a range of possible implementations. Voisey and Crete (1973) used a transducer to measure the amplitude and rate of forces applied by subjects instructed to squeeze fruits and vegetables to determine their firmness. They found that these parameters varied with the firmness of the food and also that there were wide individual differences in the forces and rates used. Using a more indirect method, Boyd and Sherman (1975) came to a similar conclusion in a study where subjects judged food hardness in the oral cavity. The authors found that no single force-compression curve (see Fig. 10) could be successfully correlated with subjects judgments of hardness, suggesting to them that compression rates and forces must be different for different foods. For example, with soft foods, the best correlations between instrumental and sensory measures were obtained with instrumental determinations at lower forces and percentage compressions. Also supporting this conclusion are the results of studies showing shifts in masticatory rate and force patterns with different foods (see discussion in Section II,C, 1). However, other interpretations of these results are possible, as correlations between instrumental and sensory measures do not imply cause-and-

188

C. M . CHRISTENSEN

effect relationship. Rupture forces were not measured in these studies, although they may also have contributed to the perception of hardness. As already described, it is difficult to measure the relationship between instrumental and sensory measures of hardness because different measurement conditions affect force-deformation functions and because the physiological events underlying judgments of hardness are unknown. Since it is unclear whether deformation or rupture forces are more important in judgments of food hardness, it is also possible that the most appropriate physical measure(s) of food hardness are being overlooked. Harper and Stevens (1964) obtained an exponent (slope) of 0.8 describing the log-log relationship between instrumental and sensory judgments of hardness. Hardness was measured instrumentally as a force: indentation ratio and determined perceptually by subjects pressing the test material between the thumb and finger. In that study, sponge and rubber stimuli were used that could not be ruptured. Moskowitz et al. (1974) measured oral judgments of hardness and found an exponent of 0.4 ( I = 0.80) relating the modulus of elasticity and judgments of hardness and an exponent of 0.6 (r = 0.75) relating the same sensory measures to ultimate strength (a measure involving rupture). Subjects were told that hardness represented the force required to rupture the test samples, which were soy protein cubes. The perception of hardness probably includes kinesthetic receptors that signal joint position during food deformation, as well as pressure receptors that signal the resistance to deformation. As described earlier (see Section II,B,4), differences in oral judgments of hardness between individuals with normal dentition and those with dentures suggested that kinesthetic receptors were more important than periodontal pressure receptors when softer stimuli were being judged (Manly et al., 1952). The stimuli were rubber disks that could not be ruptured. It would be useful to repeat these experiments with stimuli that ruptured in order to determine the relative importance of periodontal pressure and kinesthetic receptors in oral judgments of food hardness when rupture occurred. Such a test would more closely mimic the conditions associated with food consumption. In some applications of the texture profiling technique (Civille and Szczesniak, 1973) the assumption is made that compression associated with the first bite of a food determines its hardness. Subjects in the Boyd and Sherman study (1975) reported that to be the case, but self-reports should be tested empirically. It may be that a significant portion of an individual’s overall assessment of food hardness is associated with subsequent food breakdown produced by mastication and salivation. The hardness of foods is differentially affected by mastication and salivation; thus, a comparison of the perceived hardness of a range of foods after one bite or subsequent chews should help determine the relative importance of the process of partial or total food breakdown on perceived hardness. Comparative studies of oral and non-oral assessments of hardness should be conducted because they are both theoretically valuable and behaviorally relevant.

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Individuals have the opportunity to asess food hardness by both oral and nonoral means during meals. Non-oral methods for estimating food hardness include handling the food directly or via utensils (e.g., fork or knife). Anderson et al. (1973) found a high correlation ( r = 0.87) between hardness judgments obtained by squeezing crisp breads between the fingers and biting and chewing the same samples. It would also have been valuable to determine whether the magnitude of hardness judgments differed when the same samples were judged orally and nonorally. There are many reasons for expecting oral and non-oral judgments of hardness to be different. Sensory information from different sources is expected to differ; for example, kinesthetic sensations arising from joint movements associated with food deformation should be different for fingers and teeth. Also, as Peleg (1980) suggests, deformation of the sensory tissues as well as deformation of food may be important in judgments of food hardness; and the tongue, periodontium, and fingers should be compressed to different degrees by the application of force. To the extent that rupture forces are important in hardness determinations, differences in the ability to rupture food with sharp teeth or between the fingers may also produce differences in perceived hardness. In addition, oral and non-oral judgments of hardness may be different because, as suggested for solution viscosity, the “test conditions” for the determination of hardness may be different. If different compressive and shearing force amplitudes and rates are used in oral and non-oral judgments, then the perceived hardness of the same food may be different because the “test conditions” are different for oral and non-oral determinations. D.

ROUGHNESS

Roughness is a property of the surface of materials and is perceived by contact between the skin and material surfaces. The movement of one surface across the other sets up vibrations in the skin which are hypothesized to be the critical sensations in the perception of roughness. There is some debate as to the importance of vibratory frequency in the perception of roughness (Taylor et al., 1973), but the role of vibratory energy (i.e., amplitude) has been supported. For example, increasing the amplitude of skin displacement over a surface of parallel grooves by enlarging groove widths (Lederman and Taylor, 1972) or by increasing the force applied over the grooved surface by the fingers (Lederman, 1974) will increase the perception of roughness. A considerable array of solid and liquid materials produce roughness. Irregularities in solid materials such as grooved, pitted, or raised surfaces will produce sensations of roughness; in liquids, dispersions of particulate matter and emulsion droplets presumably contribute to a decrease in solution smoothness. In addition to geometric characteristics, chemical interactions between surfaces

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should also contribute to sensations of roughness and smoothness. Cussler et ul. (1977) have demonstrated that the amount of frictional force that develops between skin and test materials can be used successfully to predict the perceived roughness of a variety of different solid and liquid materials. Thus, the amount of frictional force appears to be good physical correlate of roughness and possibly the principal physical stimulus. Stevens and Harris (1962) found that a slope of 1.5 described the log-log relationship between the grit size of emery cloth and perceived roughness when subjects stroked the cloth samples with their fingers. The inverse stop of - 1.5 was found for the perception of smoothness, which suggests that smoothness lay at the opposite pole on the same continuum as roughness. Other investigators (Ekman et ul., 1965) have confirmed a reciprocal relationship between roughness and smoothness but also have noted large individual differences in the slope of the function relating physical (static coefficients of friction) and psychological measures of roughness or smoothness, although the slopes were greater than 1.O in most cases. There are few studies investigating the oral perception of roughness. Frictional forces operating in the mouth were estimated for a series of liquids and used successfully to predict their perceived smoothness (Kokini et al., 1977). The perception of rough surfaces using different areas of the oral cavity was investigated by Ringel (1970b), as described earlier (Section 11,8,1). Though the comparative data are lacking, oral and non-oral sensations of roughness should be different. A layer of solid or liquid food rubbed by the tongue against the palate is probably the chief means by which roughness is perceived in the oral cavity. As previously described, the dorsal tongue has a different sensitivity to vibratory stimuli than the hand (Verrillo, 1966); therefore, one would expect that the perception of roughness would also be different. The interaction of saliva with the skin and food surfaces should also produce different sensations of roughness in the oral cavity. Saliva is a lubricant and would be expected to reduce the frictional forces between oral and food surfaces.

E. FRACTURABILITY Fracturability describes the textural property of certain foods to fracture and disintegrate upon the application of force. Fracturability does not represent a single fundamental measure of materials; hence, it is difficult to characterize and measure, particularly in foods (Mohsenin, 1977). As an applied concept, the texture classification system developed by Szczesniak (1963) describes brittleness (later termed fracturability, Szczesniak, 1975) as a secondary mechanical property of food related both to low cohesiveness and to varying but higher levels of hardness (see Table I). Fracturability is a frequently identified and salient sensory characteristic of

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solid foods (Szczesniak and Kleyn, 1963). In the vernacular, the terms crispness, brittleness, and crunchiness are used to describe the fracturability of foods. The terms are highly correlated (Moskowitz and Kapsalis, 1976) although there are some differences in their application to different foods (Sherman and Deghaidy, 1978). The perceptual basis of fracturability is poorly understood; for example, the degree to which foods are perceived to exhibit fracturability may possibly be reflected in the amount of force required to initially cause fracturing and/or the amount of fracturing or disintegration that occurs in response to some constant force. In some applications of the texture profile method, subjects are instructed to determine food fracturability by evaluating the force with which the food sample moves away from the molar teeth when it fractures (Civille and Szczesniak, 1973). Instrumental measures of fracturability have been only partially successful in predicting the presence and degree of perceived fracturability in foods (Vickers and Bourne, 1976a). As a result, physical measures of food fracturability have not been a helpful guide to researchers studying the perceptual variable, as has been the case for other textural variables such as hardness and viscosity. Physical measures have predicted successfully only the perceived crispness or brittleness of single foods or related classes of foods. For example, using a series of crisp breads, Andersson et al. (1973) found a high correlation (-0.90) between oral judgments of brittleness and disintegration measures derived from crushing the breads (percentage by weight of particles smaller than 2.0 mm). Sherman and Deghaidy (1978) found that the maximum force at fracture in a snap test (sample supported at each end by beams and snapped in the center by a dull-edged blade) predicted oral judgments of the brittleness of a series of low-moisture foods (e.g., for bread and crackers r = 0.93), but for high-moisture foods (e.g., vegetables) the initial portion of the force-deformation curve was a better predictor of sensory crispness (brittleness was not a term subjects applied to highmoisture foods). The correlations between instrumental and sensory judgments are much lower when a wide range of test foods are used. For example, when a series of both low- and high-moisture foods were used, correlation coefficients between the parameters associated with the snap test (including peak force, slope, and deformation to fracture) and oral crispness judgments were relatively low: r = 0.25-0.55 (Vickers and Christensen, 1980). The perception of food fracturability probably involves receptors sensitive to vibration. Vibrations are likely to be produced when food is fractured because of the cellular nature of most fracturable foods (Vickers and Bourne, 1976b). The stiff-walled cells are air filled in low-moisture foods and liquid filled in highmoisture foods. When sufficient force is applied, there is a serial bursting of the cell walls. Lending support to the hypothesis that vibratory sensations are involved is the finding that sound, a product of vibration, occurs when cellular foods are crushed (Vickers and Bourne, 1976b). The authors found that a broad

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frequency band of sounds of irregularly varying amplitude was produced by crushing low- and high-moisture foods identified as being crisp. A pronounced decrease in the amplitude of the crushing sounds occurred when crispness was reduced by blanching high-moisture foods and by increasing the water content of low-moisture foods. No characteristic changes in the frequency of food crushing sounds for crisp and noncrisp foods were noted, but formal analyses of the acoustical patterns were not conducted. The complex vibrations produced upon fracturing foods may be sensed by either the auditory or tactile sensory domains, or both. The products of food disintegration may also contribute to the perception of food fracturability as suggested by the data of Anderson et al. (1973) described above. The role of sound in judgments of food texture was first examined by Drake (1963, 1965). He proposed that food crushing sounds may convey textural information to individuals because the amplitude and frequency of food crushing sounds produced by chewing varied between foodstuffs and in the same foodstuff prepared to have different textures. In those studies, food crushing sounds were recorded by placing a microphone in the ear canal of subjects. This method does not represent the true complement of sounds heard by the individual who is chewing food because the bone-conducted sounds of chewing are not recorded. Kapur (1971) recorded bone-conducted sounds during chewing at a site close to the source of chewing sounds (the mandible near its angle) and at distant sites including the mastoid process near the ear canal. Figure 1 1 presents the averaged results from subjects with natural dentition chewing peanuts for five masticatory strokes. It is evident that bone-conducted sounds do reach the auditory canal but they are considerably attenuated (approximately 50%), especially the lower frequency sounds. Kapur also found that the amplitude of bone-conducted sounds was lower when soggy rather than crisp crackers were chewed, and this result paralleled the result obtained from air-conducted food crushing sounds described above (Vickers and Bourne, 1976b). Other investigators (Christensen and Vickers, 1981) found that when subjects rated the perceived crispness of a series of low- and high-moisture foods and the loudness of their own chewing sounds for the same series foods during separate test sessions, the correlation between the two sets of sensory judgments was very high ( r = 0.98). Thus, both the recorded and naturally perceived sounds of chewing indicate that amplitude differences can signal differences in food crispness. Based on studies demonstrating that food crushing sounds differ among foods of varying crispness, it is reasonable to hypothesize that subjects use auditory cues in the determination of food crispness. This hypothesis was tested by obtaining judgments of food crispness from subjects both with and without a loud masking noise that blocked the sounds of biting and chewing (Christensen and Vickers, 1981). The results of that study are illustrated in Fig. 12. In the absence of chewing sounds, subjects demonstrated neither a loss in their ability to dis-

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F R EOUE N C Y

FIG. 11. Amplitude measurements for different frequencies of the audiograms of bone-conducted sounds recorded at three sites: mandible angle, forehead, and mastoid process. The averaged data are obtained from individuals chewing peanuts for five masticatory strokes. From Kapur (1971).

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Perceived Crispness (No Auditory Block)

FIG. 12. Judgments of perceived crispness in the presence or absence of an auditory block. Each point represents a different low- or high-moisture food and is the geometric mean of normalized magnitude estimation scores obtained from 16 subjects. From Christensen and Vickers (1981). Reprinted from Journal of Food Science 46, 574-578. Copyrignt 0 by Institute of Food Technologists.

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criminate crispness nor a rise in the variability of their judgments. Thus, these results demonstrate that auditory cues are not essential for judgments of food crispness, and further suggest that oral tactile sensations associated with food fracturing may be prepotent. This finding is surprising in light of the high correlation between the loudness of chewing sounds and the crispness of foods. However, a high correlation does not necessarily imply a cause-and-effect relationship. The vibrations that are producing sounds are also producing tactile sensations, and these tactile sensations may provide the critical sensory cues for judgments of food crispness. Another finding in the same study was that judgments of the crispness of the 16 food samples were comparable ( r = 0.97) whether subjects bit each sample only once or repetitively chewed each sample. This finding suggests that the bulk of the sensory information about food crispness is contained in the initial fracture of food, and that the perception of the disintegration products of fractured food may contribute little additional sensory information. There may be more reliance on auditory cues of food fracturability when nonoral judgments are used because tactile sensitivity to food-crushing vibrations may be lower, although this has not been tested. Vibrations in the food may be better conducted and amplified by the teeth than by the fingers or by utensils held with the fingers, both because the teeth are firmer than the tissues of the finger and because the vibrations produced by crushing foods may be amplified by the jaw. Oral and non-oral sensations of food fracturability may possibly demonstrate qualitative differences, although Andersson et af. (1973) found that the correlation between brittleness judgments of crisp breads that were handled or chewed was r = 0.94.

IV.

FUTURE RESEARCH NEEDS

To develop acceptable food products it is necessary to do more than measure the sensory and hedonic responses of consumers. One must also utilize available knowledge of the sensory and motor mechanisms underlying an individual’s responses to foods in order to identify successfully the physical elements of food that are critical to the production of the relevant sensations. If knowledge is sufficient, then product development can be guided by predictions of consumer responses rather than by reacting to them in a blind “look and see” approach. It should be apparent from this review of the literature that information about food texture perception is insufficient to predict successfully consumer responses. This ignorance is especially striking when one compares the list of textural characteristics of food delineated by Szczesniak (1963, 1979) and the much smaller list of these same categories in which any sensory research has been conducted, albeit meager.

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A need clearly exists for more research in all areas of food texture perception. There is also a need for a different type of research that stresses a more analytical and rigorous approach. Food texture perception research is emerging from its infancy; a stage that was characterized by simple descriptions of sensory phenomena associated with food texture and the enunciation of largely untested conjectures about their mechanisms. The structure and hypotheses of a research field have been created; it is now time to directly test these conjectures. Where possible, indirect measures of perception should be replaced by direct measures, and correlative studies should be supplemented with studies that critically test hypotheses. Food texture perception spans several different disciplines including physiology, engineering, psychology, and food science; thus, an appreciation of each of these fields is a prerequisite to conducting valid research in this area.

ACKNOWLEDGMENT The author expresses appreciation to Dr. Barry Green and Dr. Alina Szczesniak for their thorough critiques of an earlier draft of this article. Their knowledgeable comments considerably improved its quality.

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Szczesniak, A. S. 1973. Instrumental methods of texture measurements. In “Texture Measurements of Foods” (A. Kramer and A. S. Szczesniak, eds.), pp 71-108. D. Reidel, Dordrecht, Holland. Szczesniak, A. S . 1975. General Foods texture profile revisited-Ten years perspective. J . Text. Stud. 6 , 5-17. Szczesniak, A. S. 1979. Classification of mouthfeel characteristics of beverages. In “Food Texture and Rheology” (P. Sherman, ed.), pp. 1-20. Academic Press, New York. Szczesniak, A. S . , and Bourne, M. C. 1969. Sensory evaluation of food firmness. J . Text. Stud. 1, 52-64. Szczesniak, A. S . , and Kleyn, D. H. 1963. Consumer awareness of texture and other food attributes. Food Technol. 17, 74-77. Szczesniak, A. S . , Brandt, M. A . , and Friedman, H. H. 1963. Development of standard rating scales for mechanical parameters of texture and correlation between the objective and the sensory methods of texture evaluation. J . Food Sci. 28, 397-403. Taylor, M. M., Lederman, S. J., and Gibson, R. H. 1973. Tactual perception of texture. I n “Handbook of Perception” (E. C. Carterette and M. P. Friedman, eds.), Vol. 3, pp. 251-272. Academic Press, New York. van Steenberghe, D., and de Vries, J. H. 1978. Psychophysical threshold level of periodontal mechanoreceptors in man. Arch. Oral Biol. 23, 1041-1049. Verrillo, R. T. 1966. Specificity of a cutaneous receptor. Percept. Psychophys. 1, 149-153. Vemllo, R. T., and Grescheider, G . A. 1979. Psychophysical measurements of enhancement, suppression, and surface gradient effects in vibrotaction. I n “Sensory Functions of the Skin of Humans” (D. R. Kenshalo, ed.), pp. 153-181. Plenum, New York. Vickers, Z . , and Bourne, M. C. 1976a. Crispness in foods-a review. J . FoodSci. 41, 1153-1 157. Vickers, Z . , and Bourne, M. C. 1976b. A psychoacoustical theory of crispness. J . Food Sci. 41, 1158-1 164. Vickers, Z. M., and Christensen, C. M. 1980. Relationships between sensory crispness and other sensory and instrumental parameters. J . Text. Srud. 11, 291-307. Voisey, P. W., and Crste, R. 1973. A technique for establishing instrumental conditions for measuring food firmness to simulate consumer evaluations. J . Text. Stud. 4, 371-377. Wictorin, L. 1972. Masticatory function in cases with denture, in cases with natural teeth and the importance to digestion. In “Oral Physiology” (N. Emmelin and Y. Zotterman, eds.), pp. 223226. Pergamon, Oxford. Wood, F. 1968. Psychophysical studies on the consistency of liquid foods. SCI Monogr. 27,40-49. Yamada, M., Maruhashi, J., and Miyake, N. 1952. The distribution of sensory spots on the oral mucous membrane. Jpn. J . Physiol. 2, 328-332. Yoshikawa, S . , Nishimaru, S . , Tashiro, T., and Yoshida, M. 1970. Collection and classification of words for description of food texture. 111. Classification by multivariate analysis. J . Text. Stud. 1, 452-463. Yurkstas, A,, and Curby, W. A. 1953. Force analysis of prosthetic appliances during function. J . Prosther. Dent. 3, 82-87. Yurkstas, A. A . 1965. The masticatory act. J . Prosther. Dent. 15, 248-260.

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ADVANCES IN FOOD RESEARCH. VOL . 29

BAKING QUALITY OF WHEAT FLOURS F . MAcRITCHIE C U R 0 Wheat Research Unit North Ryde. New South Wales. Australia

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Evaluation of Baking Quality ............... A . Limiting Variables in Baking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . The Baking Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Breeding for Quality ...................................... D . Small-Scale Rapid Tests for Quality . . . . . E . Approaches to Investigating the Basis of Baking Quality . . . . . . . . . . . . 111. Fractionation and Reconstitution Techniques .......................... A . Extraction of Flour Lipid . . . . . . . . . . . . . . B . Separation of Gluten, Starch, and Water Solubles . . . . . . . . . . . . . . . . . C . Fractionation and Reconstitution of Gluten Protein . . . . . . . . . . . . . . . . . D . Fractionation of Water Solubles . . . . . . . . . E . Fractionation of Starch . . . . . . . . . . . . . . . . F . Reconstitution Experiments . . . . . . . . . . . . . IV . Dough Properties and Mixing Behavior .............................. A . Factors Affecting Dough Development . . . . . . . . . . . . . . . . . . . . . . . . . . B . Unmixing and the Effects of Rest Periods . C . Interaction of Oxidants, D . Dough Development by ....................... V . Role of Flour Components . . ....................... A . Protein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Lipid . . . . . . . . . . . . . . . . C . Water Solubles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D . Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Theories of Baking Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Protein-Lipid Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Molecular Weight Distribution ................. C . Gluten Protein Amide Content . . . . . . . . . . . .......... D . Sulfur-Containing Amino Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Gliadin Composition ......................................... F. Glutenin Subunits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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I. INTRODUCTION The quality of wheat flours from the point of view of bread baking has been ably reviewed by Pomeranz (1968) in this series. This article will concentrate on advances that have been made since then. Because the world depends on baked products of wheat for a large part of its food requirements, flour quality assumes high importance. The greatest use of wheat flour is for breadmaking, with lesser quantities utilized for cookie, pastry, and cake production. As a result, breadmaking aspects will be emphasized, but recent contributions to the understanding of cookie and cake quality will also be discussed. Different criteria are used to assess quality in each of these products, and furthermore, different baking procedures and permitted additives are employed in different countries. These factors make difficult the problem of defining baking quality and arriving at fundamental generalizations concerning its basis. In this article, we are concerned mainly with the quality of the final product and how it relates to the properties and composition of flours. We thus approach the subject from the point of view of the cereal chemist. However, before the wheat grain is obtained, many other scientific disciplines are involved in the selection of suitable strains for different areas, environmental conditions, and end uses. Thus, the plant breeder, geneticist, and plant physiologist are concerned with other quality parameters besides those considered by the cereal chemist. It is important to produce strains that are resistant to disease and give high crop yields as well as fulfill the requirements of the miller and baker. Nutritional aspects are also encompassed in the question of quality (PAG, 1976). Contents of protein, vitamins, minerals, and essential amino acids are important in addition to the food value in joules. The gluten protein, which is the main protein of wheat, is low in certain essential amino acids, particularly lysine, so that a protein diet based on wheat flour needs to be supplemented by protein from other sources such as legumes, milk, and meat. The nongluten proteins (albumins and globulins) have a higher lysine content, but the quantity of these is usually found to be fairly constant in wheat flour. Raising the protein level of wheat is generally synonymous with increasing the gluten protein level and therefore aggravating the lysine deficiency of the total flour protein. One obvious line of research is to try to breed wheats with protein of higher lysine content, but to date this has not proved to be particularly viable. Just as attempts to raise protein levels of wheat are in opposition to efforts to improve the nutritional value, a similar nexus exists between protein content and crop yield. Increased yield is achieved by increased rate of starch synthesis and therefore depressed protein levels in the grain. In general, therefore, the aim in breeding for quality has to be a compromise. Apart from all these factors, the baker is concerned with the functionality of flour, e.g., whether or not a flour fulfills criteria for good dough handling properties or adequate volume and

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texture in the baked product. To satisfy these requirements, a minimum protein content is usually needed. However, because of the attractiveness of obtaining higher crop yields with consequent depression of protein levels in the grain, there is a tendency at present for a lowering of protein levels in flour (Jackel, 1979). Because of the ever increasing world demand for food supplies, the accent in breeding programs has tended to be on increasing crop yields. The introduction of dwarfing genes, principally at the International Maize and Wheat Improvement Center (CIMMYT), Mexico, has had considerable success in producing varieties that have appreciably increased grain yields in many countries. The improvements in wheat varieties tend to be seen, therefore, as the maintenance of quality standards with decreasing flour protein contents. The cereal chemist has several roles to fulfill. First, he is responsible for the testing of new varieties that are grown in trials and for passing relevant information on to the plant breeder. Early in the breeding program, the plant breeder has a very large number of crosses to evaluate. Because of this, only small plots of each cross are grown, and the total grain harvested from each plot may be only of the order of grams. Such quantities are insufficient to carry out normal testing procedures such as baking tests and physical dough testing. For this reason, one of the tasks of the cereal chemist is to introduce small-scale tests suitable for assessing quality. Several evaluation tests have been tried with a certain degree of success (see Section 11). As more knowledge is gained by the research cereal chemist on factors that determine baking quality at a chemical level, it should be possible to devise more fundamental small-scale tests. For example, if quality can be related to specific chemical components and if these can be readily identified, the cereal chemist is then in a position to advise the plant breeder exactly what to breed for. If the synthesis of the quality components can be assigned to specific chromosomes, this provides an even more powerful base on which to plan. Another area in which the cereal chemist works is as a trouble-shooter. F’roblems can and do occur with flours at the flour mills and bakeries. These may arise because of unfavorable climatic conditions during growth or to introduction of varieties with undesirable properties. The cereal chemist hopes to prevent varieties of poor quality from becoming widely grown, but this is not always possible. For example, recent years have seen the planting of high-yielding wheat varieties in Europe that have created problems during milling and baking. It is on these occasions that the cereal chemist is called upon to advise millers and bakers on how they can best cope with the problems. Central to both this task and that of elucidating the chemical basis of flour quality is the need to separate and fractionate flour components without altering their functional properties. For this reason, Section 111 is devoted to a thorough discussion of methods for separating, fractionating, and reconstituting flour components. Section I1 discusses the meaning of baking quality, how it is assessed, and some procedures that should

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be followed in order to relate quality to chemical parameters. Recent developments in the understanding of dough properties are reviewed in Section IV with particular reference to dough mixing. The specific roles of different flour components are discussed in Section V. Section VI reviews the various theories that have been proposed to explain baking quality in molecular terms. Finally, some of the most interesting areas of current research are summarized in Section VII together with suggested areas where work may be usefully pursued. It appears that the subject of wheat flour quality will continue to present a challenging research field for cereal scientists as it has done in the past.

II. EVALUATION OF BAKING QUALITY From the viewpoint of functionality, the assessment of baking quality may be separated into three aspects: 1. Mixing behavior and dough or batter structure and properties. 2. Performance in proofing and oven baking. 3. Retention of freshness or shelf life. Suitable mixing and dough-handling properties are essential for trouble-free processing as well as for ensuring quality of the final product. This facet is particularly relevant to breadmaking and is discussed more fully in Section 111. Breadmaking has traditionally relied on the development of appropriate dough structure by long fermentation periods. In recent times, there has been a change toward more labor-saving processes that employ shorter fermentation times. Development of doughs is achieved by more intensive mixing (mechanical development) and/or chemical additives (chemical development). However, many different baking procedures are in use around the world (Ponte, 1971), varying from long fermentation overnight sponge dough procedures to short fermentation processes such as the Chorleywood Bread Process (CBP; Axford et al., 1963). The great number of different baked products, processes, and criteria for evaluating quality in various countries precludes a completely general explanation of baking quality. We can, nevertheless, aim to arrive at certain generalizationsthat are widely applicable. A.

LIMITING VARIABLES IN BAKING

Many variables that affect quality of grain and flour are outside the control of the baker. For example, the climatic conditions during growth of the wheat plant are of vital importance. Deficient rainfall during critical stages of the growing season can be detrimental, although damage caused by excessive rain, particularly near harvesting, is a more common problem. Rain damage leads to high

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concentrations of a-amylase and proteolytic enzymes in the flour. This results in breakdown of starch and protein, respectively, producing weak, sticky doughs that perform poorly in baking. In severe cases, sprouting occurs. The falling number and amylograph are standard methods used in cereal laboratories to gauge the degree of weather damage to flours. Methods for directly measuring amylase activity, which can be used in the field, have also been developed in recent years (Marchylo and Kruger, 1978; Mathewson and Pomeranz, 1979). Shortage of fertilizer and specific soil ingredients can affect quality, for example, lack of soil nitrogen may lead to low protein content while sulfur deficiency has been shown to affect protein quality (Moss et al., 1981; Timms et al., 1981). Diseases in the field, such as rust, can affect quality. Insect infestation may have bad effects as also may the use of insecticides or heat treatment to combat the problem. It is well known that the nature of the milling process can affect the baking quality of the flour; e.g., excessive grinding is detrimental to flour quality. Recent work has shown how the properties of a flour may be controlled by varying the intensity of the grinding (N. L. Stenvert, private communication). Large shear forces and high temperatures at the roller surfaces are responsible for the changes in flour properties. In the case of cake flours, the degree of bleaching is an important factor. The conditions under which a flour is stored as well as the duration of storage may have large effects on properties. Generally, the rate of deterioration increases with increasing temperature and moisture content. Having obtained the flour, there are many variables over which the baker can exert control in order to achieve a satisfactory product. These include the water addition, gassing power, levels of ingredients, dough mixing intensity and time, and the baking procedure itself. For bread, the correct water addition is critical for a good result. An estimate of the optimum water can be made from the farinograph water absorption. This is the amount of water required to be added to a flour to bring the maximum in the trace to the 500-Brabender Unit (BU) line. The optimum water can be found more exactly by baking a number of loaves with a range of water additions and choosing the one corresponding to the maximum loaf score, consistent with suitable dough handling properties. Water requirement depends on the protein content ( P ) and amount of starch damage ( S ) of a flour (Meredith, 1966). One equation that has been proposed to relate water absorption (W) to these two parameters is W = 41.6 + 1.32P + 0.34s (Moss, 1961). Fiber content and quality of the protein also exert their influence on water absorption. For leavened products, gassing power is an important consideration. As well as affecting water absorption, the degree of starch damage determines the gas production potential of a flour at a given level of a-amylase activity. The aamylase activity may be low as in flours milled from sound wheats, or there may be little starch damage as occurs for soft wheats. In these cases, supplementation

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by a source of a-amylase or addition of sucrose may be required to ensure sufficient gas production throughout fermentation, proofing, and the early stage of baking. Gas production also depends on the activity of the leavening agent and how this may be affected by dough ingredients. For this reason, the activity in the dough system needs to be continually checked to ensure maintenance of a constant level. An instrument such as the fermentograph can be used to monitor gas production in doughs. Oxidants, shortening, and surfactants (or emulsifiers) are the most common additives used in breadmaking, and they are particularly important in short-time processes. Oxidants are required to obtain optimum loaf volume and crumb grain in bread. Potassium bromate and ascorbic acid are the two most widely used in countries where they are permitted, and the amounts employed range between 0 and 100 ppm by flour weight. Shortening (mainly saturated triglycerides) improves loaf volume, texture, and sliceability, and is added in amounts up to 2% by weight. Surfactants are used to improve volume and texture of bread. Other factors which the baker can control are the molder settings, temperatures and moisture contents of initial ingredients, proofing cabinet, and oven, and lengths of fermentation times. B.

