Advances in Food Research, Volume 6

ADVANCES IN FOOD RESEARCH VOLU,ME V I

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ADVANCES IN FOOD RESEARCH VOLUME VI Edited by

E. M. MRAK

G. F. STEWART

University of California Davis, California

University of California Davis, California

Editorial Board E. C. BATE-SMITH

S. LEPKOVSHY

W. H. COOK

B. E. PROCTOR

W. F. GEDDES

EDWARD SELTZER

M. A. JOSLYK

P. F. SHARP

A. J. KLUYVER

W. M. URBAIN 0. B. WILLIAMS

1955

ACADEMIC PRESS INC., PUBLISHERS NEW YORK, N. Y.

Copyright 1955, by ACADEMIC PRESS INC. 125

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CONTRIBUTORS TO VOLUMEVI JOHNC. AYRES,Food Processing Laboratory, Iowa Agricultural Experimental Staiion, Iowa State College, Ames, Iowa GEORGBORGSTROM, Swedish Institute for Food Preservation Research ( S I K ) , Goteborg, Sweden

H.T. H. FARRER, Research Laboratories, Kraft Foods Ltd., Melbourne, Australia

W. 0. HARRINGTON, Western Utilization Research Branch, Agrzcultural Research Service, U . S. Department of AgrLculture, Albany, California P. W. KILPATRICK, Western Utilization Research Branch, Agricul..ural Research Service, U . S . Department of Agriculture, Albany, California E. LOWE, Western Utilibation Research Branch, Agricultural Research Service, U . S. Department of Agriculture, Albany, California L. F. MARTIN, Sugarcane Products Division, Southern Regional Research Laboratory, U . S . Department of Agriculture, New Orleans, Louisiana

R. L. OLSON,Western Utilization Research Branch, Agricultural Research Serrice, U . S. Department of Agriculture, Albany, California W. B. VAN ARSDEL, Western Utilization Research Branch, Agricultural Research Seruice, U. S. Department of Agriculture, Albany, California REESEH. VAUGHX,Department of Food Technology, University of California, Davis, California

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FOREWORD I n keeping with the editorial policy of Advances in Food Research, seven articles covering five commodity and three functional areas are included in this volume. The contributions are exhaustive, critical, integrating, and of fundamental importance t o the development of food research in the wide areas of the food industries. I n general, they are concerned with microbiology, processing, retention of nutrients, and engineering. Several of the reviews cover rather neglected areas and one presents a new and interesting analysis of the much discussed subject of vitamin destruction. A classical example of a neglected area is that concerned with applications of research t o problems of candy manufacture. Although the candy industry in the United States involves the use of approximately 1% million tons of materials annually, candy manufacture has developed as an art and continues primarily as an art, employing “rule of thumb’’ methods. Research in this field is in its infancy, although it is being conducted t o a n increasing extent in industry and research institutions. Nevertheless, most of the information applicable t o the field has been developed as a result of research in related fields, hence pertinent literature is widely scattered in a great variety of journals and public,ations. I n his review, Dr. Martin has brought together this widely distributed information in a n excellent manner, has outlined clearly the wide diversity and complexity of the fundamental problems of candy manufacture, and shows the great opportunities for fundamental research in this field. The review by Vaughn on the “Bacterial Spoilage of Wines” is timely, since his interpretations clarify many points of confusion. Although the wine industry is a very old one, of importance in many countries throughout the world, confusion exists concerning nature, causes, and nomenclature of microbial spoilage of wines. Taxonomic interpretations frequently have been in error, and in many cases have persisted over a period of many years. Furthermore, common terminology for the microbiological spoilage of wines is often inappropriate although it has persisted t o a large extent because of the historical background of this field. It is, therefore, most appropriate t h a t Dr. Vaughn should review the literature of the last 100 years in a critical manner and interpret existing concepts in light of recent knowledge. I n previous volumes of Advances in Food Research, the physioIogy vii