THE BAKING TEST

The proof of whether a flour is of good quality or not is determined by its performance in a baking situation. Thus, experimental baking tests have been devised in which standardized procedures are followed to allow an objective comparison between flours. Baking tests are used by laboratories associated with breeding to evaluate potential baking quality, by flour millers and bakers for the purpose of quality control, by bakery ingredient manufacturers to evaluate ingredients, and also by wheat exporters and buyers and by laboratories concerned with research (Tipples, 1979). 1 . Bread

The steps in a baking test are mixing of the dough, then fermentation, and finally baking in the oven. Basic dough ingredients are flour, water, salt, and yeast. Other additives frequently used are sucrose, oxidants (potassium bromate, ascorbic acid), shortening, and malt flour. Between the dough mixing and baking stages, numerous other operations may be introduced depending on the particular test. These include sheeting and molding, scaling and rounding, successive passages through sheeting rolls, weighing of dough pieces, and placement in baking tins. Evaluation includes assessment of dough handling properties, measurement of loaf volume, and qualitative assessment of loaf shape, crust color and crumb grain, softness, and color. Other measurements that are at times

T

carried out are height of dough piece during proofing, gas evolution during baking (Daniels and Fisher, 1976), as well as taste panel evaluation and crumb firmness following baking. The most important parameter has traditionally been the loaf volume. This is because it is a quantitative measurement and correlates well with dough handling properties, crumb texture and freshness, and technological versatility (Pomeranz, 1980). Because baked loaves tend to change significantly in volume after leaving the oven, it is important to standardize measurements by carrying them out at a specified time after baking. Loaf volume is usually measured by a displacement method, rapeseed being the most common displacement medium. A more accurate method using water displacement following paraffin coating was devised by Gras and MacRitchie (1973) to measure volumes of small loaves. Other methods are to measure loaf height or a volume index, represented by the sum of the distances around the length and width of loaves (Short, 1971). These provide relative measures but may be converted to absolute volumes, using suitable calibration graphs. Frequently, a loaf score is adopted to quantify assessment. The score is a composite that may contain contributions from volume, texture, color, and appearance. Despite what has been said, it is possible to observe a wide range of texture in baked loaves from different flours even though the loaf volume may not vary greatly. Gas cell size distribution may differ. One loaf may exhibit a fine grain with a large number of small cells whereas another may have a coarse grain with a wide variation in cell size. Thickness of cell walls can vary; thinner cell walls are an indication of a good quality gluten, which is associated with softer crumb and better freshness retention. Examination of the crumb by an experienced observer can give information on the reasons for loaf volume deficiency. For example, it can be deduced whether failure has occurred by transfer of gas from smaller to larger cells or by simultaneous collapse of a large number of cells at a particular stage. There is a need for developing methods for obtaining rapid quantitative measures of parameters such as crumb grain and cell wall thickness. Freshness retention or shelf life is an important quality factor of bread, and measurements of crumb compressibility at different stages after baking are made by instruments such the compressimeter. Recent studies of staling have utilized the penetrometer (Maleki et al., 1980) and the Instron Universal Tester (Kim and D’Appolonia, 1977). Baking tests may be carried out under rigidly prescribed conditions for all flours, or each flour may be baked with optimum levels of ingredients and conditions. In practice, baking tests may approximate to one or other of these extremes or be a compromise between the two. Generally, flours that are to be evaluated for commercial use need to comply with certain specifications. A baking test procedure that is precisely laid down and in which all samples are tested in the same manner is therefore necessary. Some flexibility may be allowed depending on the tolerances that are deemed to be acceptable in the

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bakery. For example, water addition may be varied or a small amount of extra mixing may be permitted for stronger flours. This type of baking test is unsuitable if we want to determine the full baking potential of a flour. With this as our aim, a test in which the variables are optimized is required. Such an optimized test (Finney, 1978) is the one that is appropriate for research purposes. Here, we are interested in establishing fundamental reasons for quality. It would therefore be unfair to penalize a flour because it gave a poor result under arbitrarily chosen conditions for which it was not suited. One of the commonest reasons for failure in a fixed-conditions baking test is inadequate mixing to give optimum dough development. The mixing time selected for the baking test will often determine the order of ranking in a given set of flours. The best result will be obtained for those flours with mixing requirements corresponding closely to that chosen. Flours whose doughs are grossly undermixed or overmixed will give low rankings. If, however, optimum mixing time is chosen for each flour, the order of ranking may be completely altered. This is the order of ranking on which a solid research program to identify quality factors can be based. Rankings not based on optimized variables can lead to unfounded conclusions unless effects of different variables are taken into account. Figure 1 illustrates the contrasting results obtained from two different baking tests using Canadian wheat flours. In a baking test involving short fermentation and slow speed mixing, the variety Pitic did very well whereas Glenlea performed poorly. However, under conditions of high-speed mixing and long fermentation, Pitic did poorly due to overmixing and too much fermentation while Glenlea gave a good result. Manitoba performed reasonably well under both extremes although neither test represents optimum conditions for this flour. This demonstrates another aspect of quality, namely the ability of a flour to do well under a range of conditions. It would be ideal to have one standardized baking test for use throughout the world, but this has proved very difficult to attain. Within the European Common Market, the “Groupement des Associations Meunieres des Pays de la CEE” has recently discontinued its work aimed at developing a baking test to be used for accepting or rejecting wheat on the basis of quality. The International Association for Cereal Chemistry (ICC) Baking Test Study Group has also been trying to develop a standard method for several years. A number of obstacles lie in the path of such an endeavor. First, laboratories in different parts of the world carry out their own specific baking tests. These tests may differ widely, and there is an understandable reluctance on the part of each laboratory to change radically their own test method. Second, there is a difficulty in obtaining standard and uniform equipment and ingredients throughout the world. In a collaborative study by the ICC Study Group using the ICC Draft Standard Baking Test method, 14 laboratories participated. Samples of five flours together

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FIG. I . Loaves baked with flours from three wheat varieties using two baking processes. From Tripples (1979).

with samples of malt flour and detailed instructions were sent to each laboratory. In assessing the results, it is necessary to distinguish between the ability of a baking test to consistently rank samples in the same order and the ability to consistently give the same results. With respect to ranking, agreement was quite good but there was poor agreement between collaborators on absolute loaf volume values. For one flour, the total spread represented 36.5% of the mean volume for that flour, while in the best case, the difference between the high and low values represented 24.7% of the mean. The discrepancy between loaf volumes could not be ascribed to one or more obvious reasons, although oven baking conditions may have been a major source. Mitchell (1977) has commented on the difficulty of reproducing test-baking results and lists a number of possible causes. There are numerous ingredients, some of which, like yeast, are inherently variable. A test-bake also involves many steps and manipulations. Some of these, such as molding, are difficult to do in a reproducible manner and yet have a major effect on the end result. Several steps are available for helping to achieve greater reliability in baking

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tests. First, it is advisable to have on hand one or more flours that can act as controls over a period of time. Large samples may be stored at low temperature for long periods without alteration in baking properties. They can then be used as calibration standards to check for variability in day to day baking. The precision of baking test parameters such as loaf volume is measured by the standard deviation, which can be reduced by increasing the number of replicate baked loaves. Comparison of flours under conditions of more than one type of baking test is highly recommended to avoid spurious conclusions from one test. At the Canadian Grain Research Laboratory, as many as 10 or more baking test variations may be used when assessing the quality of new wheat cultivars (Tipples, 1979). Some precautions such as maintenance of the correct temperatures and relative humidities of ingredients and dough pieces at all stages are critical but often overlooked. Losses of dough sometimes occur during molding operations. It is therefore advisable to check the weight of dough pieces before oven baking. The volume can then be adjusted for a constant dough weight (MacRitchie, 1976). Finally, the input of the test baker is of great importance. A dedicated experimentalist who is prepared to take painstaking care in carrying out the operations as reproducibly as possible and not introducing bias (albeit unintentionally) is required. The results of the ICC Study Group emphasize the difficulty of obtaining comparable absolute values from baking tests carried out in different laboratories. The relatively good agreement between rankings of flours, however, is at least reassuring, and this may well be the most that can be expected from a standard international baking test. Because sample size is often a limiting factor through shortage of material from new varieties or from laboratory-scale preparations of flour components, small-scale “pup” loaf tests with 100 g and microtests with as little as 10 g of flour have been devised. Baking quality is a function of both protein quantity and quality, and for this reason, it is usual to try to separate these two aspects. The Canadian workers have introduced a Baking Strength Index (BSI; Tipples and Kilborn, 1974) by making use of the linear relation between loaf volume and flour protein content (Finney and Barmore, 1948). They obtained a calibration graph for a large number of flours milled from the good quality Canadian Westem Red Spring (CWRS) wheats of varying protein contents using the Remix Baking Test (Irvine and McMullan, 1960). The BSI was calculated from the Remix loaf volume expressed as the percentage of the loaf volume of a CWRS wheat flour at the same protein content. MacRitchie (1979) introduced a loaf volume index (LVI) defined as LVI =

loaf volume (cm3) x 200 protein in flour (%) x weight of flour (g)

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The value of 200 was arbitrarily chosen so that flours of good performance would give values near 100. As well as separating contributions of protein quantity and quality this index, in theory, enables comparisons between samples that are baked in tests using different quantities of flour. In practice, loaves baked from smaller amounts of flour tend to have lower specific volumes than their counterparts from larger amounts of flour because of the higher crust to crumb ratio. This needs to be considered in any comparison between differently scaled baking tests. A more pertinent comparison between baked loaves is the volume of expansion, i.e., the volume less the volume of the unexpanded dough piece. Because in most baking tests either loaves are proofed to a constant height or the volumes of dough pieces are approximately the same prior to oven baking, it is the volume of oven expansion that gives the truest measure of baking potential. An oven spring index may therefore be the most useful comparative measure. This would provide a much greater proportional range of values as oven spring is found to vary from zero upward.

2. Cukes and Cookies Standardized baking tests to assess flour quality for manufacture of cakes and cookies are in common use. Cakes are assessed, similarly to bread, by their volume, texture, and freshness retention. In place of dough properties, however, we are concerned with whipping quality and batter structure, for these parameters govern the volume, appearance, and crumb structure of the baked product (Seibel et al., 1980). For evaluation of cookie flour, there are available several methods that use standard equipment and prescribed ingredients and procedures (Finney et ul., 1950). In one of these, termed the micro method 111, a cutter of 60-mm diameter is used to cut standard-sized dough pieces. The main measurement is the cookie spread factor, WIT. This is found by measuring the average width (W) and thickness ( r ) of six cookies after cooling for 30 min. Good cookie flours have high values whereas poor flours have limited spread. Other qualitative measurements that may be made are assessment of top grain, color, and internal structure (Clements, 1980). C.

BREEDING FOR QUALITY

The early forms of wheat breeding involved introduction of new materials and selection, but modern methods employ controlled hybridization in which successive cross-pollination is carried out to achieve varieties that will breed true to type. Breeding of wheat varieties of high quality is a lengthy process, and it can

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take over 10 years from the time of the initial cross to the commercial production of a new variety. The plant breeder relies on advice from many specialists including plant physiologists, geneticists, entomologists, agronomists, and cereal chemists. The way in which new varieties are developed has been succinctly summarized by Bushuk (1979). There are three main stages. The first involves establishing uniform and stable (true-breeding) varieties by selection and crossing. Next, the many potential varieties that result are evaluated for quality. The final stage involves purification, increase, and distribution of the new variety. Several different breeding procedures are in use by wheat breeders. Some of the most commonly adopted are the bulk, pedigree, and back-cross methods (Peterson, 1965). With the bulk method. seed from the first hybrid generation (F,) plants of a given cross is bulked and sown to produce a bulk population of F, plants. The seed from the F, plants is bulked, and the process is repeated in successive generations until individual plants are considered to be sufficiently homozygous. In the pedigree method, the seed of each F, plant is grown in a separate F, plot. Individual F, plants are then selected, and the seed of each is grown in a separate F, plot and so on through many generations. Both the bulk and pedigree methods have their advantages, and frequently combinations of each are used in practice. The back-cross method is employed when a parent is available that has good characteristics but lacks one desirable trait (e.g., leaf rust resistance). This variety is crossed with another that has the necessary trait. The latter parent, the “donor parent,” is used only for the first cross. Plants that acquire characteristics of the missing trait are selected for back-crossing to the good parent (“recurring parent”), and this process is repeated through successive back-crosses. Apart from standard methods such as these, other options involving artificial rearrangement of chromosomes are available and have been used in breeding programs. For example, it is possible to transfer a complete set of chromosomes (a genome) from one species to another, thus forming a new combination of genomes. In this way, many new species of plants have been formed, among which are the well-known Triticules from wheat and rye. Other operations that are commonly used are to add a chromosome to (addition) or subtract a chromosome from (subtraction) the normal chromosome complement of a given variety. It is also possible to remove a particular pair of chromosomes from a variety and substitute for it a pair from another variety or species (substitution). The more intricate exercise of permanently incorporating into a wheat chromosome a part of a chromosome from a related species having a desired gene (translocation breeding) is also used. Another approach is to induce mutations at localized points in chromosomes using irradiation or chemicals. Generally, the lines resulting from these chromosome rearrangement procedures are not expected to serve directly as successful new varieties. They are, nevertheless, of great importance in providing new material for hybridization and selection. A

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case for increased utilization of wild species related to cultivated wheat-like cereals in breeding has been argued by Feldman and Sears (1981). This would enable exploitation of a greater gene pool than that currently manipulated, with potential benefits in increased adaptability and disease resistance. Although the breeding program aims at selection for quality characteristics, an extensive testing phase should follow in which these characteristics are confirmed and others such as yield, disease reaction, grain characteristics, and milling and baking quality are checked. At this stage, potential new varieties are compared in performance with commercially grown varieties. If they perform satisfactorily, large-scale production of seed may then be initiated for commercial use. D.

SMALL-SCALE RAPID TESTS FOR QUALITY

Only small amounts of early generation material are available for testing in most breeding programs. For example, in the pedigree breeding method as used at CIMMYT and elsewhere, only about 5-10 g of wheat is normally obtained for the F, generation (L. O’Brien, private communication). This precludes the use of baking tests and other standard methods for assessing protein quality although tests that use small samples are available for measuring protein content, grain hardness, and milling yield. Consequently, there has been a demand for rapid tests capable of using very small amounts of material that might, at least to some degree, supplant the standard methods and serve as a guide to baking quality. The main criteria for an early generation quality test are small sample size, high precision, ability to distinguish differences between genotypes, negligible influence of growth locality on test values, and high correlation with standard measures of functional quality (O’Brien and Orth, 1977). Two tests that have been used are those devised by Pelshenke (1933) and Zeleny (1947). In the Pelshenke test, a heavily yeasted wholemeal dough ball is immersed in water and the time for it to disintegrate measured. Doughs from stronger wheats tend to remain intact for longer times. The Zeleny test measures the volume of sediment formed when a specially prepared flour is dispersed and allowed to settle in an aqueous solution of lactic acid and isopropanol. The sedimentation volume is taken as a measure of the baking quality of the flour protein. Although these two tests have been used extensively, there has been a continuous search for improved methods. In recent years, two have gained a degree of acceptance. These are the Residue Protein (Orth and O’Brien, 1976) and SDSsedimentation (Redman, 1979) tests. The Residue Protein test was developed with the aim of predicting the rheological properties of doughs from different flours. Because the strength of a dough depends on the glutenin protein (see Section 111), it was expected that the amount of protein remaining in a flour after one extraction with 0.05 M acetic acid might be a measure of strength. The test

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uses only 2 g of flour. In a study of 27 wheats, the percentage of residue protein correlated highly with farinograph dough breakdown and extensigraph maximum resistance (Orth et al. , 1976). Values of the percentage residue vaned between 20 and 40% with a standard deviation of 0.6%. The method gave better prediction of dough properties than either the Pelshenke or Zeleny tests. A separate study (O’Brien and Orth, 1977) showed that the residue test ranked wheat cultivars in a similar order at six locations. The SDS-sedimentation test resembles the Zeleny test in principle. A sample of 6 g of whole meal is dispersed in a sodium dodecyl sulfate (SDS)-lactic acid solution contained in a graduated cylinder and the volume of sediment read after 20 min standing. Flours may also be used. The sedimentation volume is claimed to be a measure of gluten quality for breadmaking. It has been found to correlate well with loaf volume and to be superior, in this respect, to the Zeleny and Pelshenke tests. In a comparison of some small-scale tests for breadmaking quality, Blackman and Gill (1980) concluded that the SDS-sedimentation volume showed the greatest potential as a screening test because of its small sample size, high throughput, and good correlation with loaf properties, growing sites, and genetic differences in protein quality. This test therefore appears to be more directly related to breadmaking potential. The Protein Residue test, on the other hand, aims to predict dough properties. For this reason, it can be used more generally to classify flours into those suitable for breadmaking and those for cookies. E.

APPROACHES TO INVESTIGATING THE BASIS OF BAKING QUALITY

One of the problems that often faces the research chemist is the lack of suitable starting materials on which to base a research program. Breeding strategies aim to produce good quality wheat varieties and, as a general rule, screening eliminates varieties of poor quality. Consequently, samples of flour available for research purposes tend to be acceptable and relatively uniform in standard. From a purely research viewpoint, it would be advantageous to deliberately breed and grow sufficient quantities of poor quality varieties so as to provide materials with a wider range of baking performance. In any program designed to elucidate the factors responsible for differences in baking quality, a wise selection of the starting materials is of prime importance. A series of flours spanning the whole range of quality is desirable. The first step is to thoroughly characterize the flours. This usually requires determination of protein and moisture contents and requirements for mixing, water addition, and oxidant level. Then, a baking test establishes loaf volume potential and other loaf characteristics. Preferably, more than one baking procedure should be carried

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out. This is to check the order of ranking of the different flours and verify that the order reflects genuine differences in quality. The entire success of the enterprise depends on this initial quantitative characterization of flour quality, and no amount of careful chemical work is of any use unless it can be related back to reliable measurements of quality. Much good experimental work is wasted because of the failure to observe this simple requirement. Many workers are too ready to accept data from baking tests at face value. This is particularly risky when the baking test is not an optimized one. Ideally, the research chemist should carry out his own flour assessment or, if not, at least have sufficient familiarity with the test baking procedure to be able to critically examine the results that are passed on to him. The skill required to carry out a baking test effectively is often underestimated. There are many facets such as the appearance or “feel” of a dough at various stages or its response to a given treatment that provide an experimentalist with insight into the nature of baking quality and the reasons why a dough piece might fail in a given test. Some researchers, in certain cases because of lack of suitable test-baking facilities, make the mistake of not assessing their flours and merely rely on reputations of wheat varieties. Although it is true that certain varieties tend to give flours of better quality than others, such a generalization may not always be valid in individual instances because of the variability of flour samples from the same wheat variety grown at different sites or different times. Of course, when comparing varieties, it is always preferable to use samples from wheats that have been grown under the same conditions. One criticism that may be leveled at the approach to research on quality by cereal chemists is that it has ofter tended to be inductive. The inductive method essentially consists of gathering a large amount of data and then attempting to arrive at some generalization by analysis of the data. Although a certain amount of exploratory data collection is warranted, especially in the early stages of a project, it is necessary at some time to begin to form hypotheses and to test them. The inductive approach is then superseded by the hypothetico-deductive method (Popper, 1963). Frequently, correlations are noted between certain properties of flours (or flour components) and quality parameters. However, it should be realized that such correlations are not always necessarily indicative of a cause and effect relationship. A good hypothesis, on the other hand, has a high degree of predictive power. This allows experiments to be designed that severely test the generality of the hypothesis and enables a greater chance of establishing causeeffect relations. Another tendency is to divide research into areas based on artificial divisions rather than attempting to directly solve problems. These areas may be based on biochemical (proteins, carbohydrates, etc.) or on disciplinary (analytical, microscopy, etc.) classifications. Once a problem is visualized and defined, howev-

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er, experimental work can be designed with the aim of solving it. When this happens, these artificially created boundaries tend to disappear and research becomes multidisciplinary.

Ill.

FRACTIONATION AND RECONSTITUTION TECHNIQUES

Two main approaches have been used in attempts to elucidate the factors responsible for differences in baking performance between flours. One has been to measure a given flour parameter and see how well it correlates with performance as measured by a baking test or physical dough testing instrument. This method can be useful but, as discussed in Section 11, has its limitations. The other approach involves separation and fractionation of flour components. Each component (or fraction) is then evaluated by varying its amount in a given flour or by interchange between flours of different baking quality. The former procedure characterizes the role and importance of the component and the latter identifies the component(s) responsible for the differences in quality. The use of fractionating techniques as tools in wheat flour research was introduced by Finney (1943). Its application, although tedious and exacting, represents the most effective means of acquiring an understanding of baking quality. The crucial point about the separation and fractionation processes is that they be performed without effecting any changes in the functional properties of the flour constituents. This can be checked by combining all the material separated from the flour and comparing the properties of the reconstituted flour with those of the original. Two tests are usually sufficient to verify that there has been no change of functional properties; the mixograms and baked loaf volume and texture should be the same. Unless this can be established, no valid conclusions can be drawn from any experiments in which the quantity of a flour component is varied or fractions are interchanged between flours. In view of the relative ease with which biological compounds can decompose or be altered chemically, great care needs to be taken in carrying out the separation and fractionation steps. Frequently, workers find that the reconstituted flour has properties very different from the original. Another vexing situation, often encountered, is that significantly large differences in baking quality between the original flours tend to disappear in the reconstituted flours. In other words, poor flours tend to improve and good flours sometimes deteriorate as a result of the fractionation steps. Because this work is time consuming and such results can be very discouraging, we will focus on the techniques for fractionation, the precautions to be observed, and the checks required to ensure satisfactory reconstitution. A general scheme for the separation and fractionation of flour components is illustrated in Fig. 2. The procedure can be made more elaborate by further fractionation of the components.

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FLOUR Extraction with chloroform

I Formation of

Centrifugation

FIG. 2.

General scheme for preliminary separation of flour components for reconstitution studies.

A.

EXTRACTION OF FLOUR LIPID

As a first step in the separation process, it is advisable to remove the nonstarch lipid (see Section V) from the flour. The reason for this is that once flour is brought into contact with aqueous solutions, a large proportion of the lipid becomes bound to the protein and cannot later be extracted (Wootton, 1966; Daniels et al., 1966). The result is that when the protein is fractionated, different amounts of lipid become associated with the protein fractions. It then becomes difficult to separate contributions of protein from lipid in subsequent interchange experiments. A number of solvents have been used to extract nonstarch lipid from flour by Soxhlet or batch extraction. In evaluating solvents, we need to distinguish between those that might affect the properties of the lipid, those that could affect the flour, and those that could possibly affect both. Chung and co-workers have studied the effects of different variables on the efficiency of extraction of flour lipids (Chung et al., 1978a, 1980). They found that lipid extraction increased with the solubility parameter of the solvent, increase of temperature (linearly), and increase of flour moisture content (in a certain range), and was higher in a regular than a vacuum Soxhlet. It has been customary to divide the nonstarch lipid into free and bound lipid. The free lipid is that which can be extracted with a

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purely nonpolar solvent such as hexane or petroleum ether (PE). The extra lipid that is subsequently extracted with a more polar solvent such as water-saturated butanol (WSB) is defined as bound lipid. The distinction between starch and nonstarch lipid is not always clear-cut operationally. It is known that hexane does not extract all the nonstarch lipid, whereas a solvent such as boiling methanol extracts an appreciable amount of the starch lipid. Between these extremes, there are solvents that, to a close approximation, appear to extract practically all the nonstarch lipid. Some results for the extraction of lipids from flours by different solvents are compared in Table I . The loaf volume-lipid content curve for a flour (see Section V,A) is a good indicator of the efficiency of the solvent for extracting lipid. This curve has a minimum at intermediate lipid contents, and as further lipid is extracted, the curve rises. Flour extracted with PE gives a loaf volume corresponding to a point on the rising part of the curve, whereas the flour that has been more exhaustively extracted with chloroform gives a much higher loaf volume (see Fig. 14).

I.

Functionality of Lipid

Most of the common organic solvents, or mixtures of them, have proved suitable for extraction of flour lipid without causing any detrimental effects to the complex mixture of lipid components. After extraction with aqueous mixtures (e.g., WSB), the extract is usually evaporated to dryness and redissolved in a more inert solvent such as PE or chloroform for subsequent analysis or storage. Lipids are very susceptible to decomposition. The most familiar example of this is the hydrolysis reaction, which splits off free fatty acids from triglycerides and

TABLE I EFFECTS OF DIFFERENT SOLVENTS ON LIPID EXTRACTION AND DOUGH MIXING REQUIREMENTSa

Solvent

Lipid in extracted flour (%, by hydrolysate method)

Lipid extracted (% of total lipid in flour)

Unextracted flour Petroleum ether Benzene Chloroform Water-saturated butanol Acetone-water (90: 10) Ethanol-water (955)

2.20 0.89 0.83 0.67 0.74 0.99 0.57

0 60 62 70 66

From MacRitchie and Gras (1973).

55 74

Peak dough development time (min) 4 4

4 4 8 10 24

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is the primary cause of rancidity in fats. This reaction can easily be followed by observing changes in thin layer chromatogram (TLC) patterns if lipid samples are left in contact with the air at ordinary temperatures. The large spot corresponding to the triglycerides decreases in size, giving way to a larger spot for the free fatty acids that, as the sample is left for longer times, changes to an intense smear on the TLC plate. However, well before this occurs, the functionality of the lipid has been altered, and very small changes, scarcely detectable by TLC methods, can be reflected in significant effects on the baking performance of a flour. A simple check to ascertain whether a lipid extract has been altered in functionality is to compare the loaf volume-lipid content curve of defatted flour to which increasing additions of the lipid have been made with that obtained by mixing defatted and whole flours together in varying proportions (MacRitchie and Gras, 1973). Satisfactory reconstitution requires that the two curves be the same. If any deterioration of the lipid has occurred, this often results in lower loaf volumes at lipid contents near the natural lipid content of the flour. For evaluation studies in baking, speed and low temperature favor the use of batch methods for extraction of lipid. The simplest procedure is to stir the flour with a quantity of solvent and filter, using a Buchner funnel. All the lipid that the solvent dissolves usually goes into solution rapidly so that subsequent extractions are essentially washes of the flour. Therefore, a high liquid to solid ratio in the first extraction ensures removal of most of the lipid and reduces the number of required washes. The extracted lipid may then be dried in a rotary evaporator. Evaporation should be carried out at as low a temperature as possible (preferably not above 35°C) and should not be extended longer than is necessary for evaporation of the solvent. Traces of remaining solvent may be removed by blowing oxygen-free nitrogen gas into the flask. If the lipid sample is not to be used immediately, it should be stored in solution (e.g., chloroform) under nitrogen at low temperature. If there is a time lapse between the extraction of the lipid and reconstitution with the defatted flour, it is advisable to extract a fresh sample of the lipid on the day of the experiment and use that to add back to the previously defatted flour. This eliminates the possibility of deterioration during storage. Photomicrography shows that much of the lipid is dispersed as minute particles in flour. It is probably impossible to duplicate the same state of dispersion by reconstitution. The important thing, however, is to use a technique that interferes minimally with the functionality of the lipid. Several methods of dispersion have been used. Chung et al. (1979) blended lipid fractions with defatted flour in a Stein mill. For polar fractions, because of their semisolid and sticky nature, preliminary blending in a mortar was employed before using the Stein mill. MacRitchie and Gras (1973) added back the lipid using a pipette for liquefied fractions and a spatula for semisolid fractions, relying on the dough mixing stage for dispersion. Another technique (MacRitchie, 1976) is to first dissolve the lipid in a solvent, slurry the flour with the solution, and then allow the solvent to

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evaporate. It is the author's experience that simply adding the fresh lipid in aliquots and leaving the mixer to effect the dispersion is satisfactory. Although the same degree of dispersion as the original flour is certainly not achieved, functionality as measured by baked loaf volume and texture is retained. The lack of reproducibility initially reported appears to arise from leaving warmed samples too Iong on the rotary evaporator. Slurrying flour with solution has been found to be a useful method but the drying process has, on occasions, been found to be detrimental to the functionality of the lipid, possibly because of exposure to air during this step. 2 . Functionality of Defatted Flour

In addition to not affecting the functionality of the lipid, it is equally important that the lipid extraction step does not change the properties of the flour. When assessing this, it is necessary to distinguish between the effects of removal of lipid and the effects of the solvent. A study of the effects of nine solvents on the composition of the lipid extract, mixing time, and bread loaf volume was carried out by Finney et al. (1976). The three more nonpolar solvents petroleum ether, n-hexane, and n-heptane extracted smaller amounts of lipid than the other six (see Table 11). Benzene, chloroform, and acetone were found to extract appreciable amounts of flour lipids without damaging breadmaking properties. Watersaturated butanol greatly increased dough development time, but a preliminary mixing to restore the rheological properties (Hoseney et al., 1969c) allowed

TABLE I1 EFFECTS OF SOLVENTS ON EXTRACTED LIPID COMPOSITION, MIXING TIME, AND LOAF VOLUMEn Lipids extracted (% of flour)

Lipid composition (% of total)

Solvent

Total

Bound

Nonpolar

Polar

Mixing time (min)

Loaf volume (cm')

Petroleum ether n-Hexane n-Heptane Benzene Chloroform Acetone Water-saturated butdnol Methanol 95% Ethanol

0.84 0.96 1.01 1.28 1.32 1.38 1.50 1.04 1.16

0.00 0.12 0.17 0.44 0.48 0.54 0.66

64.9 59.2 57.9 48.1 45.8 43.7 41.0 44.5 48.6

32.9 36.9 39.1 47.3 48.2 54.5 55.0 52.0 48.6

4.125 5.375 6.75 7.375 10.5 14 13.5 Infinite Infinite

82 19 11 15 14 71 53 25 24

0

From Finney et al. (1976)

BAKING QUALITY OF WHEAT FLOUR

22 1

original loaf volume to be recovered. However, doughs from flours treated with methanol and 95% ethanol could not be mixed to conventional optimum consistency, and these solvents were therefore unsuitable. Comparable results were obtained by MacRitchie and Gras (1973). Flours were treated with a series of solvents without extraction of the lipid. After evaporation of the solvent, the peak dough development times of the flours were measured and are recorded in Table I. Treatment of gluten and starch separately by ethanol-water (955) showed that the extended mixing time caused by this solvent resulted from its effect on the gluten. Evidently a conformational change in protein molecules, particularly of groups near the surface, is effected by certain amphipathic solvents such as alcohols. This retards the necessary rearrangements to produce gluten strands on contact with water. 3 . Fractionation of Lipid

Both column chromatography and batch methods have been successfully used to fractionate lipids for use in reconstitution studies. Each method has depended on the ease of elution of compounds of differing polarity from silicic acid or activated silica gel. Daftary et al. (1968) fractionated flour lipids into polar and nonpolar fractions by silicic acid column chromatography followed by subfractionation using a DEAE-cellulose column. These fractions were then tested in baking experiments. Ponte and De Stefanis (1969) introduced a batch technique for separating flour lipids into polar and nonpolar fractions for use in baking experiments. This method overcomes some of the disadvantages encountered in column fractionation such as the use of fairly elaborate equipment, small amounts of fractions, and long separation times, with the attendant hazard of degradation of labile lipids. A solution of flour lipid in diethyl ether is slurried with activated silica gel followed by elution of nonpolar lipids by four extractions with diethyl etherpetroleum ether (90: 10 v/v). Polar lipids are then eluted by four extractions with methanol-water (955 v/v). In a subsequent paper, De Stefanis and Ponte (1969) described a simple batch technique for isolation of crude mono- and digalactosyl diglycerides from wheat flour. In this procedure, five lipid fractions are prepared using several solvent systems to differentially elute the fractions from activated silica gel. The technique has been used by MacRitchie (1977) to prepare lipid fractions for evaluation in breadmaking. 4 . Analysis of Lipids

The “hydrolysate lipid” method (Wren and Wojtczak, 1964) has proved valuable for flour analysis. Analysis by solvent extraction is not possible because of the inaccessibility of the starch lipid. The “hydrolysate lipid” method, how-

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ever, enables measurements to be done on the flour before and after solvent extraction and is not influenced by nonlipid impurities in extracts. Qualitative or quick semiquantitative analysis may be most easily carried out by TLC. Ponte and De Stefanis (1969) used the solvent system of n-hexanediethyl ether-acetic acid (65:32:2 by volume) to separate nonpolar fractions and chloroform-methanol-water (90:20:2) to separate polar components. The developed plates were exposed to iodine vapors for visualization of spots. Although the intensities of the spots depend on the degree of unsaturation of the components and therefore the method strictly cannot be used for quantitative measurements, it is nevertheless convenient and surprisingly reliable for comparing different lipid samples. Chung et al. (1978b) used solvent systems of hexane-diethyl ether-95% ethanol-methanol (80:18:2:1, v/v/v) for standard nonpolar lipids and flour lipids and chloroform-methanol-water (65:25:4 v/v/v) for standard polar lipids and flour lipids. Plates were charred by spraying with a 0.6% potassium dichromate solution in 55% sulfuric acid and heated for 25 min at 180°C. The plates were photographed under ultraviolet light. This method allows a more quantitative measurement of lipid components than that using iodine vapor. Measurement of lipids on TLC plates by charring and densitometry is rapid, but the method requires considerable skill and the linear range is poor (Morrison et al., 1980). A method in which TLC plates are charred with acidic cupric acetate solution and the spot density measured in a liquid scintillation counter using an external radiation source (Weibrauch et al., 1976) appears to be a promising modification. The most satisfactory method for reliable quantitative analysis of flour lipids has been developed by Morrison and co-workers (Momson el al., 1980). Total lipids, obtained by solvent extraction or after acid hydrolysis, are converted to fatty acid methyl esters (FAME) and quantified by gas chromatography, using heptadecanoate (17:O) as internal standard. Individual lipid classes are separated by TLC; nonpolar and glycolipids are measured as FAME by gas chromatography, while phospholipids are determined from phosphorous distribution. The various classes present in wheat flour lipid and their proportions are summarized in Table 111. B.