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Vlll

FOREWORD

and chemistry of rigor mortis with special reference t o the aging of beef and the oxidative rancidity and discoloration in meat were considered. The review by Ayres on the microbiology of meat animals covers a third area of scientific problems of primary interest t o this large industry. Meat provides many, if not all, nutrients desired by most microorganisms. Preventing their growth on meat, therefore, is a serious and important problem t o the industry. The load of organisms on the living animal contributes its share t o the total count on the carcass. This count, however, also depends on factors involving the method of handling, the interrelationship between defensive mechanisms of the animal, and the enormous microbial populations which gain access t o it. Dr. Ayres has discussed the significance and sources of microbial contamination with respect t o the animal defenses against these invasions, ante-mortem changes in microbial populations, death agonies, and after death. Dr. Ayres appropriately concludes his review by pointing out the needs for further research and suggesting certain improvements in processing. Although certain frozen foods have been on the market for a number of years, the industry at present is involved in the production of greater quantities and more diversified products than ever before. This is particularly true of frozen precooked foods, the production of which is increasing at a spectacular rate. Microbial problems are always present, insofar as frozen foods are concerned, but are particularly important now because of the great variety of frozen cooked foods. Hence, Borgstrom’s inclusive review on the microbiology of frozen foods (except ice cream) is timely and in keeping with processing advances. I n Volume I of Advances i n Food Reseurch, a chapter was devoted t o the deterioration of processed potatoes. At that time, specific reference t o the processing and stability of any particular potato product was avoided, because of the need for a general discussion of the stability of all potato products. Since then, however, a large amount of work has been done on the development of potato granules. The production of potato granules coincides with the present trend toward producing foods which are easy to use, the so-called “instant ” foods. The review by Olson and Harrington is inclusive and brings together all the literature on this subject in a critical and useful manner. A very extensive literature exists relating to the destruction or stability of Vitamin B1 during cooking, processing, and handling of various food commodities. The destruction (or retention) of this vitamin is of great importance because of nutritional implications. In spite of the vast literature however, Farrer appears t o be the first t o attempt t o relate results of vitamin B1 destruction t o other food stuffs or t o any particular set of conditions. I n his review, Farrer has considered available

FOREWORD

ix

data on factors influencing thermal destruction of Vitamin B1,has shown a reliable and satisfactory approach by the use of kinetics, and has critically analyzed published data in the field-a most valuable contribution a t this time. Volume I1 of Advances in Food Research contained an article on the spray drying of foods. I n this volume, Kilpatrick, Lowe and Van Arsdale have reviewed another aspect of dehydration pertaining t o tunnel dehydrators for fruits and vegetables. While the “germ” of the idea of the tunnel dehydrator is at least a century old, and while considerable literature and information is available on these types of driers, this information has not been heretofore brought together in a critical and satisfactory manner. The material included considers type of tunnel, mechanical elements of tunnel construction, theory of tunnel dehydrators, operating procedures, and recent trends in tunnel dehydration of fruits and vegetables. This review will serve as an excellent companion article for that which appeared on spray drying in Volume I1 and as a contribution in the field of engineering and production.

October, 1955

E. M. MRAK G. F. STEWART

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CONTEXTS Contributors t o Volume VI . . . . . . . . . . Foreword . . . . . . . . . . . . . . . . . . . .

. .

.

.

.

v

.

vii

Applications of Research to Problems of Candy Manufacture B YL. F MARTIN,Sugarcane Products Division, Southern Regional Research Laboratory. U S Department of Agriculture. New Orleans. Louisiana I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Desirable Properties of Candy . . . . . . . . . . . . . . . . . . . I11. Properties and Reactions of Sugars in Candymaking . . . . . . . . . . IV . Modification of Sugar Properties by Minor Ingredients . . . . . . . . . (V . Major Ingredients Other than Sugars . . . . . . . . . . . . . . . . VI . Production Methods . . . . . . . . . . . . . . . . . . . . . . . . VII Summary . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20 29 48 50 55

Bacterial Spoilage of Wines with Special Reference to California Conditions BY REESEH VAUGHN, Department of Food Technology, University of California, Davis, California I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Historical Consequences . . . . . . . . . . . . . . . . . . . . . . 111. Types of Wine Spoilage Caused by Bacteria . . . . . . . . . . . . . . IV . Factors Affecting the Growth of Bacteria in Wines . . . . . . . . . . V . Characteristics of the Bacteria Found in California Wines . . . . . . . VI Additional Research Needs . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67 68 75 82 90 100 101 101

.

. .

.

1 6 15

.

.

Microbiological Implications in the Handling, Slaughtering, and Dressing of Meat Animals BY JOHN C . AYRES,Food Processing Laboratory, Iowa Agricultural Experimental Station, Iowa State College, Ames, Iowa I Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 I1 Defensive Mechanisms . . . . . . . . . . . . . . . . . . . . . . . 111 111 Ante-Mortem . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 IV Slaughter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 V. Post-Mortem . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 VI Improvements in Processing Practices . . . . . . . . . . . . . . . . 149 154 References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

Microbiological Problems of Frozen Food Products BY GEORGBORGSTROM, Swedish Institute for Food Preservation ( S I K ) , Goteborg, Sweden I. Introduction . . . . . . . . . . . . . . . . . . . . . . . I1. The Influence of Freezing Temperatures on Microorganisms . I11. The Influence of the Freezing Rate . . . . . . . . . . . . IV. The Freezing Death of Bacteria . . . . . . . . . . . . V . Occurrence of Bacteria in Frozen Foods . . . . . . . . . xi

Research

. . . .