SEPARATION OF GLUTEN, STARCH, AND WATER SOLUBLES

Gluten may be obtained from defatted flour by mixing a dough and either kneading by hand in water or separating out by means of a mechanical gluten washer. It is important to mix only sufficiently to form a coherent dough piece as overmixing can produce changes in flour properties. It has been the author’s experience that satisfactory reconstitution is usually assured if the temperature during gluten washing is maintained near 15°C. Higher temperatures sometimes

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BAKING QUALITY OF WHEAT FLOUR

TABLE 111 COMPOSITION OF WHEAT FLOUR LIPIDSO Lipid

Weight (Yo)

Steryl ester Triglyceride 1,2-Diglyceride 1,3-Diglyceride Free sterol Free fatty acid Monoglyceride 6-O-Acyl-monogalactosyldigl ycende 6-O- Acyl-sterylglucoside Monogalactosyldiglyceride Sterolglucoside + ceramidemonoglycoside Monogalactosylmonoglyceride Digalactosyldiglyceride Ceramidediglycoside Digalactosylmonoglyceride N - Acyl-phosphatidylethanolamine N - Act y 1-1 y sophosphatidylethanolamine Phosphatidylethanolamine Phosphatidylcholine Lysophosphatidalethanolamine Lysophosphatidalcholine Phosphatidylinosito1 Phosphatidylserine

7.5 20.8 6.2 6.0 2.1 7.0 I .3 3.6 1.6 4.9 1.8 0.4 13.5 0.03 0.6 4.9 2.9 0.8 5.8 0.9 7.1 0. I 0.2

From MacMurray and Morrison (1970).

cause weakening of dough properties, possibly due to proteolytic enzyme activity. Very low temperatures, on the other hand, reduce coherence of gluten, making it difficult to knead. The total amount of water required for washing gluten from flour is not critical, except that if nongluten components are to be recovered by drying, it is desirable to minimize the quantity. Using 200 ml of water per 100 g flour and five separate washes will generally produce a gluten of about 70% protein content. Further washes will produce a higher grade gluten (i.e., with a higher protein content), but this is offset by the larger volumes of water that need to be removed. Although higher grades are obtained with defatted flours than with whole flours, it is usually difficult to achieve glutens with protein contents much greater than 80% (dry weight). Some occluded starch always remains that cannot be easily removed. This small amount does not interfere with evaluation in subsequent reconstitution studies. Following washing out of the gluten, the aqueous suspensiorls are collected together and centrifuged to produce water solubles (supernatant) and starch (sedi-

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F. MACRITCHIE

ment) fractions. All fractions are then freeze-dried, a technique that has been found to be most valuable for preserving the functional properties of the fractions. C.

FRACTIONATION AND RECONSTITUTION OF GLUTEN PROTEIN

The separation and reconstitution of lipid, gluten protein, starch, and solubles present no major problems if the simple precautions already mentioned are observed. The greatest problems occur when attempts to fractionate and reconstitute gluten proteins are made. Satisfactory methods of carrying out these operations are essential because gluten protein is the flour component that appears to control dough properties and baking quality (see Section V,A). Column chromatography and electrophoresis have been used in attempts to correlate baking quality with various gluten proteins (Simmonds and Winzor, 1961; Koenig et al., 1964; Lee and Wrigley, 1963). As a means for preparing fractions suitable for use in baking tests, these methods suffer from the disadvantage that it is difficult to produce appreciable amounts, and the long times are undesirable in view of the labile nature of the proteins. On the other hand, it offers the advantage of separating purer fractions than the batch methods. Batch methods for'gluten protein fractionation have depended mainly on differential solubility of the constituent proteins. The solvents most commonly used have been ethanol-water, urea solutions, dilute acetic acid, dilute hydrochloric acid, and dilute sodium hydroxide. Of these, ethanol-water affects the properties of the protein, and urea solutions require exhaustive dialysis for removal of the urea. Both dilute acetic and hydrochloric acids have been found to be effective solvents for fractionation. It is important, however, not to leave gluten protein in acid solution (pH < 4) for any significant time as changes in functional properties can occur (Goforth et a / . , 1977). In order to avoid long residence times in acid solution, the dry, powdered gluten may be dispersed in solution by a high speed mixer. Immediately following agitation, the dispersion is centrifuged and the Supernatant separated from the sediment by decanting. Alkali (equivalent to the molar quantity of acid) is then added to neutralize both fractions. Although the total amount of alkali required to be added is known, the amounts to be added to each fraction have to be estimated. Measurement of pH during the neutralization is a useful means of checking the required amounts. if hydrochloric acid has been used, a small amount of sodium chloride will be present in the freeze-dried fractions as a result of the neutralization. This can be allowed for and presents no problem in the subsequent baking experiments. Fractionation of gluten protein depends on varying partitioning of the different protein constituents between two phases. With column chromatography, the partitioning occurs between the solution and an adsorbate; this allows sharp separation of fractions. Batch methods have mainly depended on partitioning

BAKING QUALITY OF WHEAT FXOUR

225

between supernatant and sediment. Each molecular entity has a different partition coefficient for a given solvent. The result is that there are different distributions of each species between the two phases but a sharp separation is difficult to attain. There are two parameters that mainly control partitioning of proteins between supernatant and sediment in aqueous solutions, one of which is the molecular weight. The lower the molecular weight, the greater the solubility and therefore the greater the concentration in the supernatant phase (Hory, 1953). The other important parameter is the electrical charge on the molecule. This can be varied by altering the pH of the solution. The concentration of gluten protein in the supernatant as a function of pH is illustrated in Fig. 3 for gluten protein samples from several flours. Solubility is a minimum in the region of the isoelectric point (pH 5-6) as found for all proteins. Because gluten protein is not a pure substance and consists of a mixture of molecular entities having different isoelectric points, the minimum corresponds to an average isoelectric point. Solubility increases much more rapidly on the acid than on the alkaline side as a result of the different ranges over which different groups ionize. At very low or very high pH, a solubility plateau is usually reached. This is because the electrical charge approaches constant values or its effect is counteracted by the increasing ionic strength. Extremes of pH can cause irreversible changes in protein molecules and should be avoided. h general, polymers may be separated into different average molecular weight ranges by fractional extraction or fractional precipitation. Fractional precipitation

PH

FIG. 3. Solubility (i.e., concentration in supematant) of gluten protein from different flours as a function of pH. A constant ratio of 3.3 g protein per 100 ml of solution was used for each measurement.).( Gluten protein from good baking flour; (0, 0) gluten protein from poor baking flours. From MacRitchie (1979).

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F. MACRITCHIE

requires that all the protein is initially in solution, a goal that has never been successfully achieved for gluten protein without modification. Fractional extraction is therefore the most effective method. Narrower molecular weight distributions of fractions are favored by high liquid to solid ratios. Because of the problem of drying large volumes of water, a compromise has to be made in order to prepare sufficient quantities of fractions for use in baking tests. Gluten protein samples from different flours may differ markedly in their solubility in dilute acid (Fig. 3). If we are to test the effects of comparable fractions, the most satisfactory fractionation procedure is to use concentrations of solvent that extract equal amounts of protein. For this, a graph of the equilibrium amount of protein against acid concentration is first established for each gluten protein sample to be compared. With these curves as a basis, acid concentrations can be fixed for each sample so as to extract a fixed proportion of the total protein from each. The use of dilute hydrochloric acid to separate protein fractions followed by neutralization has been found by the author to give satisfactory results as judged by reconstitution criteria. However, on occasions, it has been observed that, although mixing characteristics are retained, changes in baked loaf volume occur. The baking test is thus a very sensitive test for reconstitution. Some explanations can be suggested. The vigorous agitation needed to rapidly dissolve protein has two obvious effects. First, a large amount of air may be beaten into the solution and provide conditions for oxidation of protein groups by oxygen in the air. This problem can be avoided by carrying out the agitation in an inert atmosphere such as nitrogen and degassing the water before use. The other effect that can result from air incorporation during agitation is surface coagulation of the protein. Whether this has any significant effect on functional properties is not known.

D.

FRACTIONATION OF WATER SOLUBLES

The aqueous solution that is separated from starch by centrifugation following gluten washing contains a mixture of compounds. These include a small amount of gluten protein, albumin and globulin proteins, soluble carbohydrates, amino acids, peptides, and minerals. Several schemes have been devised to separate or at least concentrate the various components (Pomeranz, 1980; Baldo and Wrigley, 1978), one of which is depicted in Fig. 4. The first step consists in dialyzing out the small molecules and ions. This reduces the ionic strength, causing the globulins to precipitate out and allows their separation by centrifugation. The albumin fraction is then removed by heat precipitation, a step that unfortunately alters the properties of this component. The residual supernatant is rich in water-soluble pentosans and glycoproteins. An alternative procedure (Baldo and Wrigley , 1978) designed specifically to

BAKING QUALITY OF WHEAT FLOUR

227

Water Solubles ( 3.5% )

Dialyze Dialysate

1

Boil, 1000 g

Centrifugate (denatured albumins)

FIG. 4.

Supernatant (

wsos, 1.9% )

Scheme for separation of water soluble components of wheat flour. From Pomeranz

(1980).

prepare albumin and globulin fractions begins by extracting flour with 10% sodium chloride followed by dialysis to precipitate the globulins and centrifugation. The supernatant, containing albumins, is freeze-dried. The residue is redissolved in 10% sodium chloride, centrifuged, and again dialyzed to produce the globulin fraction. This method avoids the heat denaturation step in preparing the albumin fraction.

E. FRACTIONATION OF STARCH The separation and fractionation of starch are fraught with less difficulties than that of the other more labile components. As prepared by centrifugation after gluten washing, starch contains about 0.5% lipid (within granules) and a small amount of protein that adheres to the surface of the granules. Several washings with dilute acid or alkali extract much of this protein. Centrifugation normally produces two layers. The upper layer, called the tailings, is darker and includes damaged starch, insoluble pentosans, and glycoproteins. The lower layer is white and is termed prime starch. Wheat starch occurs in flour as small granules with a range of sizes. Two main types of granule exist. The primary granules have diameters mostly between 20 and 40 pm. The secondary granules mainly fall in the size range of 1-10 p m diameter. The heterogeneity in granule size permits fractionation into sharper size ranges by settling under gravity. A homogeneous suspension of starch is allowed to settle for a given time in a cylinder fitted with a tap. Aliquots from the lower (coarser granules) and upper (smaller granules) parts are then removed. These aliquots are then rehomogenized, and the settling process is repeated. Proceeding in this way, fractions with varying average granule sizes can be

228

F. MACRITCHIE

prepared rather simply. One precaution to be observed with starch samples is that their suspensions should not be heated. At temperatures above 50T, starch granules begin to swell, and the process of gelatinization, essentially a melting phenomenon, proceeds. Fractionation of starch into its amylose and amylopectin components is not viable for reconstitution studies as this involves disruption of the granules. For evaluation of starch samples in interchange experiments, it is important to check the protein contents to confirm first that they are not significantly different and second that they are not sufficiently high to obscure the conclusions.

F. RECONSTITUTION EXPERIMENTS Two criteria need to be satisfied in experiments in which flour components are varied in amount or interchanged between flours. The first, previously mentioned, is that components be reassembled to give flours in which the functional properties (mixogram, baked loaf volume, and texture) of the original or parent flours are preserved. Second, because it is never possible to obtain absolutely pure samples, the effects of contamination of one component by others need to be considered when evaluating the contribution of a given component. For example, starch preparations always contain small amounts of protein and lipid even after extensive purification steps. Similarly, gluten contains both starch and lipid, while gluten protein fractions prepared by differential solubility contain a spectrum of molecular entities. Both gluten protein and lipid have relatively large effects on loaf volume (see Section V). Therefore, if starch fractions that might contain 1% or more of protein, or gluten protein that may contain up to about 8% lipid if the latter has not been removed, are interchanged, misleading results may be obtained if the experimenter is not aware of the dangers in interpretation. An important requirement is that water content of doughs be accurately controlled. Moisture contents of flour fractions, after freeze-drying, therefore need to be determined to enable water contents of doughs from reconstituted flours to be brought to the same values as the controls (normally the parent or whole flours). Another problem that may arise when comparing flours of different baking performance is differences in their protein content. Because baking performance is directly related to the protein content of a flour (see Section V,A), it is preferable to perform the interchange experiments at a constant protein level. Comparison of results at several constant protein levels is advisable. When the amount of a component in a flour sample for test baking is being varied to determine its effect, it has to be decided whether to (1) maintain the amounts of all other components the same (this will therefore lead to a varying total amount of flour) or (2) maintain the amount of flour (dry weight) constant for the baking test (this will mean that we are varying the amounts of all components).

BAKING QUALITY OF WHEAT FLOUR

229

If we are varying the amount of a minor component (e.g., lipid) of which we use relatively small quantities, the difference between the two methods is not so critical. In this case, method 1 is probably the more convenient. However, most baking tests rely on a fixed weight of flour. In general, there is an approximately linear relation between the weight of flour (considering constant composition) and loaf volume. Therefore, when varying the amount of one of the major components (starch, gluten), it is desirable to maintain a constant flour weight. In this case, for example. if the amount of gluten protein in a flour is being increased, the amount of starch is proportionately being decreased. We thus need to make a judgment about which component is mainly responsible for any variations that are observed. The rationale behind interchange experiments may be illustrated by the results in Table IV (MacRitchie, 1978). Two flours, A and B, of good and poor baking performance, respectively, were each separated into three fractions, gluten, starch, and solubles. Prior interchange experiments had established that lipid was not responsible for the differences between the two flours. The minimum number of combinations required to determine which component(s) mainly controls the differences between the two flours is eight. The first two are simply the reconstituted parent flours. The mixograms and loaf volumes of these coincided closely with those of the original flours. The results clearly show the gluten protein component to be the cause of the differences. We need to consider, however, the second criterion for drawing valid conclusions from interchange experiments, i.e., that results are not influenced by contamination of one component by others. A separate experiment, in which the gluten protein content for each of the two flours was varied, showed that loaf volumes fell on two roughly parallel TABLE IV EFFECTS ON LOAF VOLUME OF INTERCHANGING COMPONENTS BETWEEN TWO FLOURS, A AND BO

Loaf volume Gluten

Starch

Solubles

(cm3)

A A A A

A

A B

B B

A

B

182 I82 183 180

B

B B A

B A B A

161 160 I63 I63

B B B a

A

A

From MacRitchie (1 978).

230

F. MACRITCHIE

lines, flour A outperforming flour B at all protein levels. Thus, an independent check confirmed the conclusions drawn from Table IV. Once the component mainly responsible for differences in performance has been identified, it is then possible to proceed with its fractionation. Further interchange experiments can then determine whether any particular fraction is more important than others or whether the quality factor is spread throughout the whole component.

IV.

DOUGH PROPERTIES AND MIXING BEHAVIOR

Wheat flour doughs exhibit a wide range of properties when different flour samples are compared. Dough properties influence both the efficiency of throughput in the manufacturing plant as well as the quality of the final baked product. In discussing the state of a dough prior to baking, we need to differentiate between the contributions from the individual flour and those from the various treatments, which include added ingredients, mixing, and intermediate punching and molding steps. Dough is a complex material from a rheological point of view. Knowledge about its structure comes from fundamental rheological studies, from standard physical dough testing including mixing, and from microscopy. A comprehensive review of rheology and how it relates to wheat products may be found in papers of a symposium given at the 59th Annual Meeting of the AACC (American Association of Cereal Chemists, 1975). Because of the trend toward greater use of baking processes involving intensive mixing and short fermentation times, much more attention has been given to basic studies of dough mixing in recent years. These studies have highlighted the sensitivity of dough structure to the conditions to which the dough has been subjected during its development. It is clear that meaningful interpretations of rheological measurements can be made only if the state of dough development is well understood. For this reason as well as because of its direct relevance to bread manufacture, this section is mainly devoted to a discussion of dough mixing and dough development in the light of recent work. Some negative effects in breadmaking such as impaired aroma and flavor may arise as a result of mechanical dough development processes. Calve1 (1979) has discussed the reasons for this and how the baker can benefit from the progress offered by high speed ‘mixing without the adverse effects on bread quality. A.

FACTORS AFFECTING DOUGH DEVELOPMENT

The first step in converting flour into bread is the mixing of flour, water, and other additives. As mixing proceeds, the initially incoherent dough mqss develops viscoelastic properties and finally acquires a sheen, this stage being termed

BAKING QUALITY OF WHEAT FLOUR

23 1

“clearing.” Prolonged mixing past this point causes the dough to lose its strength and become plastic and very sticky to the touch. If the torque on the pins, blades, or arm of the mixer is monitored over time, the trace rises, reaches a maximum, and then falls steadily. This is illustrated in Fig. 5 for doughs mixed in a mixograph. Optimum bread baking performance is usually achieved in a region at or slightly past the peak development point. 1. Critical Mixing Intensity and Critical Work lnput

Two parameters are critical for optimum development and hence best performance. These are that the mixing intensity must be above a minimum critical value and that the total work imparted to the dough must be above a certain minimum value (Kilborn and Tipples, 1972). Both parameters vary according to the flour used. Before discussing these requirements more fully, it is useful to consider the mechanical and mixing efficiencies of different dough mixers. The

FIG. 5. Mixogram traces. The sequence from A through to D may be. used to represent either doughs of increasing mixing requirements mixed at a fixed intensity, one dough mixed at increasing intensities, or one dough mixed at a fixed intensity but containing increasing additions of cysteine.

232

F. MACRITCHIE

distinction between these two quantities is illustrated in Fig. 6. A percentage of the total energy consumed by the mixer motor is available for doing mechanical work on the dough. This is called the mechanical efficiency. However, not all of this energy is utilized for mechanical development of the dough. That portion that is utilized is denoted as the mixing efficiency of the mixer. A comparative study of the mechanical efficiency of various laboratory dough mixers was made by Kilborn and Tipples (1973a). The mechanical efficiency vaned with the load on the mixer but was between 40 and 80% for the four mixers that were studied. Voisey and Kilbom (1974) used a strain-gauge transducer attached to the Grain Research Laboratow pin mixer (GRL-200) to record mixing curves electronically. This instrument enabled accurate determinations of the mechanical efficiency and the torque and energy used to mix the dough. The mechanical efficiency was calculated from the equation Mechanical efficiency =

Torque at pins Torque at motor

X

MA

where MA is the mechanical advantage relationship between the motor and the mixing pins.

TOTAL ,GROSS ENERGY

/

I

/

/

MECHANICAL EFFlCtENCY

\

\

\

* MOTOR

'. . '

/

LINKAGE

GEARS.etc

MIXER OUTPUT MIXING EFFICIENCY

i \ MECHANICAL

FIG. 6. Diagram illustrating distinction between mechanical and mixing efficiencies. From Kilbom and Tipples ( 1972).

BAKING QUALITY OF WHEAT FLOUR

233

In view of the wide range in mechanical efficiency found for laboratory mixers of different design, it seems likely that this would be the case also for large commercial mixers of different design. It therefore follows that when a fixed level of gross energy is used, doughs from a given flour will be mixed to different degrees of development depending on the mixer and its mechanical efficiency. Furthermore, because the available (net) energy that contributes to mechanical dough development also varies from one mixer to another (mixing efficiency), this also needs to be taken into account when optimum dough development is required. For example, the GRL-200 has a low mechanical efficiency, ranging from 54 to 57% at mixing speeds between 70 and 130 rpm (Kilborn and Tipples, 1973a). However, it has a high mixing efficiency. Mixing requirements of flours vary widely. Figure 5 illustrates the mixing behavior of four flour types. The mixing curves are for doughs that have been mixed at a constant and moderate speed. Flour A has high mixing requirements, and the rate of mixing used is insufficient to develop the dough; such a dough would be a failure in a baking test. Hour B is a moderately strong flour that does develop at this mixing speed, although there is an initial induction period before the dough begins to exert any appreciable torque on the mixer. Flour C is of medium strength, and Flour D is a comparatively weak flour that develops quickly. We may also make use of Fig. 5 to illustrate the effect of varying the mixing intensity (i.e., mixing speed) on the mixing curve of a given dough. At a low speed the dough does not develop (trace A). As the mixing speed is increased the dough develops after a lag phase (trace B) and thereafter develops more easily (traces C and D). With increasing speed, and providing the amount of work applied to the dough is above a certain level (corresponding to a point at or slightly past the peak in the trace), the baked loaf volume increases up to a critical mixing speed above which it plateaus. For every dough there exists a critical mixing speed (that may vary for each mixer) below which dough handling properties and baked loaf characteristics are unsatisfactory. In addition it is necessary to have imparted a minimum level of work input to the dough in order to bring it to an optimal developed state. If the mixing speed or work input level are below the critical values, unsatisfactory baking results are obtained (Kilborn and Tipples, 1972). Baking results are less affected by exceeding the critical values than when these values are not reached. However, dough handling properties deteriorate and doughs become sticky. Weaker flours are more sensitive to overmixing. 2 . Activated Dough Development Because some flours have very long mixing requirements or have critical mixing speeds that exceed the capacity of many dough mixers, methods of overcoming these limitations have been devised. One method involves the addi-

234

F. MACRITCHIE

tion of reducing agents, particularly sodium metabisulfite (SMS) and L-cysteine hydrochloride (cysteine) to doughs. Cysteine reduces the energy level required to achieve peak dough development and also the critical mixing speed necessary to produce bread of satisfactory loaf volume. We may again make use of Fig. 5 to illustrate the effects of increasing additions of cysteine in altering the mixing characteristics of a strong dough. As the amount of added cysteine is increased, the nature of the mixing curve changes from A through D. Cysteine also increases slightly the rate of energy input at a given mixing speed and increases the tolerance to undermixing, i.e., enables satisfactory bread to be produced with energy levels less than those required to achieve peak dough development. There is an optimum level of cysteine addition, and too high a level causes deterioration of loaf characteristics. In their detailed study, Kilborn and Tipples (1973b) found that the optimum level increased with increasing flour mixing requirements and with decreasing mixing speed. Optimum values ranged from zero for a flour with a relatively short mixing requirement mixed at high speed, to 240 ppm for a flour with long mixing requirements mixed at slow speed. It should be noted, however, that cysteine is not a permitted additive in many countries. 3. Mixing at Constant Rate of Energy tnput Although flours (e.g., flour B, Fig. 5) have long mixing requirements, this does not always mean that high levels of energy must be imparted to the dough, rather that the dough does not become elastic and have a high consistency during a long period at the commencement of mixing. Kilborn and Tipples (1972) found that by mixing these doughs at a constant rate of energy input, using a programmed mixing unit to vary the mixing speed as required, appreciable savings in mixing times were achieved. Interesting comparisons were made for several flours using “constant speed” and “constant energy” mixing. The savings, expressed as percentage mixing time, varied with both speed and flour, ranging from 13 to 22% at 140 rpm and from 6 to 12% at 110 rpm for the flours examined. Thus savings are greater at higher speeds and for flours that normally remain at a low consistency level for a long time before developing to a high consistency. A computing dough mixer that measures torque, speed, and power (Frazier et a l . , 1975) promises to be a valuable instrument for research into mechanical dough development. B.

UNMIXING AND THE EFFECTS OF REST PERIODS

During their studies of mechanical dough development, Tipples and Kilborn (1975) discovered an interesting behavior of doughs that they denoted by the term “unmixing.” This phenomenon is observed to occur when a dough that is

235

BAKING QUALITY OF WHEAT K O U R

mixed to peak consistency at high speed is then mixed for a further time well below the speed required for optimum development. The dough changes in character and assumes properties similar to a dough that has been undermixed; i.e., it changes from a shiny elastic mass capable of being stretched into thin sheets to a rough, lumpy dough lacking in cohesiveness. Bread baked from the unmixed dough has lower loaf volume and poorer external appearance and crumb texture similar to bread from undermixed dough. These detrimental effects become more notable with slower unmix speed and longer unmixing time. The unmixing effect is more pronounced for stronger flours. Weak flours with low critical speed requirements are less affected. As a result, decreases in loaf volume of loaves baked from stronger flours tend to be greater following unmixing (Tipples and Kilborn, 1979). An unmixed dough may be redeveloped to peak consistency by speeding up the mixer to form a dough that produces bread of quality equal to that of the initial dough. This sequence of unmixing and remixing may be repeated several times. The behavior is illustrated in Fig. 7. As can

-.. 160rpm

a

-

C

85

160

b

I

105 r p m

d

FIG.7. Effects of unmixing and remixing. For (a), (b), and (c) a mixing and remixing speed of 160 rpm is used with unmixing speeds of 55, 85, and 105 rpm, respectively. Curve (d) shows the trace obtained using a mixing speed of 105 rpm for the same dough. From Tipples and Kilborn (1979).

236

F. MACRITCHIE

be seen, the mixing curve during the slow mixing or unmixing stage resembles the reverse of a normal development curve where dough consistency increases to a maximum. Tipples and Kilborn have used the phenomenon of unmixing to determine the critical mixing speeds of doughs. By mixing a dough initially at a high speed then mixing at progressively lower speeds, it is possible to pinpoint the critical mixing speed by observing the commencement of deterioration in the baked loaf volume. This method is more reliable than that of using different mixing speeds from the beginning because, in the latter method, results tend to be more inconsistent in the region of the critical mixing speed. The Canadian Grain Research Laboratory uses, as one of several supplementary baking tests employed for variety testing and evaluation, a baking method that incorporates an unmixing step. This test is able to distinguish flours that have doughs with critical mixing requirements outside the desired range. As well as being of fundamental interest for understanding dough behavior, the phenomenon of unmixing has drawn attention to the care required in handling of doughs after mixing. The influence of rest periods on dough properties has also been studied by Tipples and Kilborn (1977). When a rest period was introduced between the end of mixing to peak consistency and slow speed unmixing, dough relaxation during the rest period appeared to augment the unmixing effect. This indicated that the dough was more easily disorientated than a developed, unrested dough. For example, when rest periods of 2 and 8 min were introduced between initial mixing and slow speed unmixing, the remixing time required to reachieve peak consistency increased to 0.9 and 1.2 min, respectively. When dough was mixed to peak consistency, rested and remixed at the same speed (above the critical mixing speed), remixing produced an initial high level start-up trace followed by a decrease in consistency (“hole”) before consistency again increased to a peak level (Fig. 8). Micrography of dough shows that a certain degree of contraction of protein occurs during resting (Moss et al., 1979). The effects on mixing curves were not restricted to pin mixers and were accentuated for doughs at high water absorption. Because dough is more fluid and offers less resistance to the mixer pins under these conditions, a higher speed is required to subject the doughs to a given degree of stress. Therefore, when a constant speed is used, the mixing curve becomes flatter and the lag phase longer. Consequently, the unmixing effect following the rest period is more marked (i.e., the “hole” effect is amplified), consistent with a higher critical speed requirement at the higher water absorption. Another effect observed was that, as the rest time after an initial short premixing was increased, subsequent development time was reduced. For a dough with a water absorption of 100%, an increase in rest time from 0 to 128 min reduced the development time from 9.9 min to 2.3 min. At least some of the observed effects of rest time may result from

BAKING QUALITY OF WHEAT FLOUR

6i"

237

6pr

A

1

B C

8 MIN REST

8 MIN REST

mMINUTES

FIG.8. Effect of rest periods between mixing to peak consistency and slow-speed unmixing on mixing curves and baked loaf volume (indicated by the numbers above the arrows) at different stages during, the mixing process. From Tipples and Kilborn (1977).

protein breakdown caused by proteolytic enzymes that are invariably present. Mixing behavior was similar when the flour-water dough systems were replaced by doughs containing the full baking formula. Dough pieces were taken at different stages during mixing and baked. The results are well summarized in Fig. 8 for a dough that was mixed at 136 rpm on the GRL-200 mixer with an 8min rest period introduced. The detrimental effects on the baked loaf volume of the unmixing stages following the rest periods are clearly demonstrated. When sheeting rolls were used to develop doughs following the rest periods, the destructive effects that were found in the case of development by mixers did not occur.

C . INTERACTION OF OXIDANTS, WORK REQUIREMENTS, AND PROOFING TIME It is widely agreed that in a normal short-time bread process, optimum bread is produced when dough is mixed to near peak consistency as judged by a mixing curve. Heaps, Frazier, and co-workers (Heaps ef al., 1976; Frazier et al., 1975; Daniels and Frazier, 1978), by contrast, have reported that best results were found when dough mixing was continued to points considerably past the peak consistency. In the baking process used by these workers, however, no oxidants were used. This has been shown to be the source of the apparent disagreement and has caused Kiiborn and Tipples (1979) to make a detailed study of the interactions between ofidant level, intermediate proof time, and work requirements in short-process breadmaking. It was shown that when the levels of oxidant are appropriate, optimum bread and greatest tolerance to intermediate proof

238

F. MACRITCHIE

time occurred when doughs were mixed to peak consistency. As the oxidant levels in doughs were decreased below the optimum, mixing needed to be progressively increased beyond peak consistency for best results. In the absence of oxidants, intermediate proof time requirements were also greater. The combined effects on loaf score of energy input and intermediate proof time for doughs containing no oxidant are summarized in Fig. 9. In general, an intermediate proof stage was found to be very important in a short baking process and could not be adequately compensated for by extending final proof. Moss et al. (1979), in a study of the time-dependent interaction of oxidizing and reducing agents in short-process breadmaking, have stressed the importance of processing steps subsequent to mixing in influencing bread quality. Effects on dough microstructure and baked loaf characteristics of varying floor-time (time between mixing and dough dividing) and levels of oxidants and of the reducing agent SMS were studied. Moderately high levels of SMS (60 ppm), as well as decreasing mixing time, increased the range of floor-times a dough could tolerate and still yield bread of acceptable quality. Low levels of SMS (15 ppm) decreased mixing time but did not alter floor-time requirements, while excessive levels (120 ppm) required very long floor-times to produce satisfactory bread. D.

DOUGH DEVELOPMENT BY SHEETING ROLLS

An alternative means for developing doughs involving repeated passages through sheeting rolls has wide application in a number of parts of the world including Peru, Spain, Southeast Asia, and parts of Africa (Moss, 1980). Although, as practiced, this is a labor-intensive process because of the manual operation, it nevertheless is very efficient in terms of the mechanical energy required to develop the dough. Because of its efficiency, more attention has been given to the method in recent years, and a number of studies has increased our basic knowledge in this area. Kilborn and Tipples (1974) devised a means for determining the mechanical efficiency of a set of modified rolls, allowing the calculation of net energy values that could be compared with net energy measurements made during mixer development. They selected a system of two rollers (National Manufacturing sheeting rolls) with gap settings of 7/32 and 5/32 in. so as to impart maximum work without tearing the surface of the dough. Use of dough brakes is generally associated with low water absorption (50 instead of 60%)arising from handling difficulties, but it was found that up to 64% could be used provided care was taken, particularly during the first few sheetings. The dough was given a short premixing at slow speed, then sheeted in the same direction through two rollers, folded in half, rotated through 90°, and returned. At this stage, it had been subjected to four sheetings and one fold. For the flour used, optimum conditions as judged by dough handling properties and bread quality were achieved with

239

BAKING QUALITY OF WHEAT FLOUR

LU

LT 0

60

-

50

-

40

-

30

-

20

-

10

-

% w

1

0

zIn

I

I

I

1

I

I

FIG. 9. Combined effects on loaf score of energy input and intermediate proof time (shown by numbers at ends of lines) for doughs containing no oxidant. From Kilbom and Tipples (1979).

15-20 folds. The total time that the dough was worked was 45 sec, and the net energy imparted was only 0.86 W hr kg- I , corresponding to about 15% of that required for peak development by the conventional pin mixer. This implies that the type of work being done on the dough with the sheeting rolls is extremely useful and efficient work. Because dough is rotated after each fold, layers tend to become cross-hatched, emphasizing the two-dimensional rather than unidirectional nature of the sheet-like structure being promoted within the dough. Kilbom et al. (1981) investigated several ways that dough brake rolls might be used for production of bread dough, using a pilot-scale dough brake and a North American pan bread formula. Using the dual gap multisheeting method following an initial short pre-mix, bread of high volume and fine crumb structure was obtained. This method, however, is limited by the size of the dough piece (- 1.5 kg) that can be conveniently handled. Larger quantities of dough (e.g., 2.5-10 kg) are possible using a single gap method. The main factor that limits the size of the dough piece is the reduction of energy per unit weight of dough that occurs as the gap is increased to accommodate a larger dough. However, both techniques use markedly less energy than dough development by a conventional mixer. For example, 6.5 W hr kg - of dough was required to produce bread of high quality using the GRL-200 mixer, whereas an energy level of 1.O W hr kg - produced

240

F . MACRITCHIE

bread at least equal to the optimum bread from the mixer using the dual gap and single gap techniques. This represents a 70% saving in energy. Two other variations of the sheeting roll process was tried. The finishing-off technique in which an undermixed dough is passed four or more times through the rolls also gave relatively large savings in energy yet produced bread of high quality. Another method in which the dough brake was used as a “mixer” involved allowing the pre-mixed dough to wrap around the top roll of the brake initially with a fairly wide gap which was then gradually reduced until the dough spread the full width of the top roll. Observations of the appearance of the dough surface allowed recognition of the stage at which development had occurred. This method used about the same amount of energy to develop dough as a conventional mixer. The rate of energy input depends on the speed of the rollers and the gap width. As in conventional mixing, the rate of energy input needs to be above a certain value for optimum development. Well-defined mixing curves were obtained. Times to reach peaks varied from 20.5 min at a roll speed of 7 cm sec- to 1.7 min at 52 cm sec- l . The time required to achieve a given work level decreased with decreasing gap width, e.g., 1.1 min at 2.5 mm and 1.8 min at 2.9 mm for a work level of 5.5 W hr kg-’ of dough. One possible way of utilizing sheeting rolls, suggested by the authors, is to have a line of rolls (say, four sets) between a continuous premixer, which could impart about 25% of the work requirement, and the dough divider. This would allow the baker to effect savings in energy and to have more control over his product. Stenvert et al. (1979) have studied the effects of sheeting rollers on doughs that have been well mixed. Three flours with a range of dough strengths and two baking processes were compared. Using a no-time (chemically activated) dough process, bread became finer in crumb structure with increasing number of passes through the rollers. The more stable the dough properties of the flour, the greater were the number of passes required to produce optimum crumb structure and the greater the tolerance to excessive numbers of passes. With fermented doughs (which had additional bowl times of 1.5 hr), greater numbers of passes were required to achieve optimum dough properties, but only the most stable flour gave the desirable crumb structure and maintained loaf volume. The considerable amount of work required to remove the fermentation gases appears to cause structural breakdown of the protein for less stable doughs with detrimental effects on bread quality. Increasing oxidant levels improved bread from the less stable doughs for the no-time process but not in the case of fermented doughs. These general trends were confirmed by Moss (1980) in an extensive survey of flour samples in which results from a conventional fermentation baking test were compared with those from a sheeted dough procedure. The main conclusion reached was that flours with very strong characteristics (extensigraph resistances above 320 BU) performed better in the sheeted dough than in the conventional procedure, giving bread with high volume and fine crumb structure. The performance of stronger flours in the conventional process was limited by their mixing

BAKING QUALITY OF WHEAT FLOUR

24 1

I

Resistance ( BU 1

FIG. 10. Loaf volumes as a function of extensigraph resistance compared for a conventional fermentation baking test and a sheeted dough procedure. From Moss (1980).

requirements. Results for loaf volume as a function of extensigraph resistance are shown in Fig. 10 for a sample of 47 flours with protein contents between 12 and 14%. The studies of dough development that have been discussed draw attention to the delicate requirements of doughs for optimum performance in baking. They also emphasize the wide range of dough behavior shown by flours. Although the effects of many of the observed phenomena are known, their relation to structural changes in dough are not well understood. Tipples and Kilborn (1977) have stressed the interaction between time and mechanical work in producing constructive and destructive changes in dough properties. They have also speculated on whether some of the effects are entirely related to changes in the physical characteristics of flour components or whether the state of dispersion of water plays a significant role. Certainly there is an appreciable proportion of dough water which is “free” (MacRitchie, 1976). It seems reasonable to expect that the manner in which this water is distributed as well as how the distribution changes will have a prominent role in determining dough properties, gas cell structure, and ultimately baking performance.