163 . . . . . 164 . . . . . 168 . . . . . 170 . . . . . 172

xii

CONTENTS

VI . Pathogenic Bacteria in Frozen VII . Defrosting Problems . . . . . VIIJ . Packaging Problems. . . . . I X . Cooking . . . . . . . . . . X . Hygienic Aspects . . . . . . X I . Practical Aspects . . . . . . References . . . . . . . . .

Foods . . . . . . . . . . . . . . . . 201 . . . . . . . . . . . . . . . . . . . 204 . . . . . . . . . . . . . . . . . . . 209 . . . . . . . . . . . . . . . . . . . 210 . . . . . . . . . . . . . . . . . . . 210 . . . . . . . . . . . . . . . . . . . 211 . . . . . . . . . . . . . . . . . . . 213

Potato Granules, Development and Technology of Manufacture BY R . L . OLSONA N D W. 0. HARRINCTON, Western Utilization Research Branch, Agricultural Research Service, U . S . Department of Agriculture, Albany, California I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 I1. Related Nongranular Products . . . . . . . . . . . . . . . . . . . 233 I11. Spray Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 . . . . . . . . . . . . . . . . . . 235 IV . Direct Dehydration-Two-Stage V . “Freeze and Squeeze” Method . . . . . . . . . . . . . . . . . . . 237 VI . Cold Tempering . . . . . . . . . . . . . . . . . . . . . . . . . . 237 VII . Solvent Extraction . . . . . . . . . . . . . . . . . . . . . . . . 238 VIII . Add-Back Method . . . . . . . . . . . . . . . . . . . . . . . . . 238 I X . General Considerations . . . . . . . . . . . . . . . . . . . . . . . 243 X . Qiality Evaluation of Potato Granules . . . . . . . . . . . . . . . . 249 References . . . . . . . . . . . . . . . . . . . . . . . . . 253 The Thermal Destruction of Vitamin B, in Foods BY X . T . H . FARRER,Research Laboratories, Kraft Foods Ltd., Melbourne, Australia I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 I1. Factors Influencing the Thermal Destruction of Vitamin B1 . . . . . . 258 I11. Thermal Losses in Cereals . . . . . . . . . . . . . . . . . . . . . 264 IV. Thermal Losses in Meats . . . . . . . . . . . . . . . . . . . . . . 275 V. Losses in Processing Vegetables . . . . . . . . . . . . . . . . . . . 285 VI . Thermal Losses in Other Foodstuffs . . . . . . . . . . . . . . . . . 293 VII . Thermal Losses on Storage . . . . . . . . . . . . . . . . . . . . . 294 VIII . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Tunnel Dehydrators for Fruits and Vegetables

BY P. W . KILPATRICK, E . LOWE,A N D W . B. VAN ARSDEL,Western Utilizztion Research Branch, Agricultural Research Service, U . S . Department of Agriculture, Albany, California I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Classification of Tunnel Dehydrators. . . . . . . . . . . . . . . . 111. Mechanical Elements of Tunnel Construction . . . . . . . . . . . . IV . Typical Commercial Tunnel Dehydrators . . . . . . . . . . . . . . V. Criteria for Selection of Tunnel Dehydrators . . . . . . . . . . . . VI Basic Theory of Tunnel Dehydrators . . . . . . . . . . . . . . . VII . Operating Procedures for Tunnel Dehydrators . . . . . . . . . . . VIII . Recent Trends in Tunnel Dehydration of Fruits and Vegetables . . . IX . List of Symbols Used . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . .

314 . 316 . 326 . 339 . 345 . 347 . 363 . 367 369 369 373 390

Applications of Research to Problems of Candy Manufacture

BY L. F. MARTIN Sugarcane Products Divisivn, Southern Regional Research Laboratory, U.S. Department of Agriculture, New Orleans, Louisiana Page I. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 11. Desirable Properties of Candy. . . . . . . .... . . . . . . . . . . . . . 6

111. IV.

V.