V.

ROLE OF FLOUR COMPONENTS

Wheat flour is a complex material chemically. The major components are protein, carbohydrate, and lipid, but each of these major constituents are made

242

F. MACRITCHIE

up of large numbers of individual chemical entities. The ranges of amounts of each commonly found in dry flour are 8-18% protein, 70-80% carbohydrate, and 1.5-2.5% lipid. The protein moiety is heterogeneous, as shown by the very large number of discrete bands that can be separated by electrophoresis, and the number of bands increases as greater resolution is obtained (Wrigley, 1976). The positions of the bands are characteristic for protein from a given variety, reflecting the genetic control of protein synthesis. This property has been utilized in recent years for the identification of cultivars in quality control (Autran et al., 1979). Variations in electrophoretic patterns from one variety to another indicate that different protein entities occur in unrelated varieties. Starch accounts for the main portion of the carbohydrate with relatively small quantities of sugars, pentosans, and celluloses. Within the lipids of wheat flour, more than 20 distinct classes of chemical compounds have been identified (Morrison et a l ., 1980). Other substances present in normal wheat flour that are important are water, minerals (analyzed as ash), and vitamins.

A.

PROTEIN

The protein in wheat flour may be divided conveniently into two groups, the water-soluble proteins (comprising albumins, globulins, and some enzymes) and the gluten protein. The water-soluble class accounts for roughly 2% of the flour and is relatively constant from one flour to another. As the protein content of flours increases, the extra protein appears as gluten (or storage) protein. The two groups of proteins are quite different in their chemical composition, solubility, and other physical properties as well as in their role in dough behavior and baking, as summarized in Table V. TABLE V COMPARISON OF PROPERTIES OF GLUTEN AND NONGLUTEN PROTEINS Property Amounts in wheat flour Chemical composition

Solubility

Physical properties

Gluten proteins 618% High glutamic acid mainly glutamine

(-40%) High proline (- 14%) Low lysine (- 1.5%) Insoluble in neutral solution, salts Soluble (partially) in acid, alkali, urea Solubility increases with increasing temperature Responsible for viscoelasticity of dough and baking properties

Nongluten proteins -2% Glutamic acid (- 12%) Proline (-8%) Lysine (-3%) Soluble in neutral solution Precipitated by heating

Not important for dough properties or in baking

BAKING QUALITY OF WHEAT FLOUR

I.

243

Gluten Protein

Gluten protein imparts the coherent and viscoelastic properties to a dough that are essential for the production of leavened products, especially bread. The rheological properties of dough are largely controlled by the molecular weight distribution of the gluten protein as is the case for polymer systems in general. It is customary to divide gluten protein into two classes, gliadin and glutenin. The original classification was based on the extractability of gliadin by a 70% aqueous ethanol solution, the insoluble residue being designated glutenin (Osborne, 1907). Since then there has been a tendency to define the classes in terms of molecular weight. According to this, a cut-off molecular weight of 100,000 is normally used, protein of lower molecular weight being called gliadin and of higher molecular weight, glutenin. Gluten protein consists of a mixture of proteins of relatively similar amino acid composition but with a broad range of molecular weights. The complexity appears to have arisen from gene mutation, duplication, and combination (Simmonds, 1981). Because of experimental difficulty, few accurate measurements of molecular weight distributions have been made. Any fractionation procedure using differential solubility cannot be expected to yield sharp fractions. Therefore, distinction between gliadin and glutenin based on solubility or molecular weight estimates is usually an approximation. Recent work has shown that a certain proportion of gluten protein has at various times been denoted as either high molecular weight gliadin or low molecular weight glutenin.

2 . Effects on Dough Properties Despite the diffuse nature of the distinction, gliadin and glutenin behave quite differently in regard to their contributions to dough properties, this behavior serving as a basis for classification. This was clearly shown by Lee and MacRitchie (1971) who fractionally extracted gluten protein from flour using urea solutions and assessed the functional properties of the fractions by addition to a base flour and measurement of mixograms and alveograms. The initially extracted fractions caused weakening of the dough, i.e., faster breakdown of the mixograms and alveograms with lower resistance and greater extensibility. At a certain point in the successive extraction procedure, the later extracted fractions produced the opposite effects. It is now fairly well established that gliadin contributes extensibility to a dough and glutenin contributes strength and elasticity. This knowledge may be used to produce flours with predictable properties of strength and elasticity by varying the proportions of early and later extracted gluten protein in a given flour (MacRitchie, 1973). Similarly, the mixing requirements of a dough are sensitive to the relative proportions of gliadin and glutenin. Figure 11 illustrates how the time to reach

244

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220

1 14;+-\

5.9 7.8

25

160 1

20

40

60

80

Percent supernatant protein FIG. 1 1 . Loaf volumes as a function of the percentage of supernatant protein for two flours of similar protein content but differing baking potential. Arrows represent supernatant protein contents of original flours. Numbers show the times in minutes to reach peak mixograph development. From MacRitchie (1980a).

peak development increases with increasing glutenin content for two flours of different mixing characteristics. That glutenin is the fraction that determines the dough mixing requirements of a flour is shown clearly by the results of Fig. 12. Here, gliadin and glutenin fractions have been interchanged between flours of long and short mixing requirements (Finney, 1979). Finney believes that differences in flour mixing times are attributable to differences in glutenin quality and not quantity. This appears to be often the case. On the other hand, Huebner and Wall (1976) showed that the amount of highest molecular weight glutenin in flour was directly related to mixing time requirements and stability. Their results for three flours, using agarose gel filtration to measure the molecular weight profiles, are illustrated in Fig. 13. Similar findings have been reported by Hamada et al. (1982). It therefore appears that differences in mixing properties may be related to the quantity as well as the nature of the glutenin. A medium to medium-long dough mixing requirement is generally the most desirable for a good bread flour (Finney, 1979). Flours with short mixing times tend to be weak, have low tolerance to overmixing, and usually do not perform so well in breadmaking. Such flours may be suitable for cookies. At the other extreme, flours that require very long mixing times or very high rates of energy input to promote adequate development, although they may perform well under optimum conditions, are undesirable under practical bakery conditions. Optimum mixing properties appear to require first a suitable balance between the quantities of gliadin and glutenin and second a good quality glutenin. The requirements for a good quality glutenin, at a molecular level, have not been precisely determined by cereal chemists. In general, gliadin fractions ap-

BAKING QUALITY OF WHEAT FLOUR

245

FIG. 12. Effects of interchanging gliadin and glutenin between flours of long (top left) and short (top right) mixing requirements. Mixograms at middle left and bottom right contain glutenin from long mixing flour. Mixograms at middle right and bottom left contain glutenin from short mixing flour. From Finney (1979).

pear to behave very similarly from one flour to another in terms of their effects on dough properties. Nevertheless, the presence of an adequate amount of gliadin is essential to obtain the required dough properties. Furthermore, the presence of excessive amounts of very low molecular weight gluten protein that may arise through proteolysis following rain damage to the grain or to severe laboratory treatments (such as exposure to acid solutions for long periods) produces marked effects on dough properties. These are evident as reduced strength, development time, and stability together with increased stickiness. This susceptibility to the presence of low molecular weight material reflects the well-known phenomenon in polymer chemistry that small quantities of components at the

246

F. MACRITCHIE

Volume of eluate i mi )

FIG. 13. Agarose gel filtration chromatograms of protein from three flours of differing mixing requirements. The decrease in the first peak on the left (corresponding to highest molecular weight glutenin) correlates with decrease in the flour mixing requirements. From Huebner and Wall (1976).

ends of the molecular weight spectrum can cause disproportionately large effects on physical properties (McGrew, 1958). A synthetic flour made from starch and gliadin alone forms a very extensible and sticky dough with a minimum of mixing. A flour containing starch and glutenin, on the other hand, cannot be developed under nonnal conditions and gives a dough mass with no sheet-forming properties. Neither type of flour is suitable for baking purposes. Gliadin is essential to contribute plasticity and ease of dough development whereas glutenin imparts the strength and elasticity necessary for good dough handling properties and capacity to support stresses such as those that occur during baking. Evidently, gliadin molecules are easily stretched by shearing and tensile forces. By contrast, much more energy is needed to extend glutenin molecules in order to incorporate them into the continuous protein matrix to contribute elasticity to the dough. 3 . Breadmuking In an optimized baking test, loaf volume is a linear function of protein content in the range of 8-20% protein. Furthermore, the slope of the regression lines is characteristic for a given wheat variety. These relationships were established during detailed studies of a large number of varieties grown over several crop years in the Great Plains area of the United States (Finney and Barmore, 1948; Fifield et ul., 1950). Experiments in which different flour components are systematically interchanged between flours of good and poor baking performance have in most cases confirmed that the gluten protein is responsible for the

BAKING QUALITY OF WHEAT FLOUR

247

differences (Finney, 1943; MacRitchie, 1978; Booth and Melvin, 1979). Similarly, water absorption by flour increases linearly with increasing protein content, and the slope of the regression line varies between varieties. Finney (1979) has reported water absorptions varying between 55 and 62% for flours of 10% protein content and between 63 and 78% for flours of 18% protein. The oxidation requirements of flours (i.e., the amount of potassium bromate to be added for optimum performance) also increase substantially with increasing protein content and are greater for short mixing than medium-long mixing flours. For example, the bromate requirement varied from 1.5 mg per 100 g flour containing 8% protein to 6 mg at 16% protein for a short mixing flour. Corresponding values for a medium-long mixing flour ranged from zero at 8.5% protein to 3 mg at 16% protein. Thus loaf volume, water absorption, and oxidation requirement are a function of both protein quantity and quality. The question of whether the quality factor in gluten protein is distributed evenly throughout its components or whether baking quality can be identified with specific protein fractions has been tackled by a number of workers, using fractionation and reconstitution methods. The problem has not been so simple as that of relating protein fractions to the mixing and mechanical properties of doughs, nor does it appear that the factors responsible for differences in dough properties are identical to those that control baking performance, even though good dough handling properties are obviously a prerequisite for a satisfactory baking result. In view of the importance and challenging nature of the problem, an appreciable part of this section will be devoted to reviewing the publications that have been concerned with the preparation and testing of gluten protein fractions in breadmaking. There are four main ones that will be considered. A casual reading of these may induce the conclusion that the results from the four separate studies are somewhat contradictory. However, closer inspection reveals that there is a good deal of consistency between the results of the different workers. Hoseney et al. (1969b) solubilized gluten in 0.005 M lactic acid (pH 4.7) and recovered fractions by adjusting to pH 6.1 with 0.1 N sodium carbonate. Some 5-870 of the total protein remained insoluble, and when this portion was interchanged between a good and a poor variety flour, loaf volume potentials were unchanged. Also, when the insoluble fractions were omitted in the reconstituted flours, loaf volumes were at least equal to those of the corresponding unfractionated flours. It was concluded that this insoluble fraction had no specific role in breadmaking. The remaining protein (soluble at pH 4.7) was fractionated by centrifuging at 100,000 g for 5 hr. This produced two fractions, the sediment protein comprising about 15% of the total soluble protein and the supernatant protein accounting for the remaining 85%. When the ratio of sediment to supernatant protein was varied from its natural value in the flours of 15:85, loaf volume was maintained within a certain range but was lower when the ratio was

248

F. MACRITCHIE

0: 100 (i.e., sediment protein absent) and began to drop significantly when the ratio was increased above 5050. With pure sediment protein (1OO:O ratio), doughs could not be developed and, as a result, gave very low loaf volumes. When sediment and supernatant fractions were interchanged between a good and a poor baking flour, the difference in loaf volume was completely accounted for by the supernatant fraction. The supernatant protein comprised 85% of the soluble and thus about 78-81% of the total protein. It therefore contained practically all the gliadin and an appreciable portion of the glutenin. The authors suggest that if we assume that the glutenin contained in the supernatant fraction behaves similarly to the glutenin of the insoluble protein and the sediment fraction of the soluble (i.e., does not account for loaf volume differences between flours), then differences in loaf volume potential would reside in the gliadin. Two points are worth noting here. First, the experiments did not establish that the glutenin from the supernatant fraction did not contribute to the differences in loaf volume. Hoseney et al. (1969b) found that the ratio of gliadin to glutenin was practically constant at 53% gliadin to 47% glutenin for four wheat varieties that varied widely in breadmaking quality. Thus, the supernatant fraction could be expected to contain between 25 and 28% of the total protein as glutenin, i.e., more than half the flour glutenin. Second, loaf volumes for flours reconstituted with only supernatant protein were significantly lower than those containing both sediment and supernatant fractions (providing the ratio of sediment to supernatant was well below 5050).This shows that the sediment fractions, although not distinguishable when interchanged between flours, were important to the baking performance of the original flours. The insoluble glutenins, on the other hand, were not. The superiority of low and medium molecular weight glutenins over the high molecular weight glutenins in baking has been commented on by Finney (1979). The factors responsible for the poor breadmaking quality of the high-yielding wheat cultivar Maris Hunstman (MH), grown widely in Europe in recent years, has been studied by fractionation and interchange of flour components with a good quality Canadian Western Red Spring (CWRS) wheat (Booth and Melvin, 1979). Lactic acid (0.005 M ) extracted 83% of the protein from MH but only 51% from CWRS gluten. Increasing the proportion of the MH insoluble gluten fraction in reconstituted MH flour produced no improvement in baking performance. Reducing the proportion of insoluble protein in the CWRS reconstituted flour did produce some increase in loaf volume. Because lactic acid extracted differing amounts of protein from the glutens of the two flours, Booth and Melvin carried out interchange experiments at both ratios of insoluble to soluble gluten protein, i.e., at ratios of 17:83 and 4951. The results showed that differences in quality resided approximately equally in the soluble and insoluble gluten fractions. Because alteration of its own gluten composition produced no improvements in MH quality, the authors concluded that the poor performance of

BAKING QUALITY OF WHEAT FLOUR

249

this flour was not due to a poor balance of gluten proteins but to qualitative factors present in both soluble and insoluble fractions. MacRitchie (1978) separated glutens from two flours of different baking performance each into two fractions by extraction with 0.1 M acetic acid. The soluble fractions contained -60% and the insoluble fractions 40% of the total gluten protein. Although reconstituted loaves were of lower volume than controls, interchange experiments clearly indicated that the quality factor resided in the insoluble gluten fraction, Improved reconstitution techniques using dilute hydrochloric acid as extractant and neutralizing the acid immediately after separation of fractions enabled more definitive interchange experiments to be carried out (MacRitchie, 1980a). In this subsequent work, it was found that the same concentrations of acid extracted unequal amounts of protein from glutens of two flours chosen on the basis of their differing baking performance. This is similar to the findings of Booth and Melvin. To overcome this problem, calibrations of acid concentration as a function of quantity of protein extracted were made for both gluten samples. Acid concentrations were then chosen so as to extract equal proportions of protein from the two gluten samples. When 33% of the total gluten protein from each flour was extracted into the supernatant and these fractions interchanged between the flours, no effect on the original loaf volumes was observed. Interchange of the sediment fractions (67% of total protein) accounted for all the difference in loaf volume. However, when acid concentrations were increased to extract 70% of the gluten protein into the supernatant, interchange of fractions showed that the supernatant fraction accounted for 65% of the loaf volume difference and the sediment (30%of total protein) accounted for about 35%. If we assume contributions from fractions to be additive, then we may conclude that the additional protein extracted in the supernatant (i.e., some 40% of the total protein) accounts for 65% of the loaf volume difference. This was approximately verified when three fractions were prepared by successive extraction of the glutens from the two flours. The results are summarized in Table VI. Another study of the effects of acid-soluble and acid-insoluble gluten proteins on rheological and baking properties of wheat flours was carried out by Preston and Tipples (1980). The approach was slightly different to the previous ones discussed in that fractions were tested by addition of varying amounts to two base flours rather than by interchange. Two flours were studied using two types of baking test. The soluble and insoluble gluten protein fractions made up approximately 70 and lo%, respectively, of the total protein, using 0.05 M acetic acid as the extracting solvent. Mixograms and farinograms of flours to which fractions were added suggested that dough-strengthening effects were mainly due to proteins present in the acid-soluble fraction, whereas the acid-insoluble proteins at higher levels had a slight dough-weakening effect. Addition of increasing amounts of acid-soluble gluten progressively increased loaf volumes of both

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F. MACRITCHIE

TABLE VI CONTRIBUTION OF GLUTEN PROTEIN FRACTIONS TO DIFFERENCES IN LOAF VOLUME OF TWO FLOURS FOUND BY INTERCHANGE OF FRACTIONS"

Fraction

Percentage of total gluten protein

Contribution to loaf volume difference (%)

1. Mainly gliadin 2. Mainly more soluble glutenin 3. Mainly more insoluble glutenin

37 40 23

70 30

0

" From MacRitchie (1980a)

flours. In contrast, except for a slight increase at an addition of 0.5% in the case of the weaker flour using the GRL Chorleywood process, increasing additions of the acid-insoluble protein (up to 2.0%) caused progressive lowering of loaf volume. In summarizing the data from these various studies, it is important to realize that the amounts and compositions of protein fractions prepared from a flour or gluten vary depending on the nature and concentration of the extracting solvent and the particular flour or gluten used. Furthermore, conclusions reached from interchanging fractions between two flours only cannot be adopted as generalizaLions. However, as more work of this nature is carried out, it might be expected that a general pattern should begin to emerge. All the studies point to the result that the quality factor in gluten, although it has not been identified with any sharply defined fraction(s), is not uniformly distributed throughout the protein but is more concentrated in certain fractions. Despite the correlations that have been found between baking quality and the amount of residue protein left after extraction of flours (Orth and O'Brien, 1976), most of the studies indicate that the most insoluble gluten protein (i.e., the remaining 10-20% or so of the total protein left after acid solubilization) either contributes little towards quality differences (Hoseney et al., 1969a), has less importance than portions of the solubilized protein (MacRitchie, 1980a), or has detrimental effects (Preston and Tipples, 1980; MacRitchie, 1973). Some of the detrimental effects can almost certainly be attributable in some cases (MacRitchie, 1973) to failure to provide the much longer or more intensive mixing requirements introduced by this fraction. The results of MacRitchie (1980a) indicate that the earliest extracted gluten protein (i.e., the first one-third) did not account for differences in baking quality between the flours studied. In the studies of Hoseney et al., Booth and Melvin, and Preston and Tipples, larger percentages of the total protein were extracted.

BAKING QUALITY OF WHEAT =OUR

25 1

There is thus a need for confirmation of this result by extraction of smaller proportions of the total protein for use in interchange experiments. If we accept that the earliest extracted portion of the gluten protein (mainly a-,p-, and ygliadins) is not very important in explaining baking quality differences, we are led to the conclusion that gluten quality is concentrated in the protein of intermediate solubility. This would include the more insoluble gliadin and the lower molecular weight glutenin. The results of all the studies considered are consistent with this interpretation. Preston and Tipples (1980) postulate that the glutenin proteins that disaggregate during dough mixing and not the insoluble proteins resistant to mechanical disaggregation are of major importance for determining breadmaking quality. It is interesting to note that Bushuk and Wrigley (1971) found that the presence of low molecular weight glutenin in gel filtration profiles appeared to be related to baking quality.

B.

LIPID

Although the lipid component makes up only about 2% of wheat flour, it can exert very large effects in baking. Roughly one-quarter of the total flour lipid is associated with the starch. This comprises mainly lysolecithin and is thought to be distributed throughout the starch granules. Its absence from the continunus matrix of the dough therefore precludes it from playing a role in the proofing and early stages of baking. When assessing the effect of lipids on dough properties, it is important to establish that the solvents used for lipid extraction do not alter the functional properties of the flours. Failure to observe this has led to false interpretations of the effects of lipids on dough properties in the past. It is the author’s experience that removal of flour lipid by chloroform has very little effect on the dough development requirements of a flour. It does, however, cause some increase in strength as measured by instruments such as the alveograph or extensograph. This effect is reversible; i.e., the original dough properties are recovered on addition of the extracted lipid. The dry defatted flour is also a noticeably finer powder than the untreated flour. Evidently the lipid causes some degree of stickiness between flour particles. 1. Bread

Figure 14 illustrates how baked loaf volume changes when the nonstarch lipid is removed from a flour (e.g., by extraction with chloroform) and added back to the flour in increasing amounts. Loaf volume, which is high for the defatted flour, decreases to a minimum at a lipid content intermediate between the defatted and untreated flour, thereafter increasing. At lipid contents near or above that of the untreated flour, the curve begins to flatten out, approaching a plateau value. The changes in volume are accompanied by corresponding changes in the

25 2

F. MACRITCHIE

200

\ al 5

-

180-

9

160J 0

140

-

I

__________-__ 0

200

400

------------600

800

1000

Added lipid ( mg )

FIG. 14. Loaf volume as a function of lipid addition to a chloroform-defatted flour in a baking test Whole lipid; (A)polar lipid; (0) nonpolar lipid; dashed line. volume at using 30.2 g dry flour. (0) end of proofing for all loaves. From MacRitchie and Gras (1973).

color and texture of the baked loaves. Loaves from defatted flour are white in color and have a fine texture, characterized by small and uniformly sized gas cells. Both color and texture deteriorate with addition of lipid until the point of minimum loaf volume, after which texture again improves although not usually achieving the fineness of loaves from the defatted flour. This behavior has been found to be quite general for all flours, using both short time and long fermentation bread processes. Modifications of the baking procedure may, however, induce changes in the pattern. For example, it was found that by not including the normal molding step after a preliminary fermentation in a short time process, loaf volume and texture decreased continuously as increasing amounts of lipid were added to the defatted flour (MacRitchie, 1976). The behavior is shown in Fig. 15. One result of increasing lipid content is to effect a deterioration in the gas cell structure of dough when the intermediate molding step is omitted. This is illustrated in Fig. 16. The dough made from defatted flour has a dispersion of many small and uniformly sized cells at the end of proofing. On the other hand, with lipid present, a much coarser and more heterogeneous gas cell structure is evident after final proofing. During proofing, some large bubbles usually form near the surface of the dough piece. It is apparent that gas transfer occurs from the smaller bubbles to the larger, leading to the coarse

BAKING QUALITY OF WHEAT FLOUR

253

2201

I

.

0.4

0.8 Hydrolysate lipid content

1.2 ( O h )

FIG. 15. Loaf volume as a function of hydrolysate lipid content of flour in a baking test using 30.2 g dry flour. Upper curve is for a test in which an intermediate molding step after 20 min fermentation is included. Lower curve is for a test in which the molding step is omitted. From MacRitchie (1976).

dispersion. This process occurs as a consequence of the higher internal pressure in smaller gas bubbles (Davies and Rideal, 1963). The effect of the intermediate molding step is to produce a more stable dispersion in which this gas transfer process is greatly reduced. It is convenient to classify flour lipids into two groups, termed the nonpolar and polar fractions. The nonpolar fraction includes sterol esters, mono-, di-, and triglycerides, and free fatty acids; the polar fraction is made up of glycolipids and phospholipids. Major constituents of the glycolipids are monogalactosyl diglyceride (MGD) and digalactosyl diglyceride (DGD). The main phospholipid is phosphatidylcholine (PC). Relative amounts of the two fractions have been reported by Ponte and De Stefanis (1969) as 63% nonpolar and 37% polar. These proportions are no doubt influenced by the method of extraction and would be expected to vary from one flour to another. Studies of the effects of the two fractions in baking have all agreed that the nonpolar portion is detrimental whereas polar lipids are beneficial. Daftary et al. (1968) measured volumes of loaves baked from PE-extracted flours as a function of the amount of nonpolar lipid at several constant levels of polar lipid. In the absence of polar lipid, loaf volume dropped rapidly with increasing addition of nonpolar lipid to reach a plateau value at about 0.5% by weight of lipid. As the constant polar lipid content was increased from 0 to 0.5%, increasingly higher plateau values for the loaf volume were obtained, showing that the deleterious effects of the nonpolar lipid were being counteracted. Ponte and De Stefanis (1969) assessed the effects of their polar and nonpolar lipid fractions on loaf volume, grain score, and loaf compressibility of an untreated

254

F. MACRITCtlIE

FIG. 16. Effects of different treatments on gas cell structure of microloaves (30.2 g dry flour) after final proofing prior to oven baking. Loaves were frozen, sliced, and freeze-dried before photographing. (1) Defatted flour; (2) flour of lipid content corresponding to a minimum in the loaf volumelipid content curve; (3) whole flour, remolding step (after 20 min) included; (4) whole flour, remolding step omitted. From MacRitchie (1981).

control flour. The polar lipid fraction produced sinall grain score improvements and substantial loaf volume increases, particularly when lard was omitted from the formulation. In contrast, the nonpolar lipid fraction markedly decreased loaf volumes and increased crumb firmness; however, the crumb grain appeared to be finer and more uniform. MacRitchic and Gras (1977) measured thc effects of increasing amounts of' polar and nonpolar lipid fractions on loaf volume for a Ilour that had been defatted with chloroform. The nonpolar fraction produced a

BAKING QUALITY OF WHEAT FLOUR

255

progressive decrease in loaf volume, while the polar fraction depressed loaf volume initially (up to 0.3% by flour weight) but increased volumes at higher increments (Fig. 14). When the intermediate molding step was omitted from the baking test, loaf volumes were slightly lower for both lipid fractions (MacRitchie, 1977). However, it was interesting to note that the shape of the curve for loaf volume vs polar lipid content was unchanged, in contrast to the radical change observed in the loaf volume-total lipid relations (Fig. 15). Daftary et al. (1968) separated flour lipid into individual components by silicic acid column chromatography. In a preliminary study of the effects of individual components, it was found that tri-; di-, and monoglycerides had no significant effect on the loaf volume of bread baked from PE-extracted flour without added shortening. Glycolipids improved loaf volume the most, both with and without shortening. Phospholipid had no effect in the absence of shortening but increased loaf volume substantially with shortening in the formulation. In order to determine which components of the nonpolar fraction are responsible for the deleterious effects in baking, De Stefanis and Ponte (1976) separated steryl esters, triglycerides, diglycerides, and free fatty acids from the mixture and measured the effects of each on loaf volume. Three dough systems were compared, flour that had been defatted with PE and no lard added, intact flour with no lard added, and intact flour with 3.0% lard. The results, summarized in Table VII, indicated that the free fatty acids were the compounds responsible for reduction in loaf volume, although the effect was noticeably masked in the system that included lard. In order to identify more closely the type of fatty acid responsible for the decrease in loaf volume, the unsaturated linoleic acid (which constituted about 66% of the free fatty acids in flour lipid) was compared with the saturated palmitic acid at two different levels of addition. The results, also included in Table VII, clearly show the unsaturated fatty acids to be mainly responsible for loaf volume reduction. MacRitchie (1977), using batch fractionation methods introduced by De Stefanis and Ponte (1969), separated flour lipid into five fractions and measured the effects of increasing additions to the parent defatted flour. Thin layer chromatograms of the five fractions are shown in Fig. 17, and corresponding curves for loaf volume as a function of lipid content are shown in Fig. 18. Fractions 1 and 2 are distinctly nonpolar in nature and fractions 3, 4, and 5 contain mainly polar components. The different effects of the two sets of fractions on loaf volume are striking. Results may be explained on the basis of beneficial effects of polar lipids and detrimental effects of free fatty acids. The steepest decrease in loaf volume per percentage of lipid occurs for fraction 2, which has a relatively higher proportion of free fatty acids than the others. Fraction 3 also contains a relatively strong band for free fatty acids. This fraction causes an initial depression of loaf volume, but at higher additions, the polar lipids that are present evidently nullify the effects of the fatty acids and cause loaf volume to increase.

256

F. MACRITCHIE

TABLE VII EFFECTS OF VARIOUS LIPID FRACTIONS ON LOAF VOLUME0 Loaf volume (cm3) Lipid system

Defatted flour, 0% lard

Intact flour, 0% lard

Intact flour, 3% lard

Control Nonpolar fraction (intact) Steryl esters Triglycerides Diglycerides Free fatty acids

2434 2684 2639 268 1 2561 2376

2606 2581 2590 2639 2610 2569

272 1 2647 2712 274 1 2704 2712

Control Palmitic Palmitic Linoleic Linoleic

2602 2512 2569 2458 2196

2639 2647 2598 2577 2278

2733 2733 2725 2782 2655

a

acid acid acid acid

(1X)b (3X) (1 X ) (3X)

From De Stefanis and Ponte (1976). x natural level in flour.

A

C D

E

W

1

2

3

6

5

BAKING QUALITY OF WHEAT FLOUR

257

b

a

P

I

I

I

0.5 1.0 1.5 Hydrolysate lipid (%I

I2(

1

2.0

Hydrolysate lipid (%)

FIG. 18. Loaf volume (a) and foam half-life (h) as a function of hydrolysate lipid content of flour (a) and of solution (h), for five lipid fractions separated from the flour. Fractions are those for which thin layer chromatograms are shown in Fig. 17. (A) 1; (m) 2; (0) 3; (0)4; (0)5 . From MacRitchie (1977).

In summary, recent work has provided a fairly coherent picture of the effects of flour lipids in baking. Many of the apparently conflicting results obtained in earlier studies may be rationalized in terms of the destruction of functional properties of flours by certain solvents, the interactions between different lipid components, and the unusual nature of the loaf volume-flour lipid content relation. For example, different solvents may extract the nonstarch lipid from a flour nearly completely (e.g., chloroform) or incompletely (PE). It can be deduced from Fig. 14 that the defatted flour responds very differently depending on the degree of completeness of extraction. The roles of different lipid compounds are now better understood. The glycolipids and phospholipids exert beneficial effects in baking. Free fatty acids (in particular the unsaturated ones) have decidedly deleterious effects, although in the presence of added shortening (saturated triglycerides) these effects are minimized. It seems possible that the shortening acts as a solvent for the fatty acids, which constitute roughly 10% of the total flour lipid. Other nonpolar components do not appear to contribute greatly to loaf volume depression. FIG. 17. Thin layer chromatograms of five lipid fractions obtained by selective elution from silica gel. Assignments of bands are (A) neutral components including triglycerides; (B) free fatty acids; (C) glycolipids; (D) digalactosyl diglycerides; (E) phospholipids. Band B was incorrectly assigned in original reference. From MacRitchie (1977).