VI. VII.

1. Physical Properties.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Chemical Composition and Properties.. . . . . . . . . . . . . . . . . . 3. Preservation of Desirable Properties. . . . . . . . . . . . . . . . . . . . . . . . . Properties and Reactions of Sugars in Candymaking.. . . . . . . . . 1. Effect of Heat upon Sugars.. . . . . . . . . . . . . . . . . . . . . . . . . . Modification of Sugar Properties by Minor Ingredients. . . . . . . . 1. Protein Whipping Agents.. . . . . . . . . . . . . . . . . . . . . 2. Gelatin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Pectin . . . . . . . . . . . . . . . . . . . . . . . . .. 4. Natural G u m s . . . . . . . . . . . . . . . . . . . . . . . . . Major Ingredients Other than Sugars. . . . . . . . . . . 1. Milk Products.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Fats Other than Dairy Butter and Cocoa Butter. . . . . . . . . 3. Starch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Cocoa and Chocolate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Nutmeats and Fruits.. . . . . . . . . . . . . . . . . . . . . . . . . Production Methods. . . . . . . . . . . . . . . . . Summary.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 9 10 15 16 20 20 24 26 29 29 30 32 33 37 45

48 50 55

I. INTRODUCTION Candymaking continues t o be primarily an art t o a greater extent than most other modern food-processing operations. The popularity of candies is attributable largely t o their great variety and the scope afforded for originality in developing a wide diversity of qualities that do not lend themselves readily to standardization by a simple set of scientific principles. Despite increasing use of precise methods for control of raw material and product quality and extensive adoption of improved machinery and techniques of mass production, the candymaker’s skill in applying “rule of thumb” procedures remains the principal guide in formulation and cooking. Research is being conducted to an increasing extent within the industry, but far more information applicable t o its 1

2

L. F. MARTIN

operations has been developed by research in related fields. Major improvements are the result of advances in the science and technology of the food materials used as ingredients of candy. A review of recent investigations bearing upon the solution of candymaking problems must include such related research as well as studies devoted specifically t o various aspects of candy manufacture. The confectionery industry, including all types of candy and chocolate goods, produces about three billion pounds of these products annually t o rank as one of the largest food industries in the United States. It is one of the largest industrial consumers of sugar, t o which it adds some eighty or ninety other ingredients in making more than a hundred distinct items. Table I presents a broad classification of major confectionery raw materials together with statistics relative to the quantities used and their value. The numerous individual ingredients may be grouped conveniently into six major classes of food materials in addition t o the miscellaneous pectic and protein products used as gel-forming or whipping agents. Ingredients in each group possess common properties and present similar problems when used in making different types of candy. One property common t o all candies is their high sugar content, attained by cooking t o relatively high temperatures. The effective use of any particular ingredient depends more upon its properties and behavior under these conditions of heating and concentration than upon the type of candy in which it happens t o be incorporated. Chemical reactions of sugars are fundamentally the same, but differ in the extent t o which they proceed in the preparation of marshmallows a t relatively low temperatures, or in cooking to high temperatures for the production of hard candy. Interactions of ingredients of different classes are primarily important in many cases such as, for example, the combination of sugars and milk solids t o produce caramel. The recently acquired knowledge of the chemistry of each class of ingredients will be considered in the following sections, with emphasis upon its practical application in improving the methods of production and the quality of candies of all types. It should not be inferred th at chemical aspects of candy manufacture have been neglected entirely until recent years in favor of more immediately profitable developments in engineering of mass production methods, mechanizing the handling of materials, automatic process control, and machine packaging. The excellent reference book of Jordan (1930) summarizes the practical knowledge of candy chemistry acquired prior t o its publication. I n earlier studies Paine (1924, 1928) sought t o explain some of the processes involved in terms of the properties of sugars as they were known at t ha t time. Subsequent discoveries have made it evident th a t reactions which may occur in the production of even the simplest, types

3

CANDY MANUFACTURE

TABLE I Estimated Amounts and Average Cost of Ingredients Used by the Candy Industrye Quantity ( X 1000 pounds)

___ Ingredients

1947

1950

Sugar and Sweetenrrs Cane and bept sugar 1,020.055 1,218,032 14,517 Corn sugar 30,061 Corn sirtip 676,864 678,683 Other (honey, maple, etr.) 3,368 20,837 30,936 Corn starch 20,074 Cocoa products 198,666 178,75:3 Cocoa beans Cocoa powder 5878 9776 Cocoa butter 20.989 23,098 8993 Chocolate liquor 27,430 Coatings 2!i6. 882 249,718 \[ilk products 258,241 Fluid milk 194,177 Condenscd and evaporated 96.343 72,268 4141 Butter 3164 Other (dried, cream, 33,562 46,949 etc.) Eggs and egg products 6735 6368 Fats and oils 39,062 31.921 Essential oils and 3495 flavors 3965 I’eanrits 199.975 136.337 Almonds and other 39 ,:3xi 14,660 nuts Coconut 18,570 64,201 Fruit products 12,543 15,193 Miscellaneous other 34.521 56,807 ingredients Total ingredicnts

2 990 548

3,175,617

Cost

(x1000 dollars)

Average cost per pound

__.

1947

85,138 2207 35,096

1950

1947

1950

99,272 SO. 084 073 1030 052 32,642

$0.082 071 053

2357 1307

1439 1980

.069 .065

.069 .064

49,377 1082 20,099 11,507 90,220

53,130 2274 13,548 3685 80,770

.276 184 670 420 338

.267

7200

9741

.037

.038

12.018 2332

763 1 2691

125 737

106 650

11,124 2893 11,261

8307 1607 7141

237 430 288

247 252 221

5779 33.660

45G8 25,111

1 653 168

1 152

28,514 5316 3933

20,345 12,467 4924

638 286 314

714

233 587 410 323

184

194 32-1

5290

7062

153

124

427,710

402,365

0 143

0 127

of candies are extremely complex, giving rise t o problems that challenge the utmost ingenuity and skill of the chemist. Earlier knowledge of sugars, fats, proteins, starches, and other complex ingredients was far from adequate for the Rolutioii of these problems. Some essential details of the structures and properties of these substances have heen determined only

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L. F. MARTIN

within the past two decades and much remains t o be learned. The progress that has been made possible by modern research methods in explaining the changes that occur in the drying of milk (Coulter et al., 1951) provides but one example. Structures and simpler reactions of the common sugars, sucrose, glucose, and fructose, have been fairly wellestablished for some time, but changes produced by heating solutions of the pure sugars are still very imperfectly understood. Obviously, the complete and accurate description of all of the reactions that occur when sugars, milk, and fats are cooked together at high temperatures t o produce caramels must await the results of future and more difficult research. Candy appeals t o the universal liking for sweetness, modified or varied by less accurately definable yet tangible qualities of flavor and texture. Unlike many other food products th a t are preserved or processed under mild conditions t o minimize alteration of their natural composition, texture, and flavor, the qualities of most candies are created by more or less drastic modification of the properties of their ingredients. These qualities must be maintained for considerable periods of time, not only for distribution but also after the products reach the consumer. Organic chemistry is the principal field in which the scientific guides to better production methods and improved quality must be sought. Improvement or control of texture calls for the application of the techniques of physical chemistry, including particularly colloid chemistry. Biochemical factors affect the composition, stability, and quality of the important natural products used as ingredients. Many of the latest methods of analytical chemistry can be applied advantageously in process control as well as in establishing and maintaining standards for the raw materials and finished products. Chemical engineering must devise the practical methods and equipment required for commercial application of improvements developed by research in the other fields of chemistry. Recent contributions of each of these fields of investigation t o progress in the candy industry will be reviewed and, of possibly greater importance, applicability of some of the more recent discoveries and techniques t o research on unsolved problems of the industry will be indicated. Many of the ingredients listed in Table I are complex systems of unstable compounds that may undergo involved reactions even under mild processing conditions. Only the sugars are comparatively simple, well-defined, organic compounds and it will be seen that they also give rise t o complex reaction systems under the conditions of concentration and temperature reached in cooking candies. When several such ingredients of different classes are combined and cooked t o final temperatures of 130" C. (266" F.) or higher, the resulting system defies complete

CANDY MANUFACTURE

5

chemical analysis by any methods yet devised. Consideration of the present knowledge of the “browning” products formed by reaction of proteins with sugars, despite the very extensive research devoted t o the study of this reaction in recent years, exemplifies the complexity of the composition of such reaction mixtures. Nevertheless, a chemical approach t o the problem of candy composition is far from hopeless. Practical results of the int.ricate processes involved in candy production can be explained only in terms of the fundamental reactions of carbohydrates, proteins, and fats revealed by the latest investigations of their chemistry. The dependence of final quality upon properties of the ingredients and processing conditions can thus be understood better, and formulation will be placed upon a more logical basis. Individual types of candy are too numerous and varied t o serve as a practical classification for systematic review of their chemistry. For this purpose they fall conveniently into three large categories, each of which possesses certain distinctive chemical characteristics. First are candies made entirely of sugars, with or without flavors or colors, such as hard candies of both the plain and pulled types, stick candies, and crystallized creams. The second category includes those made largely of sugars, the properties of which are modified by the incorporation of small proportions, not exceeding 575 and usually much less, of nonsugar ingredients. Typical examples of this category are pectin jellies, marshmallows, and nougats. The third group comprises candies such as fudges, caramels, starch jellies, chocolates, and others that contain large proportions of ingredients other than the sugars. The logic of this classification is evident from the obvious fact that the all-sugar candies are relatively the simplest, a t least in formulation if not in chemical constitution, and the introduction of increasing amounts of nonsugar ingredients modifies the behavior of the sugars, in many cases superimposing additional reactions upon those which the sugars undergo. The most fruitful contribution of research in this field will continue t o be, as in the past, the gradual introduction of more exact and scientific guides for formulation and processing. Its objective should be t o provide explanations and better understanding of the art of candymaking, rather than an attempt t o supplant that art entirely by exact science. It should not be aimed a t such rigid standardization that the play of originality in devising novel products would be restricted; on the contrary, increased knowledge of the principles underlying the chemical processes involved should lead t o development of a n even greater variety of candies made from a wider range of raw materials. Reliable methods of measuring many properties associated with candy quality are still urgently needed for use in systematic research. The effects of variations of composition