258

F. MACRITCHIE

NO satisfactory explanation has been found for the variation of loaf volume with lipid content. There are a number of interesting results, however, on which we can base conjectures. Let us first consider the results illustrated in Fig. 15. It is known that passage through rollers as occurs in the molding of dough pieces can cause improvements in structure of the gluten matrix (Moss, 1974). It may, however, be more relevant to say that the sheeting step is effecting changes in the gas cell stability. This is supported by photographs of the dough (Fig. 16). The greater stability may result from an improvement in the gas cell size distribution (i.e., more uniformly sized bubbles) or alternatively by an alteration in the composition of the stabilizing film that must be adsorbed at the gas-aqueous interface surrounding the bubbles. It is this film that governs the stability of classic foams. It has also been found that foam-stabilizing effects of flour lipid fractions correlate well with their effects on loaf volume (MacRitchie, 1977); i.e., the nonpolar fractions act as destabilizers and the polar fractions as stabilizers (see Fig. 18). The initial depression of loaf volume on adding whole lipid to defatted flour apparently reflects a contribution from nonpolar lipids (especially fatty acids). This may occur as follows. The distribution coefficient between the bulk dough and the gas cell surface varies from one lipid component to another. Consequently, the composition of the adsorbed film will change as a function of the total amount of lipid present in the dough. At low levels of total lipid, less strongly adsorbed compounds such as the fatty acids will be present in the stabilizing film. As the amount of total lipid is increased, these lipids may be gradually displaced by the more strongly adsorbing polar lipids, thus enhancing the stability of the dispersed gas phase. The passage of dough through rollers in an intermediate molding step is also likely to facilitate desorption of the more weakly surface active components, therefore concentrating the more strongly active polar lipids at the interface. Some experimental results are consistent with this explanation. For example, a minimum is found in the loaf volume-lipid content curve followed by a steady increase as more lipid is added in the case of addition of a polar lipid fraction even when no intermediate molding step is included (MacRitchie, 1977). However, when the added lipid contains larger proportions of nonpolar lipids (as in unfractionated lipid, Fig. 15), no minimum is readily observed unless the intermediate molding step is incorporated. Despite the sensitivity of baking parameters to changes in amounts and types of lipid, a number of studies have revealed that differences in baking quality of flours usually cannot be ascribed to differences in the lipid. Although varietal, seasonal, and environmental variations occur in lipid content and composition, no correlations that could be sustained over several crop years have been found (Fisher et al., 1966).

BAKING QUALITY OF WHEAT FLOUR

2.

259

Cookies

Deleterious effects are also produced on cookie quality by extraction of flour lipid with WSB (Cole et al., 1960) and PE (Kissell et al., 1971). Cookie diameter (spread) and top-grain score are seriously reduced. Restoration of extracted lipids usually leads to complete recovery of quality. Kissell et al. (1971) studied the effects on cookie quality of adding flour lipids to defatted flours. Cookie spread and top grain score increased with increasing lipid addition up to four times the natural level. Improvements were obtained with half the natural level, and with the natural level or double, cookies were equal to or better than the controls. Additions of polar and nonpolar fractions of flour lipid only partially restored baking potential, although top grain was noticeably better with the polar fraction. With the two fractions combined, cookies identical to the original were obtained. No varietal differences in lipid composition were shown by interchanging lipid between four flours of different quality. The variations in quality therefore resided in the base flour. Cole et al. (1960) obtained similar results. However, they showed that complete recovery of cookie properties could be obtained by addition of a phosphorus-containing fraction of the lipid to the flour. A phosphorusfree lipid fraction, on the other hand, gave no improvement. These results tend to agree with a recent detailed study by Clements and Donelson (1981a). To determine the source of functionality, the free flour lipids were separated quantitatively by TLC into 10 fractions. Two fractions corresponding to DGD (plus PC) and MGD gave the highest degrees of restoration. Pure commercial samples of DGD added alone at 0.10% by flour weight and PC added at 0.05% gave essentially complete recovery of original properties. However, MGD at levels up to 0.15% gave little response. There appears to be a good parallel between these results and the effect of lipids in bread baking described previously. In both cases, removal of lipid from flour causes deterioration in quality. Similarly, the polar glycolipids and phospholipids are found to be functionally beneficial, and the effects of lipids can be related to the gas cell structure and how it responds during baking. Most of the work on cookies has involved removal of only the free lipids. Whether extraction of the remainder of the nonstarch lipid would cause further deterioration or lead to an improvement as for bread is not clear, although the results of Cole et al. (1960) using WSB do not suggest that there would be an improvement. As well as comparing two different baked products, it must be remembered that the cookie formulation has an appreciable amount of shortening, whereas the effects for bread were obtained in the absence of shortening. This work has served to accentuate the role that lipids play in stabilizing the gas cell structure in baked products. Yamazaki and Donelson (1976) have drawn

260

F. MACRITCHIE

attention to the importance of internal appearance of cookies and have introduced a system of scoring for this parameter. Clements (1980) devised embedding and staining procedures that permit objective demonstrations of the deleterious effects of lipid removal on the internal appearance of cookies. Vertical cross sections showed how cell walls break down during oven expansion and cause coalescence and, in the worst cases, formation of large pockets enclosed by a thin shell. The structure solidifies early in the baking process, resulting in limited spread and almost total absence of top grain. Collapse of cell structure was observed for all flours, although it was most noticeable for flours with weak gluten and high dough plasticity (e.g., eastern United States soft white wheats).

3 . Cakes A cake differs from bread and cookies in that it enters the baking stage as a batter. Soft wheat flours are generally chosen for cake manufacture. It has been customary to bleach these flours, usually with chlorine, to enhance their cakebaking quality, especially for high-ratio cakes. Alternative methods for improving performance of cake flours by heat treatment (Doe and Russo, 1970) or air drying (Johnson et a l . , 1979) have also been suggested. Many different formulations exist for cake making. Some recent experiments with full-formula layer cakes will be described here. The conclusions are thought to be of general applicability as the lipids appear to be involved in foam formation and stabilization, a process that is basic to the development of cake structure (Clements and Donelson, 1982). Experiments in which lipids were interchanged between flours that had been chlorinated at different levels have shown that chlorination functionally modifies both the extracted lipid and the base flour (Kissell et al., 1979). This is in contrast to the conclusions of Spies and Kirleis (1978) and Johnson ef al. (1979). Improvement of cake flours has also been achieved by exposure to moving air, referred to as aging (Clements and Donelson, 1982). The degree of oven expansion depends on the flour as well as the length of exposure to air. Because the final volumes of cakes do not always provide an indication of the amount of oven expansion because of variable degrees of collapse, Clements and Donelson (198 lb) devised a method for measuring oven expansion of cakes. Using this technique, interchange experiments showed that the degree of oven expansion for both chlorinated and aged flours is a function of the lipid component. However, the degree to which oven expansion can be translated into volume depends on the nonlipid flour components (base). Expansion of an unchlorinated base often results in a cake structure that cannot support the potential volume (Kissell et al., 1979), whereas expansion generated on a bleached base is usually retained. This effect is shown in Fig. 19.

BAKING QUALITY OF WHEAT FLOUR

26 1

FIG. 19. Cross sections of cakes prepared from flours that had been sequentially chlorinated, defatted, and reconstituted with interchange of lipids and bases in all possible combinations. Numbers below are volumes in cubic centimeters (left) and grain score.

C.

WATER SOLUBLES

The aqueous supernatant that is separated from starch after gluten washing contains a mixture of water-soluble compounds. Schemes for fractionating these compounds have been discussed in Section 111. The role of water-soluble components has been studied by Hoseney et a[. (1969b). The membrane-permeable dialyzate (see Fig. 4) contributes to gas formation, but this can be replaced by synthetic yeast food. Neither the globulin nor albumin fractions were essential to produce normal bread. The residual supernatant that contains water-soluble pentosans and glycoproteins contributed to gluten extensibility and gas retention. Interchange experiments have generally shown that the water-soluble fraction does not account for variations in baking quality of normal flours (MacRitchie, 1978; Booth and Melvin, 1979). It is usually found that the water-soluble fraction may be omitted from reconstituted flours without detrimental effects on baking, providing steps are taken to ensure that gas production is not limited.

262

F. MACRITCHIE

D.

STARCH

Despite it being by far the most abundant component in wheat flour, starch has rarely been implicated in variations of quality between flours. Nevertheless, it cannot be considered simply as an inert filler (Myhre, 1970) as may be shown by attempting to replace starch granules by particles of different characteristics. It does appear, however, that replacement of wheat starch by starches from other cereals gives baked loaves of comparable characteristics, indicating that wheat starch is not unique in its baking properties (Hoseney et al., 197 1). Apart from this, starch plays two main roles in its effects on baking quality. First, the amount of starch that has been damaged during milling influences the water absorption capacity of a flour (see Section 11). A high water absorption is desirable for breadmaking, and this is provided by an adequate amount of damaged starch granules. Hard wheats, which have moderately high starch damage, are therefore suitable for breadmaking, whereas soft wheats are sought for cookie manufacture. Moss et al. (1973) report that the most useful test for soft flour quality of those commonly used is the water absorption measured by the farinograph. The second role for starch is in crumb firming, the major factor in staling and loss of shelf life in baked products, especially bread. When bread is baked, the high oven temperature causes starch granules to swell, and a melting phenomenon (gelatinization) begins to occur above temperatures of about 60°C. On cooling, the polymeric starch (or more particularly the amylopectin component) slowly crystallizes (retrogrades), and this process causes crumb firming. Evidently because of their high concentration, granules tend to fuse, forming a network structure. Crystals have far greater strength than amorphous regions, and this produces the slow increase in rigidity of the crumb. Aspects of staling are treated in recent articles (MacRitchie, 1980b; D’Appolonia and Morad, 1981).

VI.

THEORIES OF BAKING QUALITY

Research into the basis of baking quality has been undertaken with the belief that a general explanation of baking quality in terms of flour components at a molecular level is feasible. Apart from variations in quality due to environmental conditions and flour treatments, there has thus been a belief that there is an inherent factor (or factors) in flour components that determines whether a given flour performs well or not. As a consequence, a number of hypotheses have been proposed.

BAKING QUALITY OF WHEAT FLOUR

A.

263

PROTEIN-LIPID INTERACTION

As described in Section V, both gluten protein and lipid can affect baking quality as shown by response to variations of content and composition of either of these two components. Based on the unusual nature of the loaf volume-lipid content curve and the effects of altering the polar to nonpolar lipid ratio (PL:NL) (see Fig. 14), it is tempting to predict that variations in bread baking performance could be explained in terms of variations in lipid content or ratio of polar to nonpolar lipid in flours. MacRitchie (1978) studied pairs of flours, each pair having similar characteristics (protein content, starch damage level, etc.) except that one performed well and the other poorly in a baking test in which no shortening was added. Measurements of loaf volumes were made as a function of lipid content using defatted flours reconstituted with varying amounts of flour lipid. Several kinds of typical behavior were found, which are illustrated by the curves of Fig. 20. Loaf volumes for the poorer quality flours were below those of their paired samples over ranges of lipid content that varied from a small region near the natural lipid content of the flours to the complete range. Interchange experiments showed that the differences were caused not by lipid but by variations in the gluten protein of the flours.

A.

FLOUR L I P I D CONTEhT FIG. 20. Loaf volume-flour lipid content curves, which illustrate several types of general behavior found for wheat flours of differing baking performance. Arrow represents the natural flour lipid content. From MacRitchie ( 1 9 8 0 ~ ) .

264

F. MACRITCHIE

Obviously, there is an interaction between the lipid and protein in order to produce the variations in the curves from one flour to another. This does not necessarily imply that a chemical association between the two components i s responsible for the observed behavior. It seems possible that lipid and protein act independently to a certain extent; i.e., loaf volume varies with lipid content according to the general form of the curves of Fig. 20 and the effect of gluten protein quality is superimposed at each lipid level. At certain lipid levels (especially in the vicinity of the natural level in flour), differences in protein quality appear to show up more than at others. Effects of dough formation in reducing extractability of lipids (lipid binding) have been studied by a number of workers (Wootton, 1966; Daniels, 1975). The results are relevant, but no firm relationship between the extent of lipid binding and flour performance in baking has yet been established. Recent studies (Chung et al., 1980, 1982) have provoked interest in the contributions of lipids. Measurements were carried out on the lipid extracted by different solvents from 21 samples of hard red winter (HRW) wheat flours that varied in breadmaking quality. Linear significant correlations were found between loaf volume and PL, PL:NL ratio, and lipid galactose content (GAL). A comparison of the loaf volume-NL:PL plots for five HRW wheat flours using six solvent systems showed that PE gave the steepest slope and thus differentiated best between the flours (Chung et af., 1980). With WSB, which presumably extracts all the nonstarch lipid, the slope was zero, confirming that there were no differences in the total lipid between the flours. Because PE extracts more of the polar lipids from the superior flours, it may be inferred that these lipids are bound less strongly than in the flours of inferior baking performance. Factors governing protein-lipid associations in wheat flour have been discussed by Pomeranz (1973) and MacRitchie (1980~).It will be interesting to see if these preliminary correlations of Chung et al. can be extended to a greater range of flours. Although differences in lipid content and composition can be potentially a source of differences in baking performance, the weight of evidence at present suggests that gluten protein quality is invariably the controlling factor. As a result, much emphasis is currently being given to establishing the molecular parameters of gluten protein that are responsible. There are three main ways in which variations in the protein complex may occur. These are differences in molecular weight distribution, chemical composition, and molecular structure, although the three parameters may be interrelated. Two protein molecules may have the identical amino acid composition, but variations in the sequence could have great effects on both the structure and the molecular weight. The positions of proline residues, which allow protein chains to undergo changes in direction, and the positions of cystine side chains, which produce intra- or intermolecular bonds, are just two examples.

BAKING QUALITY OF WHEAT FLOUR

B.

265

MOLECULAR WEIGHT DISTRIBUTION

The effects of differences in molecular weight distribution of the gluten proteins on dough properties and baking behavior have been discussed in Section V,A. A number of independent studies have found a relationship between the amount of protein soluble under given conditions and breadmaking quality. Pomeranz (1968) showed that the amount of flour protein dispersible in 3 M urea decreased as flour quality improved, and this has been confirmed by Chung et al. (1979). Other work has shown that more protein that is soluble in dilute acetic acid solution can be obtained from poor than from good quality flours (Huebner and Wall, 1976; Orth and Bushuk, 1972), and this has formed the basis for the early generation protein residue test, discussed in Section 11. These results probably reflect molecular weight distribution differences between the flour proteins in view of the well-known relation between solubility and ’ molecular weight of polymers. This means that flours having strong dough characteristics and good breadmaking properties would have their protein molecular weights more biased toward the high molecular weight end of the spectrum (see Fig. 13). It must be remembered, however, that other factors (e.g., amino acid content and sequence) also influence solubility. The insoluble residue protein has often been referred to as gel protein. The importance, during dough mixing, of the breakdown of this protein has been discussed by Graveland ( 1980). The mechanism involves disulfide bond scission in the presence of oxygen (see MacRitchie, 1975). C.

GLUTEN PROTEIN AMIDE CONTENT

Glutamic acid is by far the most abundant amino acid in gluten protein. Together with aspartic acid, these two make up about 40% of the total protein by weight. The major portion of these amino acid residues are in the amidated form, i.e., present as glutamine and asparagine. It therefore seems reasonable to expect that the properties of gluten protein are largely governed by these residues. Deamidation results in drastic changes in gluten proteins, including increased solubility and reduced strength and cohesiveness (Beckwith et al., 1963). Lasztity (1980), in a study of the factors influencing rheological properties of gluten, found an optimum degree of amidation between 85 and 90% for gluten protein. Relaxation times were found to be a maximum in this region, falling off at lower and higher degrees of amidation. MacRitchie (1979) noted displacements in the solubility-pH curves, believed to be associated with differences in average isoelectric points, of gluten protein from flours of varying baking performance. This prompted a study of the amide contents of a range of flours. A positive correlation was shown between a loaf

266

F. MACRITCHIE

volume index and the percentage of (glutamine + asparagine) in the gluten protein from 37 Australian flours (Fig. 21). The latter figures varied between 3 1.2 and 33.7% by weight. A problem in the interpretation of these data is the uncertainty whether any bias has been introduced as a result of the gluten washing process. For example, gliadin is richer in amide content than glutenin; therefore, if for some reason more gliadin is lost to the aqueous phase during gluten washing of poorer flours, this could result in amide figures lower than the true values. Again, retention of larger amounts of nongluten protein (of much lower amide content) would have the same effect. Direct determinations of the amide nitrogen to protein nitrogen ratio of flours can also be ambiguous owing to the variable and unknown proportions of gluten and nongluten proteins. A similar study has been carried out by Ewart (1982) on 36 English wheat flours. Variations in gluten amide level were measured, but no correlation was found with baked loaf volume in this work. The absence of expected shifts in electrophoretic patterns of proteins due to amide differences and the lack of correlation between the gliadin to glutenin ratio and loaf volume also supported the conclusion. The question of whether gluten protein amide level contributes to baking performance therefore remains open, but this work draws attention to the need to include amide determinations in amino acid analyses of flour protein. Variations in the proportion of amidated residues also logically imply variations in the amounts of nonamidated acidic side chains. The significance of the proportion of non-amidated acidic side chains in contributing electrical charges

120

0

0

31.O

32.0

33.0

Glutamine + asparagine ( % in gluten protein ) FIG. 21. Loaf volume index (defined in Section II,B) vs percentage (glutamine + asparagine) in gluten protein samples from 37 flours of varying baking quality. From MacRitchie (1979).

BAKING QUALITY OF WHEAT FLOUR

267

to gluten protein, a protein noted for its lack of ionizable groups, has been stressed by MacRitchie (1980~).It is appropriate that Ewart (1981) recently discussed a method for measuring free carboxyl groups in wheat proteins.

D.

SULFUR-CONTAINING AMINO ACIDS

The sulfur-containing amino acids in gluten protein have attracted much interest in view of the great sensitivity of dough properties to additions of small quantities of oxidizing and reducing agents. Oxidants were believed to promote cross-linking of thiol groups causing the observed stiffening of dough. Reducing agents, on the other hand, break disulfide bonds and weaken doughs (see Section IV). Bloksma (1975) has drawn attention to the similar effects of thiol blocking agents to oxidizing agents in causing stiffening of doughs and, on this basis, has suggested that oxidants act by removing thiol groups. He also discusses the role of the thiol-disulfide interchange reaction in doughs and the contributions of disulfide bonds and thiol groups to elastic and viscous deformations of dough, respectively. Despite the obvious role that they play in dough, there is little evidence to date that baking quality can be related to variations in the contents of sulfur-containing amino acids. Graveland et al. (1978) determined the amounts of thiol groups and disulfide bonds in wheat flours with a range of baking quality. Levels of 5-7 kmol thiol groups and 11-18 pmol disulfide bonds per gram of flour were found, levels very much higher than those previously reported. The authors attribute this to the use of urea and SDS to unfold protein molecules and make reactive groups accessible. No correlation of the measurements with baking quality was observed. However, there is evidence that only small fractions of the analytically determined thiol groups and disulfide bonds are rheologically effective (Bloksma, 1975). Some recent studies have pointed to the deterioration in baking quality when soil sulfur levels are limiting. Moss et al. (1981) measured a number of quality parameters for a series of flours from wheat that had been grown under different regimes of nitrogen and sulfur fertilizer. Grain sulfur concentrations responded to both sulfur and nitrogen supply and varied from 0.08 to 0.18%. Flours of lower sulfur content gave doughs with excessively tough characteristics. Timms et al. (1981) found a deterioration in the expected baking quality of flours from wheat grown with high levels of nitrogen fertilizer under conditions that produced sulfur deficiency. Wrigley et d . (1980) and Timms et al. (1981) have shown that sulfur deficiency markedly alters the proportions of proteins in each of the classes, leading to an increase in the sulfur-deficient o-gliadin species as protein content is increased. As a result, there is a change in the nutritional quality owing to a reduction in some of the essential amino acids. This work has shown that baking quality is affected by limiting levels of sulfur. Whether

268

F. MACRITCHIE

differences in quality at high sulfur levels can be ascribed to variations in the sulfur-containing amino acids remains an open question. E.

GLIADIN COMPOSITION

An interesting approach to studying quality has been initiated by Wrigley et al. (1981). Electrophoretic patterns for gluten proteins were coded in numerical form to permit computer-based comparisons that could subsequently be used to search for correlations with quality. Initial work has concentrated on gliadins. and an example of the numerical assignment of band positions is shown for several wheat varieties in Fig. 22. Some 80 cultivars were examined for position and intensity of electrophoretic bands. Estimates of quality parameters such as grain hardness and dough strength were made by combining results of many years of testing. Some of the relationships found are expected to be due to similarities of pedigree between cultivars. Nevertheless, allowing for this, certain gliadin bands were found to be significantly associated with quality characteristics of grain hardness and dough strength.

F.

GLUTENIN SUBUNITS

Another very promising approach has involved study of the subunits of glutenin. Glutenin molecules consist of subunits of molecular weights varying up to about 140,000, which are connected together to form high molecular weight complexes. There is strong evidence that the connecting links are disulfide bonds, although noncovalent forces have also been implicated (Kasarda et al., 3 7 12 32

n

25

29

DURAMBA A PINNACLE TlNCURRlN

EGRET

1 4 8 1 1 15 18

20

33

24 28

31

2 l 3 l ~ l 3 l ~ l ~ l ~ l ~ l ~ l z l ~ l 2 l ~ l 3 l ~ l ~ l ~ l ~ l 4 l 3 l ~ ~ 3 l ~ l 4 l ~ l 4 l ~E G l R ~ ElT 3 l ~ l 2 l ~

FIG. 22. Example of the numerical assignment of electrophoretic bands of gliadins from different wheat varieties on the basis of position and intensity. From Wrigley et al. (1981).

BAKING QUALITY OF WHEAT FLOUR

269

1976). The subunit composition of glutenin varies according to the wheat variety (Bietz et al., 1975), and different combinations of subunits could conceivably account for differing properties of glutenin and consequently for breadmaking quality. A program to elucidate the structure of glutenin fractions in terms of their subunit composition is being pursued by Graveland et al. (1982). Among the glutenin subunits, a group of high molecular weight entities appears to be particularly associated with quality. This aspect has been investigated by Payne and co-workers (Payne et a l . , 1980, 1981). High molecular weight subunit composition was measured by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and numerical assignments given to the bands. Breadmaking quality was assessed by the SDS-sedimentation test (see Section 11) in which quality correlates positively with sedimentation volume. Some examples of the subunit compositions of cultivars are shown in Fig. 23 together with the distribution of SDS-sedimentation volumes for flours of progenies from six crosses. In this study (Payne et al., 1981) the presence of certain subunits was shown to be correlated with breadmaking quality. In particular, the presence of subunits 1, 5 , and 10, according to the nomenclature used, were beneficial whereas subunits 2, 3 , and 12 were not. Progress has been made in assigning the genetic coding of the high molecular weight subunits to specific chromosomes. The chromosome compositions of different wheats have been summarized by Wall (1979). The homoeologous group 1 chromosomes of wheat contain five major gene groupings that code for the high molecular weight subunits of glutenin. Subunit 1 is coded by chromosome lA, and subunits 5 and 10 are coded by chromosome 1D. A hypothesis, proposed by Payne et al. (1981) for the relative positions of the glutenin genes on group 1 chromosomes, is shown in Fig. 23.

VII.

CONCLUSIONS

A certain amount of positive knowledge about the complex phenomenon of baking quality of wheat flours has been acquired, and methods for further tackling the problem are now better understood. The belief of cereal chemists that quality can be related to individual chemical constituents has been vindicated, although much remains to be discovered. Studies of dough properties and mixing have greatly increased our knowledge of the requirements for successful breadmaking. Progress in improving fractionation and reconstitution of flour components has been made. Confirmation that gluten protein is the component that controls breadmaking quality has been obtained, and pointers to the specific fractions that are important have been uncovered. Gluten protein characterization and its relation to quality will assuredly continue to be the most active area in quality research. The role of lipids in baking of bread, cakes, and cookies is better understood.

Bounty x Flanders x Armarda

3

1

10

111111118

0

20 Sappo x Bounty x Flanders

10

I

0

20 vI

10 ; E

Lancota x Hobblt 10

IIIII

I2 %

O

g

20

0 c Q

8Hobbit,ox

10

Ranger

20 Maris Wldgeon x Hobbit 10

0

I

1 2 3 4 5 6 7 8 9 1 0 1 1 121314 Bad

SDS-Sedimentation Volume

4

LDng arm

Good

Shorl arm

Chromowme I A

Chromosome 18

Chromosome I D

M 8 ) \clusters

gme

X

I

FIG. 2 3 . Correlations of high molecular weight subunits of glutenin with the SDS-sedimentation volume (Section II,D) for flours from several wheat varieties. Included is a proposal for the relative positions of the glutenin genes on group I chromosomes. From Payne et al. (1981).

BAKING QUALITY OF WHEAT FLOUR

27 1

Two parameters, namely a high content and a high PL:NL ratio of the nonstarch lipid, are desirable for good breadmaking. A spinoff from the work on lipids has been the attention drawn to the importance of crumb structure of bread and cakes and the internal structure of cookies. More intensive study of cell structure and how it is controlled appears likely in the future. To date, differences in baking quality have not been shown to be normally associated with lipid content and composition. Nevertheless, variations have been noted. These variations have not been used as a basis for selection in plant breeding but would be worth considering. In this connection, work on the distribution of lipids in the different regions (germ, aleurone, and endosperm) of the wheat grain (Hargin and Morrison, 1980) is particularly relevant. In proportion to its quantity in flour, crtrbohydrate, particularly starch, has been a comparatively neglected component. A fuller recognition of freshness retention as a quality parameter in baked products appears necessary in view of the relatively large food losses associated with limited shelf life. This would encourage greater interest in studies of starches from different varieties with the aim of breeding for retardation of the staling mechanism. A particularly fascinating stage is currently being reached in research into wheat flour quality. A number of independent approaches are being used. Fractionation and reconstitution with interchange or variation of amounts of components and their fractions, discussed in Section 111 and IV, is a laborious procedure but, if carried out properly. has the advantage of providing direct answers. Another type of approach, discussed in Section VI, attempts to correlate assessments of quality with independent measurements of molecular parameters. This is typified by the work of Wrigley et al. (1981) and Payne et al. (1981). The great benefit to be expected from this work is that it will be possible not only to identify “quality” components but also to relate them to specific chromosomes or gene groupings. Once they are identified, the possibility opens for concentrating the “quality” components by conventional breeding methods, thus producing improved new wheat varieties. Future research is seen as further development of these various methods to their full potential. Because the problem of flour baking quality is such a broad one, a multidisciplinary approach is essential. The different lines of attack should be complementary, and there are signs that more cooperation between scientists studying flour quality is occurring. The comparison of international methods for the identification of protein electrophoretograms (Autran et al., 1979) is just one of many examples. Some of this cooperation has resulted from efforts of the ICC. There appears to be a need to provide greater encouragement in this direction. Many centers of excellence exist in different countries, some with expertise in analytical work, others in baking technology, others in molecular characterization, etc. More efficient research would almost certainly result from a greater coordination of efforts, without detracting from the individualism of the different scientists and research groups.

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REFERENCES American Association of Cereal Chemists. 1975. Rheology of wheat products. Cereal Chem. 52, Ir183r. Autran, J. C . , Bushuk, W . , Wrigley, C. W . , and Zillman, R. R. 1979. Wheat cultivar identification by gliadin electrophoregrams. IV. Comparison of international methods. Cereal Foods World 24,47 1-475. Axford, D. W. E., Chamberlain, N., Collins, T. H., and Elton, G . A . H. 1963. Continuous breadmaking: The Chorleywood Process. Cereal Sci. Today 5(8), 265-270. Baldo, B. A., and Wrigley, C. W. 1978. IgE antibodies to wheat flour components. Clin. Allergv 8, 109- 124. Beckwith, A. C., Wall, J. S . , and Dimler, R. J. 1963. Amide groups as interaction sites in wheat gluten proteins: Effects of amide-ester conversion. Arch. Biochem. Bioghys. 103, 3 19-330. Bietz, J. A., Shepherd, K. W., and Wall, J. S. 1975. Single kernel analysis of glutenin: Use in wheat genetics and breeding. Cereal Chem. 52, 513-532. Blackman, J. A., and Gill, A. A. 1980. A comparison of some small-scale tests for breadmaking quality used in wheat breeding. J. Agric. Sci. 95, 29-34. Bloksma, A. H. 1975. Thiol and disulfide groups in dough rheology. Cereal Chem. 52, 170r-180r. Booth, M. R., and Melvin, M. A. 1979. Factors responsible for the poor breadmaking quality of high yielding European wheat. J. Sci. Food Agric. 30, 1057-1064. Bushuk, W. 1979. Development of new varieties. Cereal Foods World 24, 588-591. Bushuk, W., and Wrigley, C. W. 1971. Glutenin in developing wheat grain. Cereal Chem. 48,448455. Calvel, R. 1979. The influence of mixing on the development of bread making and quality of bread. Parts 1, 2 and 3 . Boulanger-Patissier 48, Sept. 30-31, 31-32, Oct. 25-26. Chung, 0. K . , Pomeranz, Y., Finney, K . F., and Shogren, M. D. 1978a. Defatted and reconstituted wheat flours. 111. Effects of flour moisture content and aqueous binary azeotropes on functional (breadmaking) properties. Cereal Chem. 55, 31-43. Chung, 0. K . , Pomeranz, Y., Finney, K. F., and Shogren, M. D. 1978b. Surfactants as replacements for natural lipids in bread baked from defatted wheat flour. J . Am. Oil Chem. Soc. 55, 635-641. Chung, 0. K . , Pomeranz, Y., Hwang, E. C . , and Dikeman, E. 1979. Defatted and reconstituted wheat flours. IV. Effects of flour lipids on protein extractability from flours that vary in breadmaking quality. Cereal Chem. 56, 220-226. Chung, 0. K., Pomeranz, Y., Jacobs, R. M., and Howard, B. G. 1980. Lipid extraction conditions to differentiate among hard red winter wheats that vary in breadmaking. J . Food Sci. 45, 11681174. Chung, 0. K., Pomeranz, Y., and Finney, K. F. 1982. Relations of polar lipid content to mixing requirements and loaf volume potential of hard red winter wheat flour. Cereal Chem. 59, 1420. Clements, R. L. 1980. Note on the effect of removal of free flour lipids on the internal structure of cookies as observed by a resin-embedding method. Cereal Chem. 57, 445-446. Clements, R. L., and Donelson, J. R. 1981a. Functionality of specific flour lipids in cookies. Cereal Chem. 58, 204-206. Clements, R. L., and Donelson, J. R. 1981b. A simple device for monitoring batter expansion in layer cakes during baking. Cereal Chem. 58, 153-154. Clements, R. L., and Donelson, J . R. 1982. Role of free flour lipids in oven expansion in layer cakes. I . Effects of aging. Cereal Chem. Cole, E. W., Mecham, D. K., and Pence, J. W. 1960. Effect of flour lipid and some lipid derivatives on cookie-baking characteristics of lipid-free flours. Cereal Chem. 37, 109-121.

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Daftary, R. D., Pomeranz, Y., Shogren, M., and Finney, K . F. 1968. Functional breadmaking properties of lipids. 11. The role of flour lipid fractions in breadmaking. Food Technol. 22, 7982. Daniels, D. G. H., and Fisher, N. 1976. Release of carbon dioxide from dough during baking. J . Sci. Food Agric. 27, 351-357. Daniels, N. W. R. 1975. Some effects of water in wheat flour doughs. In “Water Relations of Food” (R. B. Duckworth, ed.), pp. 573-586. Academic Press, New York. Daniels, N. W. R., and Frazier, P. J. 1978. Wheat proteins-physical properties and baking function. Proc. Easter Sch. Agric. Sci. (Univ. Nottingham) 26, 299. Daniels, N. W. R., Richmond, J. W., Russell Eggitt, P. W., and Coppock, J. 6 . M. 1966. Studies on the lipids of flour. 111. Lipid binding in breadmaking. J. Sci. Food Agric. 17, 20-29. D’Appolonia, €3. L., and Morad, M. M. 1981. Bread staling. Cereal Chem. 58, 186-190. Davies, J. T., and Rideal, E. K. 1963. “Interfacial Phenomena,” 2nd ed., p. 9. Academic Press, New York. De Stefanis, V. A,, and Ponte, J. G. 1969. Simple method for isolation of crude mono- and digalactosyl diglycerides from wheat flour. Biochim. Biophys. Acta 176, 198-201. De Stefanis, V. A., and Ponte, J. G. 1976. Studies on the breadmaking properties of wheat-flour nonpolar lipids. Cereal Chem. 53, 636-642. Doe, C. A. F., and Russo, J. V. B. 1970. Heat treatment of flour as an alternative to chlorination. J. Food Technol. 5, 363-374. Ewart, J. A. D. 1981. Determination of carboxyl groups in wheat and barley flour proteins. J. Sci. Food Agric. 32, 572-578. Ewart, 1. A. D. 1982. Baking quality and gluten amide level. J. Food Technol. 17, 365-372. Feldman, M., and Sears, E. R. 1981. The wild gene resources of wheat. Sci. Am. 244, 98-109. Fifield, C. C., Weaver, R.. and Hayes, J. F. 1950. Bread loaf volume and protein content of hard red spring wheats. Cereal Chem. 27, 383-390. Finney, K. F. 1943. Fractionating and reconstituting techniques as tools in wheat flour research. Cereaf Chem. 20, 381-396. Finney, K. F. 1978. Contribution of individual chemical constituents to the functional (breadmaking) properties of wheat. In “Cereals ’78. Better Nutrition for the World’s Millions” (Y. Pomeranz, ed.), pp. 139-158. Am. Assoc. Cereal Chem., St. Paul, Minnesota. Finney, K. F. 1979. “Wheat Proteins. What they Do. Wheat Protein Conference,” pp. 50-61. U.S. Department of Agriculture, Peoria. Finney, K. F., and Barmore, M. A. 1948. Loaf volume and protein content of hard winter and spring wheats. Cereal Chem. 25, 291-311. Finney, K. F., Moms, V. H., and Yamazaki, W. T. 1950. Micro versus macro cookie baking procedures for evaluating the cookie quality of wheat varieties. Cereal Chem. 27, 42-49. Finney, K . F., Pomeranz, Y., and Hoseney, R. C. 1976. Effects of solvent extraction on lipid composition, mixing time and bread loaf volume. Cereal Chem. 53, 383-388. Fisher, N.. Bell, B. M., Rawlings, E. R., and Bennett, R. 1966. The lipids of wheat. 111. Further studies of the lipids of flours from single wheat varieties of widely varying quality. J . Sci. Food Agric. 17, 370-382. Flory, P. J . 1953. “Principles of Polymer Chemistry,” p. 559. Cornell Univ. Press, Ithaca, New York. Frazier, P. J., Daniels, N. W. R., and Russell Eggett, P. W. 1975. Rheology and the continous breadmaking process. Cereal Chem. 52, 106r- 130r. Goforth, D. R., Finney, K. F., Hoseney, R. C., and Shogren, M. D. 1977. Effect of strength and concentration of acid on the functional properties of solubilized glutens of good- and poorquality bread flours. Cereal Chem. 54, 1249-1258. Gras, P. W., and MacRitchie, F. 1973. An improved method of volume measurement for small loaves. Cereal Sci. Today 18, 135-137.

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Graveland, A. 1980. Ann. Technol. Agric. 29, 113-123. Graveland, A., Bosveld, P., and Marseille, J. P. 1978. Determination of thiol groups and disulfide bonds in wheat flour and dough. J . Sci. Food Agric. 29, 53-61. Graveland, A., Bosveld, P., Lichtendonk, W. J., Moonen, H. H. E., and Scheepstra, A . 1982. Extraction and fractionation of wheat flour proteins. J . Sci. Food Agric. 33, 1117-1 128. Hamada, A. S . , McDonald, C. E., and Sibbitt, L. D. 1982. Relationship of protein fractions of spring wheat flour to baking quality. Cereal Chem. 59, 296-301. Hargin, K. D., and Morrison, W. R. 1980. The distribution of acyl lipids in the germ, aleurone, starch and non-starch endosperm of four wheat varieties. J . Sci. Food Agric. 31, 877-888. Heaps, P. W . , Webb, T., Russell Eggitt, P. W., and Coppock, J. B. M. 1967. Studies on mechanical factors affecting dough development. J . Food Technol. 2, 37-43. Hoseney, R. C., Finney, K. F., Shogren, M. D., and Pomeranz, Y . 1969a. Functional (breadmaking) and biochemical properties of wheat flour components. 111. Characterization of gluten protein fractions obtained by ultracentrifugation. Cereal Chem. 46, 126- 135. Hoseney, R. C., Finney, K. F., Shogren, M. D., and Pomeranz, Y. 1969b. Functional (breadmaking) and biochemical properties of wheat flour components. 11. Role of water-solubles. Cereal Chem. 46, 117-125. Hoseney, R. C., Finney, K. F., Pomeranz, Y . , and Shogren, M. D. 1969c. Functional (breadmaking) and biochemical properties of wheat flour components. V. Role of total extractable lipids. Cereal Chem. 46, 606-613. Hoseney, R. C., Finney, K. F., Pomeranz, Y . ,and Shogren, M. D. 1971. Functional (breadmaking) and biochemical properties of wheat flour components. VIII. Starch. Cereal Chem. 48, 191201. Huebner, F. R., and Wall, J. S. 1976. Fractionation and quantitative differences of glutenins from wheat varieties varying in baking quality. Cereal Chem. 53, 258-268. Irvine, G. N., and McMullan, M. E. 1960. The “Premix” baking test. Cereal Chem. 37, 603-613. Jackel, S. S. 1979. “The Baker’s View. Wheat Protein Conference,” pp. 1-10. U.S. Department of Agriculture, Peoria, Illinois. Johnson, A. C . , Hoseney, R. C . , and Vamano-Marston, E. 1979. Chlorine treatment of cake flours. I. Effect of lipids. Cereal Chem. 56, 333-335. Kasarda, D. D., Bemardin, J. E., and Nimmo, C. C. 1976. Wheat proteins. Adv. Cereal Sci. Technol. 1, 158-236. Kilborn, R. H., and Tipples, K. H. 1972. Factors affecting mechanical dough development. I. Effect of mixing intensity and work input. Cereal Chem. 49, 34-47. Kilborn, R. H., and Tipples, K . H. 1973a. Factors affecting mechanical dough development. 111. Mechanical efficiency of laboratory dough mixers. Cereal Chem. 50, 50-69. Kilborn, R. H., and Tipples, K. H. 1973b. Factors affecting mechanical dough development. IV. Effect of cysteine. Cereal Chem. 50, 70-86. Kilborn, R. H., and Tipples, K. H. 1974. Implications of the mechanical development of bread dough by means of sheeting rolls. Cereal Chem. 51, 648-657. Kilborn, R. H., and Tipples, K. H. 1979. The effect of oxidation and intermediate proof on work requirements for optimum short-process bread. Cereal Chem. 56, 407-412. Kilbom, R. H., Tweed, A. R., and Tipples, K. H. 1981. Dough development and baking studies using a pilot scale dough brake. Baker’s Dig. 55, 18-31. Kim, S. K., and D’Appolonia, B. L. 1977. Bread staling studies I. Effect of protein content on staling rate and bread crumb pasting properties. Cereal Chem. 54, 207-215. Kissell, L. T . , Pomeranz, Y . , and Yamazaki, W. T. 1971. Effects of flour lipids on cookie quality. Cereal Chem. 48, 655-662. Kissell, L. T., Donelson, 1. R., and Clements, R. L. 1979. Functionality in white layer cake of lipids from untreated and chlorinated patent flours. I. Effects of free lipids. Cereal Chem. 56, I I- 14.

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Koenig, V . L., Ogrins, A., Trimho, H. B., and Miller, B. S. 1964. The electrophoretic analysis of flour from several varieties of hard red winter wheat grown at several locations. J . Sci. Food Agric. 15, 492-497. Lasztity, R. 1980. Correlation between chemical structure and rheological properties of gluten. Ann. Technol. Agric. 29, 339-361. Lee, J. W., and MacRitchie, F. 1971. The effect of gluten protein fractions on dough properties. Cereal Chem. 48, 620-625. Lee, J. W., and Wrigley, C. W. 1963. The protein composition of gluten extracted from different wheats. Ausr. J . Exp. Agric. Anim. Husb. 3, 85-88. McGrew, F. C. 1958. Structure of synthetic high polymers. J . Chem. Educ. 35, 178-186. MacMurray, T. A , , and Momson, W. R. 1970. Composition of wheat-flour lipids. J . Sci. Food Agric. 21, 520-528. MacRitchie, F. 1973. Conversion of a weak flour to a strong one by increasing the proportion of its high molecular weight gluten protein. J . Sci. Food Agric. 24, 1325-1329. MacRitchie, F. 1975. Mechanical degradation of gluten proteins during high speed mixing of doughs. J . Polym. Sci. Polym. Symp. 49, 85-90. MacRitchie, F. 1976. The liquid phase of dough and its role in baking. Cereal Chem. 53, 3 18-326. MacRitchie, F. 1977. Flour lipids and their effects in baking. J . Sci. Food Agric. 28, 53-58. MacRitchie, F. 1978. Baking quality of wheat flours. J . Food Technol. 13, 187-194. MacRitchie, F. 1979. A relation between gluten protein amide content and baking performance of wheat flours. J . Food Technol. 14, 595-601. MacRitchie, F. 1980a. Studies of gluten protein from wheat flours. Cereal Foods World 25, 382385. MacRitchie, F. 1980b. Physicochemical aspects of some problems in wheat research. Adv. Cereal Sci. Technol. 3, 271-326. MacRitchie, F. 1980c. Protein-lipid interaction in baking quality of wheat flours. Baker’s Dig. 54, 10-13. MacRitchie, F. 1981, Flour lipids: Theoretical aspects and functional properties. Cereal Chem. 58,15. MacRitchie, F., and Gras, P. W. 1973. The role of flour lipids in baking. Cereal Chem. 50, 292302. Maleki, M., Hoseney, R. C., and Mattern, P. J. 1980. Effects of loaf volume, moisture content and protein quality on the softness and staling rate of bread. Cereal Chem. 57, 138-140. Marchylo, R., and Kruger, J. E. 1978. A sensitive automated method for the determination of aamylase in wheat flour. Cereal Chem. 55, 188-196. Mathewson, P. R., and Pomeranz, Y . 1979. Modified chromatogenic a-amylase assay for sprouted wheat. J . Assoc. Off. Anal. Chem. 62, 198-200. Meredith, P. 1966. Dependence of water absorption of wheat flour on protein content and degree of starch granule damage. N . Z . J . Sci. 9, 324. Mitchell, T. A. 1977. Test baking systems-comparisons and philosophies. Food Technol. N . Z . , Jan. 12-15. Momson, W. R., Siew, L. T., and Hargin, K. D. 1980. Methods for the quantitative analysis of lipids in cereal grains and similar tissues. J . Sci. Food Agric. 31, 329-340. Moss, H. J. 1961. Milling damage and quality evaluation of wheat. Ausr. J . Exp. Agric. Anim. Husb. 1, 133-139. Moss, H. J. 1980. Strength requirements of doughs destined for repeated sheeting compared with those of normal doughs. Cereal Chem. 57, 195-197. Moss, H. J . , Wrigley, C. W., MacRitchie, F., and Randall, P. J. 1981. Sulfur and nitrogen fertilizer effects on wheat. 11. Influence on grain quality. Aust. J . Agric. Res. 32, 213-226. Moss, R. 1974. Dough microstructure as affected by the addition of cysteine, potassium bromate and ascorbic acid. Cereal Sci. Today 19, 557-561.

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Moss, R., Stenvert, N. L., Pointing, G . , Worthington, G., and Bond, E. E. 1979. The timedependent interaction of oxidizing and reducing agents in breadmaking. Baker’s Dig. 53, 1012, 14, 16-17. Myhre, D. V . 1970. The function of carbohydrates in baking. Baker’s Dig. 44(3), 32-39. O’Brien, L., and Orth, R. A. 1977. Effect of geographic location of growth on wheat milling yield, farinograph properties, flour protein and residue protein. Aust. J . Agric. Res. 28, 5-9. Orth, R. A., and Bushuk, W. 1972. A comparative study of the proteins of wheats of diverse baking qualities. Cereal Chem. 49, 268-275. Orth, R. A . , and O’Brien, L. 1976. A new biochemical test of dough strength of wheat flour. J . Aust. Inst. Agric. Sci. 42, 122-124. Orth, R. A., O’Brien, L., and Jardine, R. 1976. A factor analysis of bread wheat quality tests. Aust. J . Agric. Res. 27, 575-582. Osborne, T. B. 1907. “The Proteins of the Wheat Kernel,” Publ. No. 84. Carnegie Institute, Washington, D.C. PAC 1976. Protein methods for cereal breeders as related to human nutritional requirements. Adv. Cereal Sci. Technol. 1, 378-406. Payne, P. I., Harris, P. A., Law, C. N., Holt, L. M., and Blackman, J. A. 1980. The highmolecular-weight subunits of glutenin: Structure, genetics and relationship to bread-making quality. Ann. Technol. Agric. 29, 309-320. Payne, P. I., Corfield, K. G., Holt, L. M., and Blackman, J . A . 1981. Correlations between the inheritance of certain high-molecular weight subunits of glutenin and bread-making quality in progenies of six crosses of bread wheat. J . Sci. Food Agric. 32, 51-60. Pelshenke, P. 1933. A short method for the determination of gluten quality of wheat. Cereal Chem. 10, 90-96. Peterson, R. F. 1965. “Wheat,” pp. 177-197. Wiley (Interscience), New York. Pomeranz, Y. 1968. Relation between chemical composition and breadmaking potentialities of wheat flour. Adv. Food Res. 16, 335-455. Pomeranz, Y. 1973. Interactions between glycolipids and wheat flour macromolecules in breadmaking. Adv. Food Res. 20, 153-188. Pomeranz, Y. 1980. What? How much? Where? What function? in breadmaking. Cereal Foods World 25, 656-662. Ponte, J . G., Jr. 1971. Bread. In “Wheat ChemistryandTechnology” (Y. Pomeranz, ed.), pp. 675735. Am. Assoc. Cereal Chem., St. Paul, Minnesota. Ponte, J. G., Jr., and De Stefanis, V. A. 1969. Note on the separation and baking properties of polar and nonpolar wheat flour lipids. Cereal Chem. 46, 325-329. Popper, K. R. 1963. “Conjectures and Refutations,” p. 37. Routledge & Kegan Paul, London. Preston, K. R., and Tipples, K. H. 1980. Effects of acid-soluble and acid-insoluble gluten proteins on the rheological and baking properties of wheat flours. Cereal Chem. 57, 314-320. Redman, D. G. 1979. A simple wholemeal sedimentation test for the breadmaking quality of wheat. FMBRA Bull. pp. 16-27. Seibel, W., Ludewig, H. G., and Bretschneider, F. 1980. Experiments with foaming agents in sponge and sandcake batters (in German). Getreide, Mehl Brot 34, 298-304. Short, A. L. 1971. The evaluation of additives for breadmaking. 1. An improved test method. Food Technol. Aust. 23, 34-35. Simmonds, D. H. 1981. Wheat proteins: Their chemistry and nutritional potential. In “Cereal Science-Today and Tomorrow” (L. T. Evans and W. J . Peacock, eds.), pp. 149-166. Cambridge Univ. Press, London and New York. Simmonds, D. H., and Winzor, D. J. 1961. Chromatography of the proteins from wheat flour soluble in acetic acid. Nature (London) 189, 306-307. Spies, R. D., and Kirleis, A. W. 1978. Effect of free flour lipids in cake-baking potential. Cereal Chem. 55, 699-704.

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ADVANCES IN FOOD RESEARCH, VOL. 29

KIWIFRUIT B. S. LUH AND ZHANG WANG’ Department of Food Science and Technology University of California, Davis Davis, California

I. Introduction. . . . . . . . . . . . . . . . . . ........ ........... Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... ........................................... eds . . . . . . . . . . . . . . . . . . . . . . ......................... D. Harvest and Storage .........................

11.

111. Growth and Ripening Physiology.. . . . . . . . . . . . . . . . . . IV. Chemical Components. ......................... A. Acids and Sugars. .................... B . Chlorophyll and Pectin . . . . . ......................... C. Proteinase.. . . . . . . . . . . . . . . ......................... V. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ A. Peeling.. ........................................... B . Canning . . . . . . . . . . . . . . . . . . . ......................... C. Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ D. Drying ........................................... E. Kiwifruit Juice.. . . . . . . . . . . ......................... F. Kiwifruit Wine.. . . . .................................. G . Kiwifruit Leather. . . .................................. H. Candy . . . . . . . . . . ......................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I.

279 280 280 282 282 285 288 294 294 295 297 298 298 299 300 300 30 1 302 306 306 307

INTRODUCTION

Kiwifruit are produced by a large, deciduous vine botanically known as Actinidia chinesis Planch. Individual plants bear either “male” (staminate) or “female” (pistillate) flowers, so both types are planted in a vineyard, at a ratio of 8 females to 1 male, to ensure adequate cross-pollination and fruit set. Bees ’Present address: Wuxi Institute of Light Industry, Wuxi, Jiangsu 214004, China. 279 Copynght 0 1984 by Academic Press. Inc All rights of reproduction in any form reserved. ISBN 0-12-016429~9

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are necessary for pollination because wind pollination is unsatisfactory. The vine originated in the Yantze River Valley of China. The first commercial kiwifruit vineyard was planted in New Zealand about 1950 in the Bay of Plenty district near the city of Taurango. Fruit was exported to the United States as early as 1958 and was featured first as Chinese gooseberries and later as kiwifruit. In 1981, New Zealand exported 6 . 2 million flats (3.18 kg/flat) of kiwifruit to the United States, Japan, and West European countries (J. A. Beutel, personal communication, 1983). In California, there were 3.1 million flats of kiwifruit packaged in 1982. Due to the rapid increase in plantings and production, California has doubled its production capacity every 2 years in the past decade. There is a need for utilization of kiwifruit that will not meet the fresh market grades according to the U.S. Department of Agriculture (USDA, 1982). This article covers the production, ripening, chemical components, and processing of kiwifruit.

II.

PRODUCTION

Kiwifruit has a fuzzy brown skin which covers attractive, emerald green flesh filled with small, black edible seeds. It is eaten fresh in fruit salad, can be canned or frozen, and is used for making nectar, jam, preserves, and wine. For best flavor, fresh kiwifruit should be allowed to ripen and soften like an avocado. When harvested at the mature stage (6.5-8% soluble solids) and properly handled, kiwifruit can be stored under refrigeration and 90-95% relative humidity for 3 to 4 months provided that the storage room is free from ethylene gas. In major markets of the world, kiwifruit are available year-round. Acceptable climates must have winter temperatures below 7°C for about 700 hr. A frost-free season of 225-240 days is needed in California for kiwifruit since vines leaf out in March, bloom in May, and are harvested in October or early November. Temperatures below -12°C in midwinter will kill all young vines and most old bearing vines.

A.

PROPAGATION

Kiwifruit vines can be propagated by grafting seedlings or by rooting cuttings. Both methods can be used to produce good vines and crops. Both seedlings and rooted cuttings make good field nursery and container plants. Field-grown nursery stock make the largest sized (trunk diameter) plants and are transplanted to the kiwi vineyard bareroot in the winter. Container-raised plants are sold as medium to tall plants and can be transplanted to the field at any time of the year.

B . S . LUH AND ZHANG WANG

I.

28 1

Grafting Seedlings

Seeds for planting can be extracted from any ripe kiwifruit by peeling the fruit and running it through a blender at slow speed to separate the seeds from the pulp. By slowly washing the slurry of pulp and seeds in a bowl or pan, the water will float away the pulp and leave the seeds in the bottom of the container. The seeds can then be dried for storage. When needed, the seeds are placed on a wet paper, wrapped in a plastic bag, and placed in a refrigerator for 3 weeks to stratify so that they will germinate uniformly. Next, the seeds are planted in sterilized soil. They will germinate in 3 weeks at 18-24°C and can then be transplanted into 8- 10 cm pots and grown at 16-24°C. When they are 9- 13 cm high and frost is past, they can be planted 35 cm apart in nursery rows spaced approximately 1 m apart. Small (9-13 cm) seedlings can be transplanted to 24liter containers at any time of the year. Seedlings are allowed to grow one whole season in the field nursery before they are grafted to the desired variety. One-year-old seedlings can be whip grafted in January, or in April and May with dormant wood collected in January and held in plastic bags in refrigeration at 0-1°C. Budding (T-bud) is also satisfactory in April or May if dormant wood is used. Nurserymen prefer whip grafting in April and May. After grafting, field nursery plants grow until December when they are dug bareroot for transplanting. When container plants are pencil sized (0.6- 1.O em) they are whip grafted to the desired variety. When plants (0.6-1.0 cm) are 1-2 m high, they are large enough for sale and for transplanting to the kiwi vineyard. Container plants may be transplanted to commercial vineyards in late summer to fall or in winter to early spring. 2.

Rooting Cuttings

Cuttings are made from 1.3-em diameter wood taken in midsummer. Each cutting is two to three nodes long (13-20 cm). A whole leaf is left at the top node, and the leaves are removed from the bottom nodes of each cutting. The basal end of the cutting is dipped in a 4000-8000 ppm indolebutyric acid (IBA) rooting hormone solution or in a 4% naphthalene acetic acid (NAA) or equivalent rooting hormone powder. The cuttings are set in a coarse rooting medium (perlite or a mix of approximately half perlite and half vermiculite) and placed under intermittent mist (water spray). The water mist spray keeps the leaves moist to prevent wilting while roots are forming. Frequency and duration of water “on” cycle varies with temperature, propagation house, and length of time the cuttings are in the propagation house. Rooting occurs in 6-8 weeks at 21-24°C. The rooted cuttings are transplanted into pots and returned to a humid green-

282

KIWIFRUIT

house or mist to minimize transplant shock (rooting the cuttings in 10-cm deep pots avoids an otherwise necessary first bareroot transplanting). Well-established plants in pots can be transplanted to the field nursery or to 4- to 1 1-liter containers. When plants are 1.3 cm in diameter or 1 . 3 m tall, they have reached the minimum size for transplanting to the permanent kiwi vineyard. Transplanting from the field nursery can be done bareroot in the winter. Transplanting from containers can be done in the fall, winter, or spring. Dormant cuttings can be rooted in the winter if given a 24-hr soak in 200 ppm IBA or quickly dipped in 4000 ppm IBA solution or rooting powder. They are set in a coarse medium, given bottom heat, and the tops are left at an air temperature of 0-4°C. The cool tops prevent growth until the bases of the cuttings are rooted in the heated medium. After rooting is complete, the cuttings are transplanted to containers and placed in a humid house for further growth. Later, they can be moved to larger containers or to a field nursery. Hayward and most other female varieties root easily as do New Zealand males Tamori and Matua. The California or Chico male variety will not root under mist. California male plants are propagated by being grafted onto seedlings.

B . PRODUCTION AREAS AND SOIL AND WATER NEEDS In California, the main production areas with a future are the MarysvilleGridley-Chico area; Fresno, Tulare, and Kern Counties; the Fairfield-Winters area; and orchard areas on the east side of the San Joaquin Valley. Hayward kiwifruit is the only acceptable female variety, and it needs winter cold to break dormancy. Without 700 hr under 7"C, plants flower late and poorly. It is too warm most winters in Southern California for normal dormancy to develop in vines, so flowering is greatly reduced. In foothill and North Coast areas, frost limits kiwifruit culture. In the Central Coast areas, lack of winter chilling reduces production in some years. Kiwifruit vines grow best on class 1 soils, especially deep, alluvial soils. Good and sometimes excellent production is obtained on class 2 soils if properly managed. Kiwifruit vines grow and produce better on loam and silt-loam soils than on loamy sands. Kiwifruit vines require low-salt water. Chloride, bicarbonate, boron, and sodium are the most damaging. Approximate safe levels for irrigation water are as follows: less than 100 ppm chloride, less than 200 ppm bicarbonate, less than 0.8 ppm boron, less than 70 ppm sodium. C.

CULTURAL FACTORS

Using good bare root or container plants is essential for a good start. The larger sized plants are preferred. Plants should be 1-2 m high with a diameter

B . S . LUH AND ZHANG WANG

283

near the bud union of about 1.3 cm. More plants are lost in the first year when smaller plants are used, and smaller plants take longer after planting before they produce fruit. Most plants are grafted onto Hayward or Bruno seedling rootstock, although rooted cuttings can make good plants. The only acceptable commercial variety is Hayward; any good male pollinator that blooms at the same time as the Hayward is satisfactory. One may plant an outside row of all female (Hayward) plants, follow that with a row consisting of one male plant between every two female plants. Then plant two rows of all female plants, another row with one male between every two females, 2 more rows of all females, etc. This means that every third plant in every third row should be a male. Irrigation is extremely important for optimal plant growth and good production. Irrigating three times per week in the summer is usually advisable for the first three years after planting, and twice per week for bearing plants. Plants on sandy soils or in vineyards where drip irrigation is available may be irrigated daily or every other day. Excessive irrigation or poor drainage can cause crown rot, yellow vines, and plant death. Underirrigation reduces growth, causes some sunburned leaves, produces small fruit, and stunts plant growth. Severely deficient irrigation causes leaf bum and will ultimately result in plant death. Solid set, overhead irrigation is standard for irrigation and frost protection in most kiwifruit vineyards in Northern California. Drip is common in the southern San Joaquin Valley, southern California, and other areas of California. Furrow, flood, or sprinkler irrigation supplement drip irrigation in most bearing kiwifruit vineyards. Plastic minisprinklers are replacing drip and supplementing other forms of irrigation by wetting a wide area under the plants. Areas with minimum wind should be selected for growing kiwifruit because wind easily breaks canes, damages fruit, and reduces plant growth. Use of windbreaks will provide partial to adequate protection depending on the type of trees used and the severity of the winds. Poplars and certain willows are generally considered the fastest growing windbreaks. Some growers use pine and eucalyptus trees for the same purpose. Frost damage to young vines in the fall and spring can result in plant death or killing of the budded portion of the vine. Temperatures below - 1°C in the spring will damage new leaves, fruit, and tender shoots. In November, temperatures below -3°C may cause trunk damage to young vines and will freeze leaves. After two to four nights of freezing temperatures (with or without frost protection), vines usually tolerate temperatures in the -4 to -5°C range. Cold sites are not satisfactory for kiwifruit vines; although some growers are able to grow vines in these areas, they often take severe losses due to the cold. The temperatures which mature winter-hardy kiwifruit vines will safely tolerate range from -9 to - 12°C. Sprinklers can help protect vines against cold damage, but protection is limited to 3-4°C of frost by overhead sprinklers applying 190 liters/min per acre.

284

KIWIFRUIT

Sprinklers must be started when temperatures are several degrees above the critical temperature to perform as described. Pest and diseases are not serious in California. It is obvious that spraying will be necessary in older vineyards and in established kiwifruit districts. Looper worms, especially Omnivorous Leaf Roller and small white scale (Greedy, Latania, Ivy, and Oleander scales) are presenting some problems to California growers. Worms and scale are perennial problems for New Zealand growers, necessitating four to six sprays per year. Soil diseases such as Armillaria mellea (oak root fungus) and Phytophthoru sp. (crown rot) are fatal to kiwifruit vines. Vines growing in low spots, in saturated soils, or in standing water are likely to be killed by crown rot. Rootknot and lesion nematodes cause damage to kiwifruit roots if the number of nematodes is large. When kiwifruit follow crops infested with these nematodes, the soil should be fumigated with a good nematocide. Fruit in storage may show gray Botrytis mold and other forms of decay. Absence of chemicals registered in the United States, Japan, and Europe for pest, disease, and weed control is a problem of the kiwifruit industry. In 1982, there were at least two chemicals registered in California for worm control, two for scale control, and two for preemergent weed control. No contact herbicides are registered for bearing kiwifruit vineyards. Fertilization consists mostly of applications of nitrogen fertilizer two or more times per year. The grower must avoid applying fertilizer too close to the plants. It is not necessary to fertilize the first year after planting. For full-bearing vines, use 0.45 kg of nitrogen per plant or 68 kg per acre. A 0.48-kg quantity of nitrogen is equivalent to 1 kg of urea or 2.27 kg of ammonium sulfate. Younger plants should receive proportional amounts during the year depending on age. The second leaf plants can have 0.09 kg of nitrogen per plant per year; third leaf 0.18 kg, fourth leaf 0.36 kg, and fifth leaf 1.0 kg. Pruning and vine training are costly. The pruning and training techniques vary among growers. Pruning is done mostly during the winter in California, while New Zealand growers do as much summer pruning as winter pruning. In the hot California summer, shade must be maintained over the fruit, while in New Zealand growers remove part of the summer shoot growth to let light into their plants to encourage fruitbud formation for the next year’s crop. Generally, kiwifruit vines are trained to a single trunk. When the main trunk cane rises above the trellis, it is cut off just below the trellis wire and two canes are encouraged to grow in opposite directions down the center wire of the trellis, forming two main arms or cordons from which all future fruiting canes originate. Each year the cordons are headed 50-76 cm beyond the last cut to encourage cane growth from each bud on the cordon cane. Most new canes produce three to five fruits at their base and then form flower buds during the summer along the rest of the cane. Winter pruning of female vines usually leaves four to eight flower buds on most canes and removes older canes that have fruited for 2 or 3

E . S. LUH AND ZHANG WANG

285

years. Summer pruning usually consists of cutting back vigorous canes that grow beyond the T-bar into the driveways between the trellis and removing a few vigorous, upright growing “sucker-like” canes. Male vines are pruned differently from female vines. Winter pruning in male vines consists of cutting out twisted, dead, or weak canes but leaving canes about 1 m long. After flowering and pollination in May, male vines are cut back severely leaving only a few canes spread on the trellis. New growth appears soon and will grow and produce flower buds for the next year. Since male plants produce only flowers, it is best to leave them long before bloom to get maximum flowering. Pruning after bloom restricts male plant size and maintains good annual flowering canes. Post-bloom pruning must be done in May, or summer heat will cause sunburn damage to the plant. D.

HARVEST AND STORAGE

The kiwifruit are grown on a perennial vine. Cropping usually begins some 3 years after planting with full bearing 3-5 years later (Miller, 1975). The flesh is green, slightly acidic, melting, and delicate in flavor when fully ripe. Each fruit contains numerous small, dark-colored seeds. The fruit are oval to globular in shape with a rough hairy brown skin. There are many cultivars, but Hayward is the cultivar most widely cultivated (Astridge, 1975; Beutel et al., 1976; Reid et al., 1982).

I.

Harvesting

In New Zealand, the optimal harvest maturity is some 23 weeks after flowering or when the soluble solid content of the flesh has reached 8% (Pratt and Reid, 1974). In the United States, the minimum average soluble solid requirement of the kiwifruit is 6.5% at the time of harvest (USDA, 1982). The soluble solids of the fruit will increase to 14-17% upon ripening. The usual harvest time in some kiwifruit producing countries is as follows (Thompson, 1982): Australia France Italy New Zealand United States (California)

May to June (Wilson and Richards, 1973) November (Ford, 1971b; Anonymous, 1977) October-November (Sozzi et al., 1976) May to July (Greig, 1953) November (Beutel et al., 1976)

Kiwifruit are harvested by hand from the vine. They break rapidly from the fruit stalk at the abscission layer (Fletcher, 197 1). If the stalks are cut, leaving a piece attached to the fruit, damage may be caused to other fruit during handling (Padfield and Bailey, 1952). The fruit are placed in canvas bags that are worn

286

KIWIFRUIT

like aprons by the people who harvest. When the bags are full the fruit are poured into 18-kg wooden boxes or 455-kg bulk handling bins. The possibility of harvesting with a mechanical shaker has been studied by Curlee (1970), but the method is not used commercially because it increased fruit bruising. The fruit should be cooled to 0-4°C within 8-24 hr after harvest and stored at 0°C storage. Commercial storage and transport of kiwifruits are usually at 0°C under 9294% relative humidity in New Zealand (Harris and McDonald, 1975) or at -0.5 to 0°C under 90% R/H (Harris, 1976). Ethylene gas, given out by the fruits, may cause them to ripen quickly if it is accumulated in the stores. If this is a problem the stores may be ventilated for 4 hr each day (Harris, 1976) or an ethyleneabsorbing compound such as potassium permanganate or active charcoal can be placed in the stores (Strachan, 1970; Sozzi et al., 1976). Kiwifruit harvested at 6.5-8% soluble solids can be kept firm at 0°C under 90-95% R/H for three months or more. They should not be stored together with other fruit, especially those that produce ethylene gas. Presence of ethylene gas will cause fruit softening and drastically limit storage time and sale of fruit. McDonald and Harman (1982) studied the effect of controlled atmosphere storage under high CO, and low 0, on the firmness of kiwifruit during cold storage at 0°C. Atmospheres containing above 4% CO, with 15-20% 0, caused a retardation in the softening of kiwifruit. This effect increased as the CO, content of the atmosphere increased from 4 to lo%, but additional CO, above 10% had no further effect on fruit firmness. Low 0, (2-3%) with 3-5% CO, further delayed the rate of softening and increased storage life up to 3-4 months beyond normal air-storage life. Although controlled-atmosphere storage increases storage life of kiwifruit, the magnitude of the effect varied from year to year. Contamination of the storage atmosphere by as little as 50 ppb ethylene severely reduced the effectiveness of controlled-atmosphere storage in maintaining kiwifruit firmness, even at 0°C (Arpaia, 1982). 2.

Packing

Kiwifruit for export are normally packed in a single layer tray, with 40-55 fruits per tray and a net weight between 3 and 4 kg. In New Zealand, a wooden tray of simple nailed construction with dimensions of approximately 45 x 29 x 5 cm and labeled at one end is commonly used (Wolstenholme and Allen, 1973; Reid, 1977). Fruits are placed in pockets in a thermoformed plastic tray, and a polyethylene film sheet, which reduces desiccation, is passed around this and overlapped but not sealed (Miller, 1975). A sheet of corrugated paper may be placed between the film and the tray lid in order to reduce movement of fruit. In some countries, P-flute corrugated fiberboard trays of rigidly locked construction are used, with a lid of E-flute board. The lid carries printing and is perforated for use as display material. The external dimensions of the box are approximately 50

B . S . LUH AND ZHANG WANG

287

X 30 X 6.5 cm. The fruits, located in pockets in a thermoformed plastic tray, are covered with a sheet of polyethylene film. The box also contains a supply of bags with suggestions for use of the fruit. Trays may be assembled into stacks of three by wiring or by means of a cardboard sleeve or outer wooden box, but are also stacked directly onto pallets for transport (Thompson, 1982). Forklifts used in the storage rooms should be battery operated in order to avoid ethylene gas generation that would cause softening of kiwifruit. Storage rooms should be thoroughly aired to remove any residual gasses before newly packed fruit is stored. Potassium permanganate may be used in the storage room to remove ethylene gas.