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L. F. MARTIN

and cooking procedures on texture, consistency or body, color, and flavor must be determined quantitatively for rapid, sure progress to be made in experimental work. Such methods are also invaluable for standardization and control of commercial production once optimum conditions have been established experimentally. Specifications for the quality of raw materials required to produce the best results can be improved as our knowledge increases regarding the importance in candymaking of specific properties and reactions of these materials. Storage stability, or “shelflife,” is of such great economic importance in the marketing of candy that it is imperative to consider the gradual changes in composition and properties that occur after it has been packaged. There is much yet to be learned of the nature and causes of changes in stored candies so that effective means of retarding them may be devised.

11. DESIRABLE PROPERTIES OF CANDY The variety of confections generally accepted by custom as candy is so great that a precise definition of the word is both impossible and unnecessary. Individual types may be described broadly, but cannot be defined strictly by narrow limits of chemical composition or physical properties. The only truly characteristic quality common to all types is that of sweetness. The British term sweets, applied to all except chocolate candies, is more accurately descriptive than our word derived from the Arabic Khandi. It is desirable, nevertheless, to consider briefly the range of variations in composition, physical properties, and organoleptic qualities that good candies are expected to possess. The ultimate goal of the application of research reviewed here is to improve and maintain these properties. 1. Physical Properties The important quality of many candies designated as texture is the sum or resultant of several physical properties including density, hardness, plasticity or elasticity, and consistency. It varies in different types from the soft, “tender” texture of marshmallows or chocolate cream centers to the glass-like hardness of the clear varieties of hard candy. The particular property or properties of primary importance vary according to the kind of texture desired in different candies. Quantitative measurements of physical properties have been employed to a very limited extent and texture is still evaluated qualitatively or described by the candymaker’s terms, “short,” “tender,” “firm,” “chewy,” etc. Whatever the texture, uniformity and smoothness are desirable almost invariably, as grainy or gritty candies are generally unpopular. The problem of translating these qualities into precisely defined, measurable

CANDY MANUF-4CTURE

7

properties has been solved in very few cases. Adaptation of methods th a t have been applied t o other products should prove effective for determination of the intrinsic properties upon which the texture of candies depends. This is necessary particularly in experimental work on improvement of texture. a. Density. The true density or specific gravity of all sugar hard candies does not vary significantly. Apparent density can be determined readily and is of more importance in relation t o the textures of many types of candy, but n o data have been published on this property. Variations are greatest in aerated candies made with frappes such as nougats and certain grades of fudge. Nougat texture in particular ranges from light, “short” types almost like fudge t o dense, “chewy” sorts approaching the density and qualities of caramels. Gelatin marshmallows vary in apparent density with differences in gel structure and moisture content. b. Hardness. This property, associated with elasticity and brittleness, is obviously of primary importance in relation t o the texture of those all-sugar candies that have minimum moisture contents. No suitable methods have been devised or adapted for measuring the hardness or brittleness of such candies. A major difficulty in marketing them is their tendency t o become sticky because of the hygroscopicity of the sugar reaction products formed in cooking a t high temperatures. This hygroscopicity is probably related t o hardness or other physical properties, as it is not simply dependent upon a low initial moisture content which is readily attained in modern vacuum cooking equipment. c. Plasticity. The texture of a large variety of candies is governed by this property. The quality described by candymakers as ‘(tenderness” is essentially dependent upon plasticity. The maximum permissible degree of tenderness is a prime attribute of the best grades of creams, caramels, nougats, fudges, and marshmallows. Both pectin and starch jellies are rated higher in proportion t o the extent t o which they possess and retain this quality. It can be developed t o the highest degree in any of these candies prepared for chocolate coating, but some tenderness must be sacrificed in less expensive grades th at must be durable enough to be shipped and sold in bulk. Loss of moisture decreases plasticity with resultant toughening of nougats, jellies, and marshmallows which is the principal cause of their deterioration in storage. Fudges, creams, and caramels are more prone t o grain and harden on drying. Quantitative measurements of plasticity are used for control of quality, particularly in large scale manufacture of jellies and gum candies, such as gum drops. The Humboldt penetrometer is modified for this purpose by adjusting the weight of the plunger to give a penetration of