3 . Diseases Field diseases of kiwifruit have not been an important problem although leaf spots caused by Glomerella spp. and Phomopsis spp. can occasionally be a problem (Ford, 1971a). Rotting and decay during storage and marketing of kiwifruit are usually attributable to harvest of overmature fruit (Wolstenholme and Allen, 1973). There have been reports of stem-end rots of kiwifruit arriving in the United States from New Zealand. This was due to Phomopsis spp. (Beraha, 1970) and Diaporthe actinidiae (Sommer and Beraha, 1975). Some Phomopsis spp. are the imperfect state of Diaporthe, and therefore the two observations are probably of the same organisms. In infected fruit the skin becomes soft and lighter in color at the stem end. Severely infected kiwifruit may remain intact if the skin is not broken, but the flesh beneath will become very soft and a lighter shade of green than normal (Beraha, 1970). Cook (1975) reported on infection of kiwifruit by the fungus Botrytis cinerea. Myxosporium spp. were found infecting kiwifruit in New Zealand, but it was not considered a serious problem (Ford, 1971a). 4.

Standards: Grades

Standards exist for international trade on kiwifruit (Love and Ferguson, 1977). The minimum average soluble solids required at harvest time is 6.5% in the United States (USDA, 1982). The U.S. Standards for Grades of Kiwifruit are established and codified as 7CFR 51.2335 through 51.2340 as follows: Sec.51.2335 5 1.2336 51.2337 51.2338 5 1.2339 51.2340

Grades Tolerances Application of Tolerances Standard Pack Definitions Classification of Defects

288

KIWIFRUIT

Kiwifruit are classified into the following grades: (a) “U.S. Fancy” consists of kiwifruit which meet the following requirements. ( 1) Basic requirements: similar varietal characteristics; mature; not soft, overripe, or shriveled; carefully packed; clean; and well formed. (2) Free from worm holes; broken skins that are not healed; sunscald; freeze injury; internal breakdown; and decay. ( 3 ) Free from injury by bruises; leaf or limb rubs; discoloration; hail; growth cracks; scab; scars; heat; spray burn or sunburn; scale; insects; other diseases; and mechanical or other means. (4) Tolerances (Section 51.2336) (b) U.S. No. 1 (c) U.S. No. 2 The tolerances, application of tolerances, standard pack, definitions, and classification of defects are described in detail in the standards set by USDA (1982). These standards stipulate that kiwifruit should be free from leaf-roller larvae, Greedy scale, and mites and have no excessive pesticide residues. It will be important for the importers of kiwifruit to consult the appropriate plant quarantine authority.

Ill. GROWTH AND RIPENING PHYSIOLOGY The development of fruit and seeds in kiwifruit after flowering was studied in detail by Hopping (1976a,b). The growth curve as measured by increase in fresh weight with time in days after flowering is shown in Fig. 1A. The growth can be divided into three stages: stage I (0-58 days after flowering), stage I1 (58-76 days after flowering), and stage I11 (76-160 days after flowering). The three stages are (1) rapid increase in weight and volume, (2) slow growth, and (3) rapid growth, respectively. There was no similar division in the dry-weight growth curve. Fruit growth during stage I was initially due to cell division in the central core and the inner and outer pericarp. Cell division in the outer and inner pericarp ceased after 23 and 33 days, respectively, but continued in the central core at a slow rate until 110 days after flowering. Cell enlargement commenced in all three tissues immediately after flowering and continued until the onset of stage I1 (Fig. 1C). From Fig. lD, it can be seen that seed development began immediately after fertilization and continued for nearly 80 days while the nucleus reached its maximum size and was, in turn, replaced almost entirely by the endosperm and endothelium. About 60 days after flowering the two-celled embryo divided to form a spheroid and then developed rapidly to reach its final size at 110 days. Wright and Heatherbell (1967) studied the respiration rates and physicochemi-

289

8 . S . LUH AND ZHANG WANG

I-

60

-

I

2 UJ

40

3 I - ?

3

20

~

-

/

DRY.

Wt

0 44

220 3 68

OUTER

-

: '

30

1.6

-

0.8

-

O

L

2.4

CENTRAL

PERICARP

h

J

CORE INNER

PERICARP

k

FIG. 1. Fruit and seed development in kiwifruit. (A) Cumulative increases in fresh and dry weight of fruit; (B) cell number of the central core and the inner and outer pericarp; (C) mean cell size of the central core and the inner and outer pericarp; (D) cumulative increases in the length of the seed, nucellus, endosperm, and embryo. From Hopping (1976a).

290

KIWIFRUIT

cal changes of kiwifruit harvested at various stages and stored at various temperatures ranging from 0 to 25°C. They concluded that kiwifruit was nonclimacteric. In contrast, Pratt and Reid (1974) reported that kiwifruit should be climateric because each fruit of a given harvested lot showed a respiratory rise eventually, accompanied by softening, development of aroma, and a pattern of ethylene production (Fig, 2). They indicated that among 73 individual fruits examined for ethylene production, 26 showed ethylene gas rise occurring first, usually as much as a week earlier than the respiratory rise (Fig. 2; Fig. 3, one fruit); in the rest, the two rises appeared to coincide (Fig. 3), and in no case did the respiration rate increase before the rise in ethylene production. They thought kiwifruit was another example of a fruit showing a climacteric rise. Most phenomena of ripening are induced by a rise in endogenous ethylene gas. The discrepancy between the two reports may be explained by the difference in observation methods and durations. Wright and Heatherbell (1967) used samples containing several fruits, which may mask the behavior of individual fruit, and their tests were not of sufficiently long duration. Pratt and Reid (1974) suggested that one must study the respiration of individual fruit over a considerable period. They also reported that kiwifruit could remain on the vine for months after maturation commenced. A wide variation in the time at which ripening occurs was observed in fruit that set and was harvested at the same time. Matsumoto and Luh (1983) reported on the physical and chemical changes in

Time after anthesis [weeks)

FIG. 2 . Respiration and ethylene production throughout the life of all fruits of a single harvest. These fruits were approximately at the maturity approved for export marketing. Circles are averagcs of two or more fruits: the standard deviation is shown when n > 4. Other symbols represent individual specimens and corresponding solid symbols show ethylene production by that same individual. Duration of an ethylene treatment is shown by a horizontal arrow and the response to the treatment is shown by solid circles. From Pratt and Reid (1974).

B. S . LUH AND ZHANG WANG

29 1

T i m e after anthesis (weeks)

FIG. 3. Respiration and ethylene production of a fully mature kiwifruit. From Pratt and Reid (1974).

(0) COz; (0)ethylene.

ripening kiwifruit at 20°C in the presence of 5 ppm ethylene gas (Fig. 4). The effect of ethylene treatment at the 5 ppm level on the decrease in total acidity and starch and on the increase in soluble solids in the fruit was significant. Okuse and Ryugo (1981) studied the compositional changes at 20-day intervals in the developing Hayward kiwifruit in California after full bloom. Starch is the predominant carbohydrate stored in the carpellary tissue and becomes hydrolyzed as the fruit approach maturity (Fig. 5). Glucose was rich in the green immature fruit, but the level decreased while starch accumulated rapidly during late July and August. As starch hydrolysis began, glucose levels increased rapidly, attaining nearly 10% by harvest. Fructose increased gradually from the youngest stage of fruit development until harvest. The types of cytokinins changed as the fruit developed. Reid et al. (1982) studied the seasonal patterns of changes in chemical composition of Actinidia chinensis cultivar Bruno. The kiwifruits were harvested at intervals throughout the season. On a fresh weight basis, immature fruit contained high concentrations of starch, which was hydrolyzed after the fruit reached full size. Concurrently, there was a rapid increase in the concentrations of sucrose, glucose, and fructose, which were the major sugars present. This increase was reflected in a linear rise in soluble solid content of the fruit. Malic and quinic acid concentrations decreased during the early part of fruit growth,

292

KIWIFRUIT

I,

2

4

6

8 1 0 1 2

20

QJYS

FIG. 4. Changes of firmness during ripening of kiwifruit (average of 12 fruit). treated; (0)control. From Matsumoto et al. (1983).

(0) Ethylene

rose to a maximum after the fruit reached full size, and then decreased slightly. The concentration of citric acid rose linearly during fruit growth, then fell gradually after the fruit reached full size. Ascorbic acid and amino nitrogen concentrations fell during the early part of fruit development, then remained relatively constant. The relationships between patterns of chemical change and the triple sigmoid growth curve of the kiwifruit were discussed in detail. Quinic acid was the main organic acid in young kiwifruit which disappeared concurrently with the appearance of ascorbic acid. Malic acid attained a low midsummer maximum and decreased toward harvest time. The change in citric acid content followed that of starch, reaching a peak in mid-September. De-

FIG. 5 . Seasonal changes in carbohydrate composition of “Hayward” kiwifruit (dry weight basis). From Okuse and Ryugo (1981).

293

B . S. LUH AND ZHANG WANG

hydroascorbic acid was detectable in kiwifruit but always in small quantities. Ascorbic acid increased rapidly during mid-season when the seeds enlarged and fruit growth was slowed (Fig. 1). Hopping (1976a) studied the relationship between fruit weight and seed number for three kiwifruit cultivars. As shown in Fig. 6 . the increase in fruit weight for each additional seed was proportionally greater at low and high seed numbers than at 100-700 seeds (Monty), 200-800 seeds (standard), or 2001200 seeds (Hayward). The effect of various growth regulators, either alone or in combination, on the development of kiwifruit was investigated by Hopping (1976b). Auxins, such as 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic and -naphthoxyacetic acids, and indole acetylaspartate, gibberellins (GA,, GA4--7), and cytokinins (6-benzylaminopurine, zeatin) when applied alone did not stimulate fruit development, nor did combinations of auxin plus gibberellin or gibberellin plus cytokinin. However, auxin (2,4-D or 2,4,5-T) plus benzylaminopurineand auxin plus cytokinin plus gibberellin markedly stimulated fruit growth, especially when the treated fruit contained more than 200 seeds. Hopping explained his results by assuming that developing seeds produce a growth factor other than auxin, gibberellin, or cytokinin that was required for fruit development. The hypothetical seed factor(s) must be present above some threshold level before the response to exogenous auxin, gibberellin, and cytokinin can be most effective.

lo l

o Hayward Monty A

0

Standard

200

400

600

800

1000

1200

1400

Seed number

FIG. 6. The relationship between fruit weight and seed number for three kiwifruit cultivars. From Hopping (1976a).

294

KIWIFRUIT

IV. CHEMICAL COMPONENTS The constituents of fresh, frozen, and canned kiwifruit are presented in Table I (Beutel et al., 1976). The report published by Dawes (1972) indicates that the average soluble solid content of New Zealand-grown kiwifruit was 14.16% (1 1.44-17.49). There were some minor differences in chemical composition of the kiwifruit grown in California and New Zealand. The pH value of ripe kiwifruit may vary from 3.1 to 3.96 (Dawes, 1972), and the mean pH value of four varieties was 3.38. A.

ACIDS AND SUGARS

Acids and sugars in kiwifruit were investigated by Heatherbell (1975). After purification through lead salt precipitation and ion exchange, the acids and sugars were determined by gas-liquid chromatography of their trimethylsilyl derivatives. Heatherbell’s results were lower than those reported by others due to the difference in ripeness level and methods of analysis. Matsumoto et al. (1983) TABLE I COMPOSITION OF EDIBLE PORTION OF FRESH, FROZEN, AND CANNED HAYWARD KIWIFRUITo Constituent

Fresh

Frozenb

Canned

Soluble solids (“Brix at 20°C) Moisture (%) Ash (%) Fat (Ether extract) (%) Protein (%) Carbohydrate (%) Minerals (mg/100 g sample) Calcium Magnesium Iron Phosphorus Vitamins Vitamin A (IU) Ascorbic acid (mg/100 g) Thiamin (mg/100 g) Niacin (mg/100 g) Riboflavin (mgi100 g)

14.9 81.2 0.45 0.07 0.79 17.5

16.4 80.7 0.53 0.08 0.95 17.6

25. I 73.0 0.45 0.06 0.89 25.5

16 30

18 27

23 30 0.40 48

0.51

64 175 105 0.02 0.50 0.05

0.51

67 117 21% 0.01

0.22 0.03

155 I03 0.02 0.40 0.02

From Beutel et al. (1976). Fruit immersed in a solution of I % citric acid plus 0.25% ascorbic acid 3 min before freezing. Due to predip in ascorbic acid.

295

B . S . LUH AND ZHANG WANG

TABLE I1 ANALYSIS OF SUGARS BY HPLC IN RIPENING KIWImUITa Days Treatment

Sugar

Initial

1

2

3

4

5

Ethylene 5 ppm at 20°C

Fructose Glucose Sucrose Total

2.77 2.67 0.47 5.91

2.69 2.69 1.79 7.17

3.47 3.61 2.22 9.30

4.15 4.15 1.72 10.57

4.47 4.60 1.50 10.57

4.88 5.00 1.19 11.07

Initial

1

3

6

9

12

20

2.77 2.67 0.47 5.91

2.77 2.70 0.85 6.32

2.93 2.78 0.84

3.09 3.05 0.72 6.86

3.17 3.34 0.80 7.31

3.64 3.80 0.81 8.25

4.12 4.30 1.05 9.47

Control

a

Fructose Glucose Sucrose Total

6.55

From Matsumoto et al. (1983).

reported on changes of sugars in kiwifruit during ripening of kiwifruit (Table 11). The sugars in kiwifruit are largely fructose, glucose, and sucrose. Citric, quinic, and malic acids are the major acids in kiwifruit (Fig. 7; Okuse et al., 1981). Phosphoric, ascorbic, glucuronic, galacturonic, oxalic, succinic, furmaric, oxaloacetic, and p-coumaric acid are also present in minor amounts (Table 111). Heatherbell reported that 69 mg ascorbic acid/100 g kiwifruit as detected by gas-liquid chromatography was 130 mg/100 g by the 2,6-dichlorophenolindophenol titration method. Seasonal changes of ascorbic and dehydroascorbic acid in kiwifruit are presented in Fig. 8 (Okuse and Ryugo, 1981).

B.

CHLOROPHYLL AND PECTIN

Robertson and Swinburne (1981) studied changes of chlorophyll and pectin in storage and canning. Kiwifruit were harvested and stored at 0°C. Over a 92-day period fruit were removed from storage, ripened to 13"Brix by holding at 25"C, and canned as slices in a sugar syrup. Changes in chlorophylls and pectic substances were measured after storage and canning. The concentration of total chlorophyll remained virtually constant during storage and ripening. Matsumoto et a/. (1983) also found no significant change in the chlorophylls during postharvest ripening of kiwifruit. During the canning process about 90% of the chlorophylls were degraded in a first order reaction. The total pectin content increased by over 100% during cold storage but decreased significantly during canning.

296

KIWIFRUIT

30

o----: \ \

25.

\ \ \

\

-ae

20

-

b

\ \

b

a‘ Quinic \

II)

E 4 .-V

\ \ \

15-

\

m C

\

P

\

0 10-

\ 0. \

FIG. 7. Seasonal changes in three major organic acids in “Hayward” kiwifruit (dry weight basis). From Okuse et al. (1981). TABLE I11 QUANTITATIVE DETERMINATION OF ORGANIC ACIDS IN KIWIFRUITo Acids prepared as lead saltsc Acidb Oxalic Phosphoric Succinic Fumaric Malic Oxaloacetic Citric Quinic p-Coumaric Ascorbic Glucuronic and galacturonic Volatile acids Total acids Total titratable acids (as citric) 0

From Heatherbell (1975). Individual acids by GLC. Average of three determinations. Average of two determinations.

mg/100 g fruit Trace 35 Trace Trace 530 Trace 980 920 Trace I0 80 1

2619 1800

mEq/100 g fruit

Acids prepared by ion exchange“

mg/100 g fruit

1.06

Trace

7.91

468 Trace 1008 996 Trace 68 98 4 2642 1800

15.31 4.79

0.39 0.41 0.06 29.93 28.12

rnEq/100 g fruit

6.98 15.75 5.18 0.44 0.51 0.06 28.92 28.12

297

B . S . LUH AND ZHANG WANG

-

,.&--.&---A---&---

May

June

July

Aug

Sept

Oct

-

Y

FIG. 8. Seasonal changes in weight of “Hayward” kiwifruit and ascorbic and dehydroascorbic acid content. From Okuse and Ryugo (1981).

C.

PROTEINASE

An anionic proteolytic enzyme (actinidin) obtained from kiwifruit has been shown to contain a sulfhydryl group essential for enzymatic activity (McDowall, 1970). The molecular weight of the enzyme is 12,800 2 700 as estimated by gel filtration chromatograph on Sephadex G-50 column. Came and Moore (1978) studied the amino acid sequence of the tryptic peptides from the enzyme. They reported that there were 12 tryptic peptides which give a polypeptide chain of 220 amino acid residues with a molecular weight of 23,500. Anomalously low molecular weights for a variety of proteases in a gel chromatograph were investigated by Voordouw et al. (1974). The reason for the low molecular weights has not been determined yet; they suggested that caution be exercised in the use of gel chromatographic methods alone for the characterization of proteases with respect to their molecular weight. McDowall (1970) reported that the proteolytic enzyme resembled papain (EC 3.4.4.10) in its action against benzoyl L-arginine ethyl ester, having a broad pH optimum (5-7), with K,,, (Kapp)of 89 mM and K,,, of about 2.6 sec-l at pH 5.6, 25”C, in 0.3 M KCl. The enzyme has been fractionated into two active components, proteinase A and A,, by McDowall (1973). The action of proteinase A, on the p chain of oxidized insulin was studied further by McDowall. He reported that the specificity closely resembled that of papain and that proteinase A, cleaved most readily the carboxyl peptide bonds of the residues of the p chain acylated by the large hydrophobic residues valyl, leucyl, or phenylalanyl, but not tyrosyl, and also the 22-23 Arg-Gly bond. Baker (1977) reported on the polypeptide chain conformation and active site of actinidin revealed by an electron density map at 2.8-A resolution. The molecule consists of a single chain of 220 residues, with at least two, and probably three, disulfide bridges. The conformation of the polypeptide is remarkably similar to

298

KIWIFRUIT

that of papain; it is folded into two domains. The positions and orientations of the catalytically important side chains in the active centers of actinidin and papain were studied by Brocklehurst et al. (1981). The results showed that actinidin resembled ficin rather than papain in (1) the disposition of the active-center imidazole group with respect to hydrophobic binding areas and (2) the inability of the active-center aspartic acid carboxyl group to influence the reactivity of the active-center thiol group at pH 4.6.

V.

PROCESSING

Kiwifruit production is increasing rapidly in New Zealand, the United States, China, France, Japan, Chile, Italy, and Israel. Because a significant proportion of kiwifruit cannot meet the standard required for the fresh fruit market, it is necessary to develop methods for making high quality processed products from kiwifruit. In New Zealand, about half of the total amount of kiwifruit processed in 1981 (5900 tons) was canned as slices. The remainder was processed as a frozen pulp (Wilson and Bums, 1983). Other processed products have been developed commercially. Fruit are canned as whole kiwifruit in syrup or as nectars (Wildman and Luh, 1981). They can also be frozen as slices (Beutel et al., 1976) or dehydrated as whole kiwifruit (Beutel et al., 1976) and slices (Simmons, 1978). Kiwifruit may be canned as a juice blend (Wilson and Bums, 1983). Other products from kiwifruit such as wine (Heatherbell et a l . , 1980; Lodge, 1981), leather (Lodge, 1981), and jam (Dawes, 1972) have also been produced. Recovery of actinidin as a proteolytic enzyme from kiwifruit has been suggested by Wilson and Burns (1983).

A.

PEELING

Usually, kiwifruit are harvested when mature but not ripe (6.5-8% soluble solids). They are stored at 0°C under 85-90% relative humidity. Before processing the fruit should be ripened by storing at room temperature or treating with ethylene. Ripe kiwifruit is partially soft (Beutel et al., 1976) and has a soluble content of 13-16% (Wilson and Bums, 1983). Ripe kiwifruit has an attractive flavor and is suitable for fresh eating or processing into various products. Kiwifruit has a hairy skin that must be peeled before processing. Several peeling methods have been investigated by food scientists. Beutel et al. (1976) reported that a gas-flame technique for peeling kiwifruit was impractical while lye peeling worked satisfactorily. The partially soft fruit was peeled in a 15% lye solution for 90 sec at boiling temperature, then washed in cold water. The lyepeeled fruit retained a higher concentration of ascrobic acid (104 mg/ 100 g) than did fruit peeled by hand (81.3 mg/100 g). This rather surprising result was

B . S . LUH AND ZHANG WANG

299

explained by Beutel et al. (1976); a greater concentration of ascorbic acid exists near the skin than in other parts of the fruit. Peeling by knife removes some of the fruit tissue along with the skin. The lye treatment removes only the very thin skin membrane (9.0% weight loss in lye-peeled versus 13.7% in hand-peeled fruit). Simmons (1978) reported that lye peeling at 10% NaOH and high temperature (boiling) was unsuccessful, but kiwifruit could be hand peeled after treatment in boiling water for approximately 45 sec without excessive loss of flesh. The difference in results of lye peeling might be explained by the difference in concentration of NaOH solution used. Another factor that should be taken into account for lye peeling is the ripeness level of kiwifruit before peeling treatment. Dawes (1972) indicated that overmature fruit tended to peel too deeply and spoil the appearance of the product.

B.

CANNING

Most kiwifruit are preserved by canning with 40"Brix syrup as peeled whole fruit, halves, or slices. The major deterrent to processing is an inefficient method of peeling. Currently, the majority of the canned fruit is hand peeled with a small proportion flame peeled, although the latter method is not entirely satisfactory (Dawes, 1972). It has been shown that firm, ripe kiwifruit can be peeled by immersion in boiling 15% sodium hydroxide solution for 30 sec followed by high-pressure washing with water. Maturity is critical for caustic peeling as overmature fruit tend to peel too deeply, spoiling the appearance of the product. Simmons (1978) stated that peeling kiwifruit with boiling 10%sodium hydroxide was unsuccessful and offered no advantages over placing fruit in boiling water for approximately 45 sec. The fruit can then be peeled easily without excessive loss of flesh. Because of the high acidity (1.5%) and low pH (3.1-3.8), it is necessary to add sugar in canning kiwifruit and its products to improve the flavor acceptance. Wildman and Luh (1981) studied the effects of different sweetener types and of added citric acid on quality and composition of canned kiwifruit nectars. The sweetener types and citric acid caused some difference in sweetness scores, but the panel did not find a significant difference in flavor between the nectars made with sucrose and those made with other sweeteners. A similar study was conducted on frozen kiwifruit concentrates by El-Zalaki and Luh (1981). The flavor score was determined on nectars made from diluted kiwifruit concentrates. The highest score went to the product made with sucrose. Beutel et al. (1976) assessed the quality of canned whole kiwifruit stored for approximately 5 months. They found that the canned fruit had changed from a light, bright green color characteristic of fresh, peeled fruit to a darker, dull green. The texture was considerably less firm than that of raw fruit. The canned fruit had a much milder flavor than did fresh kiwifruit but a very sweet taste due

300

KIWIFRUIT

to the added syrup. They concluded that although the flavor of the canned product was not similar to that of fresh kiwifruit, the taste was considered very pleasing, and even more desirable to some judges, than that of the fresh fruit. Dawes (1972) mentioned that kiwifruit often developed a hard core after canning, which was more prevalent in overmature fruits. The phenomenon occurs at all maturities in some seasons. More research work is needed to understand the basic cause of the problem. C.

FREEZING

Freezing preservation of peeled kiwifruit slices has been reported by Beutel et al. (1976). Before freezing, 1-cm thick sections of kiwifruit should be dipped for 3 min in a solution of 12% sucrose, 1% ascorbic acid, and 0.5% malic acid to inhibit enzymatic changes. The slices were frozen by an IQF (individually quick frozen) technique in liquid Freon at -3O"C, or in an air blast at -40°C. The frozen product was packaged in polyethylene bags and stored at - 17.8"C. For better protection, the frozen product should be sealed in Saran- or aluminumlaminated pouches. They concluded that the frozen sliced kiwifruit were closest to fresh fruit in appearance and flavor, and freezing did not cause large compositional changes. Frozen sweetened kiwifruit concentrates have been studied by El-Zalaki and Luh (1981). Ripe kiwifruit can be made into attractive frozen, sweetened concentrates of 40% soluble solids. The frozen concentrates can be converted into nectars after dilution with two volumes of water. It was necessary to inactivate the polyphenoloxidase and peroxidase enzymes present in the fruit by heating the puree in a plate heat exchange to 87.8"C for 1 min, followed by cooling, sweetening, filling into containers, and freezing. Frozen kiwifruit puree is a commercial product for export from New Zealand. For this purpose, the skin can be removed economically by freezing the fruit solid and then removing the skins with a smooth abrasive peeler. The still partly frozen kiwifruit is chopped and refrozen in bulk (Strachan, 1974). This product has been used in ice cream and fountain products. Care must be exercised in prohibiting the protease activity of the fruit through pasteurization. Otherwise, the ice cream will have a poor texture. D.

DRYING

Simmons (1978) studied the effect of slice thickness and temperature on drying time, color, and visual appearance of a slice of kiwifruit. The data in Tables IV and V showed that thickness of the sliced kiwifruit has a marked effect on the drying time and appearance. The 4-mm thick slices were easy to manipulate, dried in a reasonable time, and produced an attractive product. The taste of

30 1

B . S . LUH AND ZHANG WANG

TABLE IV EFFECT OF SLICE THICKNESS ON DRYING TIME, COLOR, AND VISUAL APPEARANCE OF SLICES OF KIWIFRUITo ___________~~

Thickness of slice (mm)

Drying time (hr)

Hunter color ratio (-alb)

Appearance

2 4 8 16

6 9 24 40

0.17 0.12 -0.03 -0.04

Good, slightly translucent green, good shape Very good, green, good shape Unacceptable yellowlgreen, fair shape Unacceptable yellowlbrown, unattractive shape

~~

0

From Simmons (1978). Drying temperature was 45°C. The moisture content of dried product was 10%

dried kiwifruit should be considered as well as the color and appearance. Beutel et al. (1976) reported that the taste of dried, lye peeled, whole fruit was quite sour, but a preliminary dip in a sugar solution before drying helped to improve

the flavor. E.

KIWIFRUIT JUICE

Recently Wilson and Burns (1983) applied heat treatment techniques and ultrafiltration to processing kiwifruit juice. The partially clarified juice was heated to 90°C for 5 min, then cooled to 30-40°C in a heat exchange. After the heatdenatured protein settled, the “top” fraction and the cloudy fraction were mixed with 0.5 and 1% Celite, respectively, then filtered through washed Carlson-Ford filters (grade 2A) layered with 2-3 mm Celite. The resulting clear juice was hot filled at 85°C into cans. Ultrafiltration was carried out on partially clarified juice using Romicon hollow fiber modules with two varieties of membrane: (1) PM TABLE V EFFECT OF TEMPERATURE AND DRYING TIME ON COLOR AND VISUAL APPEARANCE OF SLICES OF KIWIFRUITn Temperature (“C)

Drying time

(W

Hunter color ratio (-alb)

Appearance

35

18 9 8 7 6

0.24 0.09 0.12 0.04 -0.11

Very good, bright green Good, slightly less green than 35°C sample Good, green to greedyellow Fair, green/yellow to yellow Even yellow color

45 55

65 75 Q

Source: Simmons (1978). Slices: 4 mm.

302

KIWIFRUIT

10, 0.09 m2 membrane area, 10,000 molecular weight cut off, and (2) XM 50, 0.10 m2 membrane area, 50,000 molecular weight cut off. Thermal stability of protein in kiwifruit juice was examined at different temperatures. The protein was found to be stable when the juice was held at temperatures of 30°C and below, however, at 35°C a slight haze formed within 15-30 min, and at 40°C haze formed within 1-2 min. Three potential advantages of ultrafiltration applied to processing kiwifruit juice were illustrated by Wilson and Bums (1983): (1) production of a clear, sterile juice retaining its kiwifruit flavor without using elevated temperature, (2) removal of most of the protein from juice thereby rendering unlikely any subsequent protein precipitation caused by heat or any other factor, and (3) concentration of actinidin as a by-product of possible commercial significance. Some trouble with clearness of juice occurred during storage. Kiwifruit juice was monitored by Wilson and Bums (1983) for formation of sediments. A fine sediment formed in juice prepared by conventional means after several weeks storage at ambient temperatures, and sediment appeared in juice produced by ultrafiltration after 1 week’s storage at ambient temperatures. Factors leading to sediment formation were investigated. The investigators excluded both interaction between phenolics and proteins as well as the formation of cation complexes with pectins andlor phenolics as the factors bringing about the sediment formation. So the nature of the sediment still was not clear. Although it was found that the juice made either conventionally or by ultrafiltration remained clear after at least 6 months’ storage, they indicated that until the factors leading to the appearance of the haze were solved it could prove troublesome in commercial juice operations. Because of the acidic nature of kiwifruit, a single-strength juice might not be accepted by some consumers. The feasibility of producing a kiwifruit-apple juice blend was investigated by Wilson and Burns (1983). They reported that on mixing equal volumes of the two juices a fairly thick haze formed immediately and suggested that an apple juice procyanidin-kiwifruit protein interaction might be the cause of the haze. An attempt to overcome the haze formation by milling equal weights of apples and kiwifruit together at the beginning of processing the juice blend was taken into consideration by them.

F.

KIWIFRUIT WINE

Wines made from kiwifruit were reported by Heatherbell et al. (1980) and Lodge (1981). The procedure of producing wine from Bruno and Hayward varieties is shown in Fig. 9 (Lodge, 1981). Heatherbell et a1. (1980) studied the effect of clarification of kiwifruit juice on the quality of wine. The wine prepared from juice clarified by pectolytic enzymes possessed an aroma described by the panel as an intense fruity “Riesling

303

B . S . LUH AND ZHANG WANG

Mill

50 mg SO, lkg 100 mg pectolytic enzymelkg 2% w/w press aid

L

I

14 mg pectolytic enzyme/kg at 45OC overnight Settling tank

Filter

I

Adjust to 18”Brix

1-b Ferment. 15OC

I

1

I

30 mg SO, lkg Saccharomyc& cerevisiae (Strain R92) Stop fermentation 5 0 ms - SO, Iks. 4’C 1

-

1 I

J

I

30 mg SO, Ikg

FIG. 9. Flow diagram of kiwifruit wine production. From Lodge (1981).

Sylvaner-type” aroma, which developed during fermentation (after approximately 4 days at 15°C); wine prepared from unclarified juice or juice clarified by settling or centrifuging, however, was described by the panel as being “grassy, green, stalky” in aroma and taste and as being of unacceptable astringency and bitterness. They concluded that the commercial pectolytic enzyme used for clarifying the juice released or produced desirable aroma compounds and/or degraded undesirable aroma compounds in kiwifruit juice, and some enzymes other than pectinases might be present in the commercial enzyme preparations. They also showed that the pectolytic enzyme preparations had removed potential bitter and astringent compounds. The relationship between SO,, ascorbic acid, and dehydroascorbic acid in juice and wine was investigated by Heatherbell et al. (1980). Table VI shows

304

KIWIFRUIT

TABLE VI CHANGES IN SO2 AND ASCORBIC ACID CONCENTRATIONS DURING JUICE EXTRACTION AND VINIFICATIONa

SOz (mg/liter) Time (treatment)

Total added

Free

Bound

1 hr (press juice)b 24 hr (ameliorated juice)= 24 days (end of fermentation) 31 daysd 38 daysd 45 daysd 52 daysd 66 dayse (bottle)

50 50 50 100 150 200 250 275

8.6 3.2 1.3 3.0 7.5 15.2 23.0 29.6

16 6.8 2.4 40 62 87 95

0

101

Ascorbic acid (rng/liter)

Dehydroascorbic acid (mg/liter)

81 1 434 380 ND 375 ND ND 336

131 25 16 ND 15 ND ND 24

From Heatherbell er al. (1980). Values are average of duplicate analyses. ND: not determined. One hour after addition of 50 mg SOz/kg fruit at crushing. Enzyme-clarified juice. Addition of 50 mg S02/liter (after determination of free and bound SOz). Addition of 25 mg SOz/liter.

that the unfermented juice has considerable SO, binding power. By the end of fermentation, concentrations of both free and bound SO, remaining in the wine are negligible. When 275 mg SO,/liter is added, the free SO, concentration in the wine has been raised to the desired level for bottling (30 mg/liter). According to their practice it was better to adjust the free SO, content gradually over a 6-8week period, thus allowing adequate time for settling and clarification. After bottling the free SO, concentration decreased while bound SO, concentration increased further (Table VI). Heatherbell et al. (1980) indicated that ascorbic acid was relatively stable during juice extraction and vinification in the presence of SO, (Table VI), and wine at bottling retained approximately 75% of the total ascorbic acid (ascorbic and dehydroascorbic acid) that was present in the press juice. The results showed that low concentrations of SO, can protect ascorbate (Fig. 10). In contrast, high concentrations of SO, may destroy ascorbate. The quality of the wines made with and without the use of SO, was investigated by Heatherbell et al. After 1 week’s storage the two sets of wines were similar in aroma and taste. After 14 weeks’ storage although the aroma of both wines was still described as very fruity, the aroma difference was more pronounced. After 20 weeks’ storage, wine made with SO, still had a fresh fruity “Riesling Sylvaner-type” aroma and a good color, whereas the wine made without SO, lost its fruitiness and developed an oxidized flavor and excessive “brown color” (Table VII).