8

L. F. MARTIN

1 cm., or 100 units, with samples of predetermined desirable plasticity (Alikonis, 1952a). This makes it possible t o detect deviations with greater speed and certainty. The standard Humboldt instrument has been used t o obtain data on the effect of emulsifiers upon the texture of starch jellies (Martin et al., 1952a). A test of the effectiveness of tempering in solid chocolate based upon measurements with a sensitive penetrometer has been suggested by Neville et al. (1950), whereas a less sensitive modification is proposed by Mickevicz (1950a) for determination of its surface hardness. d. Viscosity. Efficient tempering and application of liquid chocolate coatings are critically dependent upon viscosity, particularly in modern, continuous, enrobing machinery. The textures of finished coatings and candies are governed by plasticity of the solidified chocolate, but the two properties are related. Specifications for different grades of chocolate include the viscosity of the liquid material determined at temperatures somewhat above its melting point. The MacMichael (1915) viscosimeter has been employed generally for this purpose for many years (Stanley, 1939), although the need for more precise standardization of the procedure for making the determinations has been emphasized recently by Kempf (1949a). e. Consistency. The smoothness of texture essential for highest quality in most candies is approximated by the physical property of consistency. The exact definition of this property by Bingham (1930) includes plastic as well as fluid materials, and the properties of some candies are intermediate between the plastic and fluid states. Examples are the creamy fudges in which the formation of very small, uniform sugar crystals is induced by the use of separately creamed fondant, and the soft cream centers produced by invertase action th a t increases the ratio of the sirup to the crystal phases after they have been coated. No attempts t o measure the consistency of such candies have been reported. Neither methods used for fluids nor those applicable t o strictly plastic materials are readily adaptable for quantitative determinations in this intermediate range. Campbell (1940) has made an interesting study of the consistency of chocolate and its relation t o viscosity, employing methods and theoretical principles developed earlier by Williamson (1929). f. Color. Attractive colors are essential because color is known t o affect sales appeal directly, and t o influence the organoleptic response t o flavors, indirectly affecting their acceptability. This property of candies is assessed invariably by personal judgment. It can be standardized for many candy items with the available range of certified food colors. White candies, and those which depend upon reactions of the ingredients t o develop desirable colors, are not so readily standardized. Accurate,

CANDY MANUFACTURE

9

objective methods of measuring color have been applied t o sugar and other products, but there are no published reports of their use in candy manufacturing or research. g. Flavor. Candy flavors, like their colors, are judged subjectively. This property cannot be measured in any other way, yet it is undoubtedly the most important single physical property of the vast majority of candies. Fortunately, it is possible t o introduce almost any desired flavor by the use of either natural or approved synthetic flavoring materials. Standardization is more difficult for individual products such as caramels, chocolates, or fudges, the flavor of which depends upon variable natural ingredients or their reactions with sugars. 2 . Chemical Composition and Properties

A sufficiently high concentration of sugar t o make the finished products self-preserving is the only basic requirement of the chemical composition of all candies. This is consistent with providing the desired sweetness. Excessive sweetness is undesirable in many types of candy, requiring increased proportions of corn sirup or other less sweet ingredients t o modify the sugar present. It is also advantageous t o incorporate fats, proteins, minerals, and other natural nutritional values, although high-sugar contents make candies primarily an energy food. The range of proportions of the principal ingredients entering into the formulation of important types of candy is presented in Table 11. The composition of each type of candy varies within rather wide limits so that its optimum chemical requirements can be defined only in a general manner. Ingredients other than sugar are used for their various effects in modifying the properties of sucrose or reacting with it t o produce the desirable physical properties described in section 11, 1. All of these nonsucrose substances are effective in either preventing crystallization, or limiting it t o the formation of extremely small, uniform crystals under suitable processing conditions. Clear, white, or bright,ly colored candies must be produced under conditions that minimize possible reactions of the ingredients, whereas the flavors of caramels, taffies, brittles, and butterscotches are developed by controlled reactions leading t o the formation of complex end products. The final chemical composition of hard candies, crystallized creams, and jellies must be as nonhygroscopic as possible so that they will not become sticky. On the other hand, candies such as fudges and nougats develop undesirable textures rapidly unless they are sufficiently hygroscopic t o prevent loss of moisture. As the specific effects of individual ingredients will be considered in subsequent sections, details are superfluous a t this point.