305

B. S . LUH AND ZHANG WANG

Ferrn+-

Celar,-

-

Storage in bottle (15°C)

70

5

10

20

15

Time (weeks)

FIG. 10. Changes in ascorbic acid content during vinification and storage of kiwifruit wines made with and without the addition of SOz. Total ascorbic acid (includes 20 5 mglliter dehydroascorbic acid) for (A) wine made with SOz and (B) wine made without SOz. From Heatherbell et al. (1980).

*

TABLE VII CHANGES IN COLOR (OD42o) AND SO2 DURING STORAGE OF KIWIFRUIT WINE MADE WITH AND WITHOUT SO*" Storage time (15°C. in bottle) Treatment

1 week

Wine + SOzb OD420 Free SOz (mg/Iiter) Bound SO2 (mg/liter) Wine - SOz= OD42O

0.071 28 67 0.123

14 weeks

20 weeks

0.666

0.079 9 82

11

19

0.275

0.318

Source: Heatherbell et al. (1980). Values are average results for duplicate analyses. Wine processed as described in Table V. Wine processed as for b, except no SOz was used at any stage of the process.

306

KIWIFRUIT

G.

KIWIFRUIT LEATHER

Leather made from kiwifruit pulp was studied by Lodge (1981). A flow diagram of kiwifruit leather production is shown in Fig. 11. The relationship of drying temperature, drying time, the presence of SO,, and the leather quality was investigated by Lodge (1981). Drying at 70°C resulted in a fast drying time, but the product lost all the distinctive green pigmentation in the flesh. The sample dried at 45°C in the presence of SO, had a color closer to that of the fresh pulp than the one dried without SO,. The product dried at 70°C 'had some odd taste described as "catch" in the throat after ingestion. This effect could be eliminated by making a blend leather of 75% apple pulp and 25% kiwifruit.

H.

CANDY

Simmons ( 1978) reported that kiwifruit slices could be successfully candied. The peeled kiwifruit slices of 10 mm thickness were treated with five syrup concentrations until 72"Brix was reached. The fruit slices were brought to the boiling point at each concentration change. After the syrup treatments the fruit slices were briefly washed in hot water and dried in air at 40°C until they were no longer sticky. The resulting product had good texture and taste, but the slices

6 Finishing screen.700pm

------------- Apple pulp, 75%

T Trays

Tunnel drier (45"C115 hr)

FIG. 11.

Flow diagram of kiwifru&.leatherDmdmtbL FranLodee (1981).

B . S. LUH AND ZHANG WANG

307

darkened during candying and turned yellow-brown in color. The process can be modified with only three changes of syrup to reach 72”Brix. At each change, the syrup was heated to 75°C for 20 min, poured over the fruit, and allowed to cool. The slices were dried as described above. There was marked improvement in retention of green color in the slices. Dawes (1972) mentioned that kiwifruit can be made into jam that had a golden green color and a distinctive flavor. Kiwifruit jam blends were also made with citrus such as orange, mandarin, and lemon. More work is needed to improve the quality of candy, jam, and preserves made from kiwifruit.

ACKNOWLEDGMENT The authors thank Professors Emil M. Mark and Bernard S. Schweigert of the University of California, Davis for their guidance and suggestions in preparing this review. We appreciate the assistance of James A. Beutel, Adel A. Kader, F. Gordon Mitchell, Harlan K. Pratt, Michael S. Reid, Kay Ryugo, and Noel F. Sommer in supplying pertinent information on kiwifruit.

REFERENCES Anonymous. 1977. Conseils pratiques sur la culture due kiwifruit aprks dix ans d’expbrimentation au domaine de Ribebon en Gironde. Conditions favourables a la culture due kiwi. Arboric. Fruit. 283, 48-57. Arpaia, M. L. 1982. The ethylene problem in modified atmosphere storage of kiwifruit. In “Controlled Atmospheres for Storage and Transport of Perishable Agricultural Commodities” (D. G. Richardson and M. Meheriuk, eds.), Symp. Ser. No. 1, pp. 331-335. Timber Press, Beaverton, Oregon. Astridge, S. J. 1975. Cultivars of Chinese gooseberry (Actinidin chinensis) in New Zealand. Econ. Bor. 29, 357-360. Baker, E. N. 1977. Structure of actinidin: Details of the polypeptide chain conformation and active . resolution. J . Mol. Biol. 115, 263-277. site from an electron density map at 2.8 & Beraha, L. 1970. Stem-end rot of Chinese gooseberries (Actinidin chinensis) on the market. Plant L)is. Rep. 54, 422-423. Beutel, J. A,, Winter, F. H., Manners, S. C., and Miller, M. W. 1976. A new crop for California: Kiwifruit. Calif. Agric. 30(10), 5-7. Brocklehurst, K., Baines, B. S., and Malthouse, J. P. G. 1981. Differences in the interactions of the catalytic groups of the active centres of actinidin and papain. Biochern. J . 197, 739-746. Came, A , , and Moore, C. H. 1978. The amino aciol sequence of the tryptic peptides from actinidin, a proteolytic enzyme from the fruit of Actinidin chinensis. Biochern. J. 173, 73-83. Cook, A. A. 1975. “Diseases of Tropical and Subtropical Fruits and Nuts.” Hafner, New York. Curlee, D. 1970. Kiwi, the ugly visitor from “Down Under” finds a new home in the west. West. Fruir Grower 24(8), 8-14. Dawes, N. 1972. Processing potential and composition of New Zealand subtropical fruits. Food Technol. N . Z. 7(4), 22-27.

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El-Zalaki, E. M., and Luh, B. S. 1981. Effect of sweetener types on chemical and sensory quality of frozen kiwifruit concentrates. Food Chem. 6, 295-308. Ferguson, I. B. 1980. Movement of mineral nutrients into the developing fruit of the kiwifruit (Actinidiu chinensis Planch). N . Z . J. Agric. Res. 23, 349-353. Fletcher, N. A. 1971. Growing Chinese gooseberries. N. Z. Dep. Agric., Bull. 349. Fletcher, W. A,, Baumgart, M. G., and Piper, B. M. 1959. Handling Chinese gooseberries for long storage and export. N. Z. J. Agric. 99(1), 15-16. Ford, I., 1971a. Chinese gooseberry pest and disease control. N. Z. J. Agric. 122(3), 86-89. Ford, I. 1971b. Chinese gooseberry production in France, N. Z. J . Agric. 123(4), 20-21. Greig, A. M. W. 1953. Ecological factors affecting the commercial production of certain subtropical fruits in New Zealand. Rep. Int. Hortic. Congr. 13th, 1952, Vol. 2, pp. 1191-1 199. Harris, C. M., Covey, H. M., and Harvey, J. M. 972. Effect of harvest time, storage period and ripening time on the quality of Chinese goosebemes. U.S.Dept. Agric., Murk. Res. Rep. 940. Harris, S. 1976. The refrigerated export chain of kiwifruit (Actinidiu chinensis) from New Zealand. Annexe Bull. Inst. Inr. Froid 1976-1, 157-164. Harris, S., and McDonald, B. 1975. Physical data for kiwifruit (Actinidia chinensis). N. 2. J . Sci. 18(3), 307-312. Heatherbell, D. A. 1975. Identification and quantitative analysis of sugars and non-volatile organic aciols in Chinese gooseberry fruit (Actinidiu chinensis Planch). J . Sci. Food Agric. 26, 8 15820. Heatherbell, D. A , , Struebi, P . , Eschenbruch, R., and Withy, L. M. 1980. A new fruit wine from kiwifruit: A wine of unusual composition and Riesling Sylvaner character. Am. J. Enol. Vitic. 31(2), 114-121. Hopping, M. E. 1976a. Structure and development of fruit and seeds in Chinese gooseberry. N. Z. J. Bot. 14, 63-68. Hopping, M. E. 1976b. Effect of exogenous auxins, gibberellins, and cytokinins on fruit development in Chinese gooseberry (Actinidiu chinensis Planch). N . Z . J. Bot. 14, 69-75. Lodge, N. 1981. Kiwifruit: Two novel processed products. Food Technol. N. Z . 16(7), 35-41. Love, J. L., and Ferguson, A. M. 1977. Pesticide residue and greedy scale control in kiwifruit. N. Z. J . Agric. Res. 20(1), 95-103. McDowall, M. A. 1970. Anionic proteinase from Actinidiu chinensis preparation and properties of the crystalline enzyme. Eur. J. Biochem. 14, 214-221. McDowall, M. A. 1973. The action of proteinase A, of Actinidiu chinensis on the P-chain of oxidized insulin. Biochim. Biophys. Acru 293, 226-231. McDonald, B., and Harman, J. E. 1982. Controlled atmosphere storage of kiwifruit Acrinidiu chinensis cultivar Hayward. 1. Effect on fruit firmness and storage life. Sci. Hortic. (Amsterdam) 17(2), 113-124. Matsumoto, S . , Obara, T., and Luh, B. S. 1983. Changes in constituents of kiwifruit during postharvest ripening. J . Food Sci. 48, 607-61 1. Miller, T. 1975. Export potential of kiwifruit. N. Z. J . Agric. 131(2), 20-21, 35, 37. Okuse, A., Okuse, I., and Ryugo, K. 1981. Effects of certain processing methods substrate level and polyphenol oxidase on the stability of ascorbic acid in kiwifruit Actinidiu chinensis cultivar Hayward. Okuse, I., and Ryugo, K. 1981. Compositional changes in the developing kiwifruit Acrinidiu chinensis ‘Hayward’ in California, USA. J. Am. SOC. Hortic. Sci. 106(1), 73-76. Padfield, C. A. S., and Bailey, F. L. 1952. Chinese gooseberries (Actinidiu chinensis): A survey of their behaviour in cold-storage at all stages of harvest maturity from May to July. N. Z. J. Sci. Technol. 33(5), 113-116. Pratt, H. K., and Goeschl, J. D. 1969. Physiological role of ethylene in plants. Annu. Rev. Plunt Physiol. 20, 541-584.

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Pratt, H. K., and Reid, M. S. 1974. Chinese gooseberry: Seasonal patterns in fruit growth and maturation, ripening, respiration and the role of ethylene. J . Sci. Food Agric. 25, 747-757. Reid, M. S. 1977. New kiwifruit tray liners this season. Orchardisr N. 2. 50(3), 79. Reid, M. S . , Heatherbell, D. A,, and Pratt, H. K. 1982. Seasonal patterns in chemical composition of the fruit of Actinidia chinensis cultivar Bruno. J . Am. SOC. Hortic. Sci. 107(2), 316-319. Robertson, G. L., and Swinburne, D. 1981. Changes in chlorophyll and pectin after storage and canning of kiwifruit. J. Food Sci. 46, 1557-1559. Schroeder, C. A,, and Fletcher, W. A. 1967. The Chinese gooseberry (Actinidia chinensis) in New Zealand. Econ. Bor. 21, 81-92. Simmons, I. D. 1978. Drying and candying of Chinese gooseberries. Food Technol. Ausr. 30(6), 236-239. Sommer, N. F., and Beraha, L. 1975. Diaporthe actinidiae. a new species causing stem-end rot of Chinese gooseberries. Mycologia 67, 650-653. Sozzi, A , , Zerbini, P. E., and Gorini, F. L. 1976. Epoca ottimale di raccolta e conservabilita dei lruitti di Actinidia. Ifal. Agric. 113(10), 120-125. Strachan, G . 1970. Storage of Chinese gooseberries under adverse conditions. Orchardist N. Z. 43(1). 32-35. Strachan, G. 1974. Processing and storage of subtropical fruits. Food Technol. N. 2. 19(7), 19-23. Thompson, A. K. 1982. “The Storage and Handling of Kiwifruit Tropical Products Institute. Publ. No. G159 56/62 Gray’s Inn Road, London. U.S. Department of Agriculture. 1982. U.S. Standards for grades of kiwifruit. Fed. Regist. 47(154), 345 13-345 15. Voordouw, G., Gaucher, G. M., and Roche, R. S. 1974. Anomalous molecular weights of proteases in gel chromatography. Biochem. Biophys. Res. Commun. 58, 8- 12. Wildman, T., and Luh, B. S. 1981. Effect of sweetener types on quality and composition of canned kiwifruit nectars. J . Food Sci. 46(2), 387-390. Wilson, E. L., and Bums, D. J. W. 1983. Kiwi juice processing using heat treatment techniques and ultrafiltration. J . Food Sci. 48(4), 1101-1105. Wilson, S. J . . and Richards, R. R. 1973. The utlization, cultivation, harvesting, packing and storage of Chinese goosebemes. Tasmanian J . Agric. 44(4), 254-256. Wolstenholme, B. N . , and Allen, P. 1973. The cultivation of the Chinese gooseberry. Fruit World Market Grower April, pp. 22-23. Wright, H. B., and Heatherbell, D. A. 1967. A study of respiratory trends and some associated physiochemical changes of Chinese gooseberry fruit Actinidia chinensis (Yang-tao) during the later stages of development. N. 2. J. Agric. Res. 10, 405-414.

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A Actinidin, in kiwifruit, proteolytic activity, 297-298 Aldehydes, in smoke condensate, effect on color formation, 137- 139 Amides, in flour gluten, effect on baking, 265-267 Amino acids, sulfur-containing in flour gluten, effect on baking, 267-268 a-Amylase, in wheat flour, effect on baking, 205-206 Antioxidants, inhibition of warmed-over flavor in cooked meat, 44-46 Aromatic compounds bicyclic in curing smoke, 134 monocyclic non-phenolic in curing smoke, 132, 134 polycyclic hydrocarbons, see Polycyclic aro. matic hydrocarbons (PAH) Ascorbic acid inhibition of warmed-over flavor development in cooked meat, 41, 44 in luwifruit juice during vinification, 303305 in meat curing color stabilization, 14 flavor and odor improvement, 15 N-nitrosamine formation blocking, 14- 15 59-60 long-chain acetals, inhibition of N-nitrosamine formation in cured meat, 60 Ascorbyl palmitate inhibition of N-nitrosamine formation in cured meat, 60

B Bacon flavor, nitrite effect, 24-26

liquid smoke-treated, phenol composition, 144 Bacteria, see also specific bacteria inhibition by nitrite and nitrate during meat curing, 13-14, 46-48 lactic acid-producing, antibotulinal activity in cured meat, 55-56 Baking, see also Breadmaking, Dough wheat flour quality for, 201-271 approaches to research, 214-216 evaluation, 204-216 limiting variables, 204-206 Baking test, bread, 206-21 1 cake and cookies, 21 1 small-scale rapid Residue Protein test, 213-214 SDS-Sedimentation test, 2 13-2 14 Benzo[a]pyrene in curing smoke, 92, 135, 140-141, 147149 smoked meat product contamination, 147149 Bread baking tests, 206-21 1 loaf volume, effect of lipids in flours, 251258 Breadmaking effect of wheat flour composition proteins, 246-251 starch, 262 water solubles, 261 Breeding wheat for flour baking quality, 211-213

C Cakes baking tests, 21 1 quality, effect of lipids in wheat flours, 260-261

311

312

INDEX

Candies, from kiwifruit peeled slices, 306-305 Canning kiwifruit effect on chemical composition, 294, 299 technique, 299-300 Carboxymethylcellulose solution viscosity instrumental readings, 183 perceived, 183 Cell growth, during kiwifruit growth and ripening, 288-289 Cellulose, pyrolysis, phenol production, 123 Chelating agents, inhibition of warmed-over flavor in cooked meat, 44-45 Chewing, see Mastication Chlorophyll, in kiwifruit after storage and canning, 295 Clostridium botulinum inhibition in cured meat by fumarate esters, 52 lactic acid-producing bacteria, 55-56 nitrate, 13-14 nitrite, 13-14, 46-48 radiation, 52, 55 sodium hypophospite, 52 sorbic acid, 49-52 Clostridium sporogenes in meat, inhibition by nitrite and nitrate during curing, 13-14 Cookies baking tests, 21 1 quality, effect of lipids in wheat flours, 259-260 Crown rot (Phytophtoru sp.), kiwifruit killing, 285 Cured meat antibotulinal activity of fumarate esters, 52 lactic acid-producing bacteria, 55-56 nitrate, 13-14 nitrite, 13-14, 46-48 sodium hypophosphite, 52 sorbic acid, 49-52 research needs, 70-72 flavor evaluation, 33 N-nitrosamine formation, 57-58 reduction by blocking agents, 58-61 nitrite-ascorbate combination, 61-62

smoking, 21-23, see also Meat smoking antioxidant activity, 23 bacteriostatic action, 23 color development, 22 flavor development, 22-23, 33 volatile compounds, nitrite effect, 64-70 Curing smoke aldehydes in condensate, color formation, 137- I39 antioxidative, phenolic compounds, 145 application, 98- 100 from beech wood, phenol composition, 124-127 benzo[a]pyrene, 92, 135, 140-141, 147149 chemical composition, 104-135 analytical methods, 109-1 12 of condensate, 106-107 from different wood species, 108 electrostatically treated or untreated, 140141 for hot smoke processing, 107 condensation laboratory equipment for, 102- 103 technical equipment for, 103 formaldehyde formation, toxic effects, 148149 generation pyrolysis of wood, 93-98 from smoldering wood, 92-95 wood materials for, 90-92 identified compounds alicyclic, 117-1 18 aromatic, 118-135, see also Phenols monocyclic non-phenolic, 132, 134 heterocyclic, 115-1 17 lysine loss induction in meat, 146-147 phenols, see Phenols physical properties, 89-90 polycyclic aromatic hydrocarbons, 99, 101, 104-106, 110, 112-113, 134-136, 140, 147-148, 150 preparations liquid and solid, application, 100- 102 production, patented methods, 103-104 secondary skin formation in fish and meat products, 136 Cysteine dough development and, 231, 234

313

INDEX

D Dehydroascorbic acid, in kiwifruit juice during vinification, 303-305 Dextrose, in meat curing, 6-7 Dough development, effect of cysteine, 231, 234 loaf score of energy input and intermediate proof time combination, 238-239 mixing intensity, 230-234 oxidants, 237-238 passages through sheeting rolls, 238-241 protein in flour, 243-246 sodium metabisulfite, 234, 238 unmixing and remixing, 234-237 Dough mixers mechanic and mixing efficiencies, 23 1-234 Dry distillation of different woods, products, 93-94

E Erythorbic acid inhibition of N-nitrosamine formation in cured meat, 59 in meat smoking, 32-33 Ethylamine, reaction with glycolaldehyde, color formation by smoking, 138139 Ethylene, production by mature kiwifruit, 291

F Fatty acids, free in wheat flours, breadmaking and, 253, 255-257 Fertilization, in kiwifruit cultivation, 284 Fish, smoking secondary skin induction, 146 Food texture definibon, 162 dimensions, 176- 179 effect on masticatiw, 171- 173 salivation, 173-175 fracturability, 190- 194 auditory cues, pot essential for perception, 193-194 crushing sounds, 192- 194 instrumental measures, 191 vibratory sensations, 191-192

hardness, 185- 189 force-compression functions, 186- 188 instrumental and sensory measures, 187189 oral cavity responses to, 170-175 perception, see also Oral cavity complexity of sensations, 159- 162 future research needs, 194-195 roughness, 189-190 non-oral perception, 189- 190 oral perception, 190 viscosity, 179-185 instrumental and sensory values, 184- 185 shear stress-shear rate for samples, 180185 Formaldehyde, in curing smoke, toxic effects, 148-149 Friction smoke, steam-volatile and -nonvolatile fractions, 107-108 Frankfurters, flavor, nitrite effect, 29-30, 3233 Frost, kiwifruit damage, 283 Fumarate esters, antibotulinal activity in cured neat. 52

G Gliadin in wheat flours breadmaking and, 248, 251 composition, effect on baking quality, 268 dough properties and, 242-246 Gluten in wheat flours acid-soluble and -insoluble fractions, 249-25 1 amide content, effect on baking quality, 265-267 breadmaking and, 246-25 1 dough properties and, 242-251 fractionation and reconstitution, 224-226 gliadin, see Gliadin glutenin, see Glutenin interchanging between two samples, 228230 -lipid interaction, effect on baking quality, 263-264 molecular weight distribution, effect on baking quality, 265

314

INDEX

Gluten (continued) separation, 222-224 sulfur-containing amino acids, effect on baking quality, 267-268 Glutenin in wheat flours breadmaking and, 248, 251 dough properties and, 243-246 subunits, effect on baking quality, 268269 Glycolaldehyde, reaction with ethylamine in color formation by smoking, 138139 Glycolipids in wheat flours breadmaking and, 253, 257 cookie quality and, 259

H Ham nitrite effects on flavor, 26-29 volatile compounds, 65-70 smoked, phenol composition, 133 Hardwood lignin pyrolysis, 121-122 phenol composition, 127 Heme compounds, as catalysts of iipid oxidation in meat. 37-38 1

Iron, nonheme, warmed-over flavor development in cooked meat and, 39-40 Irrigation, in kiwifruit cultivation, 283 lsoascorbic acid in meat curing color stabilization, 14 N-nitrosamine formation blocking, 14

.I Jam, from kiwifruit, 307 Joints, temperomandibular, oral cavity sensitivity to position of, 169-170 Juice from kiwifruit chemical changes during vinification, 303-305 production, 301-302

K Kiwifruit (Actinidiu chinesis) biology and vineyards, 279-280 canning of peeled fruits, 299-300 chemical composition of fruits, 294-298 actinidin, proteolytic activity, 297-298 chlorophyll after storage and canning, 295 fresh, frozen, and canned, 294 organic acids, 294-297 Sugars, 294-295 cultivation diseases, 284, 287 fertilization, 284 frost damage, 283 irrigation, 283 pruning, 284-285 training, 284-285 wind sensitivity, 283 growth and ripening, 288-294 cell growth during, 288-289 chemical changes during, 291-294 ethylene production during, 291 fruit weight-seed numbers in cultivars, 293 physiological changes during, 290-292 respiration during, 288, 290-291 harvesting, 285-286 low-salt water requirement, 282 packing, 286-287 processing, 298-307 drying, effect of slice thickness, 30030 1 freezing preservation of slices, 300 peeling, 298-299 production of candy from peeled slices, 306-307 jam, 307 juice, procedure and vinification, 301305 leather from fruit pulp, 306 wine, procedure and composition, 302306 production areas, 282 propagation container plants, 281-282 grafting seedlings, 28 1 rooting cuttings, 281-282 soil needs, 282 standards, grades, 287-288

INDEX

L Leather, from kiwifruit pulp, 306 Lignin, pyrolysis, phenol production, 121- 123 Lipid oxidation in meat, 35-46 catalysis by heme compounds, 37-38 metal ions, 38-39 mechanism, 36-37 Lipids carbonyl compounds in country-style cured ham, 67 in meat, 33-39 composition, 33-35 oxidation, 35-46, see Lipid oxidation in wheat flours, 217-222 analysis, 221-223 effect on baking quality, 251-260 bread, 251-258 cakes, 260-261 cookies, 259-260 extraction, 217-218 fractionation, 221 functionality, 218-220 nonpolar fraction breadmaking and, 253-256, 258 cookie quality and, 259 polar fraction breadmaking and, 253-256 cookie quality and, 259 -protein interaction, effect on baking quality, 263-264 Looper worms, kiwifruit damage, 284 Lysine, smoking-induced losses in meat, 146147

M Maillard reaction, products as antioxidants inhibiting warmed-over flavor in cooked meat, 45-46 Mastication crushing sounds during, 192-194 effect of food texture, 171-173 Meat cooked flavor development, chemistry, 62-64 warmed-over flavor development, 35-36 39-41 inhibition by curing and natural antioxi-

315

dants, 41-46 lipids composition, 33-35 oxidation, 35-46 catalysis, 37-39 mechanisms, 36-37 pigments, changes during curing, 8-12 nitrate effect, 10-12 nitrite effect, 10-12 smoking effects, see Meat smoking Meat curing, 1-86 by ascorbic acid color stabilization, 14 flavor development, 15, 32 N-nitrosamjne formation blocking, 14- 15, 59-60 history, 2 by isoascorbic acid color stabilization, 14 N-nitrosamine formation blocking, 14 methods, 16-21 artery pumping, 18-19 conventional dry, 17 dry salt, 17 pickle, 18 stitch pumping, 19-20 thermal, 20 Wiltshire, 20-21 by nitrate, 7-17 bacteriostatic action, 13-14 color stabilization, 7-8, 10-12 flavor production, 11-13 by nitrite bacteriostatic action, 13- 14, 46-48 color stabilization, 7-8, 10-12 flavor production, 11-13 by nitrite alternatives, evaluation, 56 by phosphates color improvement, 16 flavor development, 32 water binding increase, 15-16 by radiation as nitrite alternative, 52-55 by sodium chloride antioxidation-prooxidation, 3-5 flavor development, comparison with nitrite, 30-32 by sodium erythorbate, 32-33 by sugars, 6-7 by sweetening agents, 6-7 Meat smoking, 87-158, see also Cured meat, smoking

316

INDEX

Meat smoking (continued) carcinogenic polycyclic aromatic compound increase, 147-149 color formation, 137- 139 flavoring, 139-144 formaldehyde formation in smoke, 148- 149 history, 87-89 lysine loss, 146-147 mutagenic phenol formation, 150 preservation, 144-146 secondary skin formation, 146 technology, 90-102 wood materials, 90-92 Metal ions, as catalysts of lipid oxidation in meat, 38-39 Myoglobin in meat during curing, 10-12 nitrosochromogen production, 10- 12 structure and properties, 8-1 1 warmed-over flavor development in cooked meat and, 39-41

N Nematodes kiwifruit damage, 284 Nitrate in meat curing, 7- 17 bacteriostatic action, 13-14 color stabilization, 7-8, 10-12 flavor production, 11-13 myoglobin conversion to nitrosochromogen, 10-12 Nitrite -axorbate combination, reduction of N-nitrosamine formation in cured meat, 61-62 bacon flavor and, 24-26 frankfurter flavor and, 29-30, 32-33 ham flavor and, 26-29 inhibition of warmed-over flavor development in cooked meat, 41-44 in meat curing bacteriostatic action, 13-14, 46-48 color stabilization, 7-8, 10-12 effect on volatile compounds, 64-70 flavor production, I 1- 13 comparison with sodium chloride, 30-32 myoglobin conversion to nitrosochromogen, 10- 12

substitution by alternatives, evaluation, 56 fumarate esters, 52 lactic acid-producing bacteria, 55-56 radiation, 52-55 sodium hypophosphite, 52 sorbic acid, 49-52 N-Nitrosamines formation in cured meat mechanism of, 57-58 reduction by ascorbate and its isomeres, 14-15, 5960 ascorbic acid long-chain acetals, 60 ascorbyl palmitate, 60 nitrite-ascorbate combination, 61 -62 a-tocopherol, 60-61 Nitrosochromogen conversion from myoglobin during meat curing, 10-12

0 Oak root fungus (Armillaria mellea) kiwifruit killing, 284 Oral cavity anatomy, 163-164 food responses, 170- 175 sensitivity to pain, 167-168 temperature, 168-169 temperomandibular joint position, 169170 touch pressure, 166-167 somesthetic receptors, 164-166 Organic acids in kiwifruit composition, 296 fresh, frozen, canned, 294 during growth and ripening, 291-292 seasonal changes, 295-297 Oxidants dough development and, 237-238

P PAH, see Polycyclic aromatic hydrocarbons Pain oral cavity sensitivity to, 167-168

317

INDEX

Pectin in kiwifruit after storage and canning, 295 Phenols in beech smoke, 124-127 temperature effect, 124-125 classification and structure, 118-121 in curing smoke antimicrobial effect as antioxidants, 144146 content, 123-127 effect on flavor, 140-144 mutagenic activity, 150 formation in curing smoke from cellulose pyrolysis, 123 from lignin pyrolysis, 121-123 temperature effects, 124-125, 140 in liquid smoke preparations, 127-130 in smoke and smoked ham, 133 in smoked products, 130-132 in solid smoke preparations, 130 Phosphates inhibition of warmed-over flavor development in cooked meat, 41, 44 in meat curing color improvement, 16 flavor development, 32 water binding increase, 15- 16 Phospholipids warmed-over flavor development in cooked meat and, 39-41 in wheat flours breadmaking and, 253, 257 cookie quality and, 259 Polycyclic aromatic hydrocarbons (PAH) in curing smoke, 99, 101, 104-106, 110, 112, 113, 117, 134-136, 140, 147148, 150 smoked meat product contamination, 147149 Proteins in wheat flours breadmaking and, 246-25 1 composition, 24 1-243 dough properties and, 243-246 gluten, see Gluten -lipid interaction, effect on baking quality, 263-264 nongluten, 242 Pyrolysis of cellulose, phenol formation, 123

of lignin, phenol formation, 121-123 in wood during curing smoke generation, 93-98

R Radiation in meat curing as alternative to nitrite, 5255 Respiration in kiwifruit during growth and ripening, 288. 290-291

S Salivation physiology, food texture effect, 173-175 Sausages curing, methods, 21 SDS, see Sodium dodecyl sulfate Sensory system for food texture perception, 160- 162 Sheeting rolls in dough development, 238-241 Smoke curing, see Curing smoke Smoke generators fluidization, 97 friction, 96 with heat supply, 95-96 laboratory according to T6th, 98 for isothermal smoke development, 97-98 two-stage, 97 steam, 96 Smoked products chemical composition, 104- 135 analytical methods, 109- 112 bologna layers, 109 compounds identified from smoke, see Curing smoke, identified compounds phenol content, 130- 132 Smoldering sawdust steam-volatile and -nonvolatile fractions, 107-108 Sodium chloride in meat curing, 3-5 flavor development, comparison with nitrite, 30-32

318

INDEX

Sodium dodecyl sulfate (SDS) -Sedimentation baking test, 213-214 Sodium erythorbate, see Erythorbic acid Sodium hypophosphite antibotulinal activity in cured meat, 52 Sodium metabisulfite dough development and, 234, 238 Softwood lignin pyrolysis, 121-122 phenol composition, 127 Solutions viscosity, three types of flow, 180 Somesthetic receptors in oral cavity, 164-166 Sorbic acid antibotulinal activity in cured meat, 49-52 mixtures with nitrite, 51-52 Sounds crushing, role in food fracturability perception, 192-194 Starch in kiwifruit during growth and ripening, 29 1-292 in wheat flours breadmaking and, 262 fractionation, 227-228 interchanging between two samples, 228230 separation, 222-224 Sugars in kiwifruit composition during ripening, 295 fresh, frozen, canned, 294 during growth and ripening, 291-292 in meat curing, 6-7 Sulfur dioxide in kiwifruit juice during vinification, 303305 Sweetening agents in meat curing, 6-7

T

Temperature effect on phenol formation in curing smoke, 124-125, 140 perception by oral cavity, 168-169

a-Tocopherol inhibition of N-nitrosamine formation in cured meat, 60-61 Touch pressure oral cavity sensitivity to, 166-167

v Vinification of kiwifruit juice, chemical changes during, 303-305 Volatile compounds in cured meat, nitrite effect, 64-70 W Water addition to wheat flour, effect on baking, 205 Water solubles in wheat flours breadmaking and, 261 fractionation, 226-227 interchanging between two samples, 228230 separation, 222--224 Wheat breeding for flour baking quality, 2 1 1-2 13 varieties, gliadin composition in flours, 268 Wheat flours baking quality, 201-271 approaches to research, 214-216 breeding for, 21 1-213 component interchanging between two samples, 228-230 conclusions and future research, 269-27 1 evaluation, 204-216 gliadin composition and, 268 gluten amidation and, 265-267 gluten molecular weight distribution and, 265 glutenin subunits and, 268-269 limiting variables, 204-206 protein-lipid interaction and, 263-264 sulfur-containing amino acids in gluten and, 267-268 defatted, functionality, 220-221 gluten fractionation and reconstitution, 224-226 separation, 222-224

INDEX

lipids, see also Lipids breadmaking and, 251-258 cake quality and, 260-261 cookie quality and, 259-260 extraction and properties, 217-222 starch breadmaking and, 262 fractionation, 227-228 separation, 222-224 water solubles breadmaking and, 261 fractionation, 226-227 separation, 222-224

319

Wine from kiwifruit juice, 302-306 WOF (warmed-over flavor), see Meat, cooked, warmed-over flavor development Wood materials for curing smoke generation, 90-92 effect on food flavoring, 139-140 main constituents, 90-91 pyrolysis during, 93-98 smoldering, 92-95 dry distillation products, 93-94 smoke compounds from various species, 107- I08

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