10

L. F. MARTIN

TABLEI1 Range of Cooking Temperatures, Moisture Contents, and Proportions of Sugars of Principal Types of Candy

Type of candy

Final Final cooking moisture temperature content range range (OF.) (%)

Sugar ingredients" range (%I

Other principal Ingredientsa

Corn ________ sirup Range Sucrose Invert solids Ingredient (%)

Hard Plain Butterscotch Brittle Creams Fondant Cast Butter

275-338 240-265 290-295

1.0-1.5 1.5-2.0 1.0-1.5

40-100 40-65 25-55

0-10

235-244 235-245 235-247

10.0-11.5 9.5-10.5 9.5-11.0

85-100 65-75 50-65

Fudge

24s250

8.0-10.5

30-70

Caramel

240-265

8.0-11.5

0-50

Nougat Marshmallow Grained Soft Jellies Starch Pectin

255-270

8.0-8.5

20-50

0-10 Starch 0-1 25-40 Egg albumin 0-0.05 - 25-40 Butter 1-5 10-20 0-17 12-40 Milk solids 5-15 Fat 1-5 0-15 0-50 Milk solids 15-25 Fat 0-10 0-15b 30-60 F a t 0-5

-

-

0-60 35-60 Butter 20-50 -

1-7 -

5-10

-

240-245 12.0-14.0 50-78 225-230 15.0-18.0 25-54

0-5 0-10

15-40 Gelatin 40-60 Gelatin

1.5-3 2-5

230-235 220-230

0-10 -

28-65 Starch 30-48 Pectin

7-12 1.5-4

14.5-18.0 18.0-22.0

25-60 40-65

.0 Adapted from "Confectionery Analysis and Composition," Jordan, S., and Langwill, K. E., &fan,,facturing Confectioner Publishing, Chicago (1946). b Honey is often used.

3. Preservation of DesiraEle Properties

The marketing of candy involves storage for considerable periods because of the manner in which it is consumed. Demand fluctuates both seasonally and on occasions such as Christmas and Easter. Even after i t reaches the consumer it may be expected t o last longer than other foods without any special storage precautions such as those normally taken with perishable items. Candies have generally been stored and distributed under the most adverse conditions. They are practically immune t o microbiological spoilage and have been assumed t o be entirely stable in other respects. Efforts are being made t o devise better storage practices that will ensure the marketing of products that retain all of their desirable properties. Research has been in progress t o demonstrate

CANDY MANUFACTURE

11

the advantages of controlling both temperatures and humidities in storing candy for extended periods of time (Heaton and Woodroof, 1952). Experience with candies required for military rations and the necessity of stockpiling such items in times of emergency has given impetus t o these investigations (Cosler, 1951). As the widespread adoption of adequately conditioned storage and marketing facilities will require considerable time and investment, other research has sought means of extending the shelf-life of candies by suitable modifications of formulation and processing conditions (Martin et al., 1952b). a. Stabilization of Texture. Alterations of texture result in the most obvious deterioration of many types of candy. Marshmallows may become tough and rubbery; fudges, creams, and mints become hard and gritty because of excessive graining; pectin jellies mag either lose moisture and become cloudy and tough, or undergo syneresis and become sticky. Loss of moisture can be prevented by judiciously increasing the extent of inversion, or by using larger proportions of corn sirup for its hygroscopicity, up t o the maximum proportions shown in Table 11. Addition of smaller percentages of more hygroscopic ingredients has been resorted t o for the purpose of still further increasing moisture retention. The use of glycerin has been noted (Anon., 1937) 10% being incorporated in fudge and from 5 t o 15% in other candies. Alikonis (1952b) found that fudge and nougat are improved by sorbitol added in amounts of about 10% of the batch weight. It appeared t o be beneficial in gum drops only after storage for 9 months t o a year, but 5% and 10% additions of sorbitol had an adverse effect on the quality ratings of both pectin and starch jellies stored for 6 months according t o Heaton and Woodroof (1952). Detrimental effects of the use of 15% concentrations of sorbitol on the strength, texture, and sweating tendency of pectin jellies during 40 days storage under various conditions have been described by Poulsen (1953). The most important contribution on the effect of moisture changes upon texture and keeping quality was that of Grover (1947) who showed that the equilibrium vapor pressures of candies may be calculated approximately from their compositions. He established conversion factors for expressing the concentrations of other ingredients in the sirup phase in terms of equivalent sucrose, and tabulated data on the relative vapor pressures for total equivalent sucrose concentrations. This makes it possible to calculate, from a given formula, whether the candy will be in equilibrium with surrounding atmosphere at a particular relative humidity, or will gain or lose moisture by exposure. Measurements on actual candies were found t o agree well with calculated values. Equations for calculating the equilibrium humidities for various candies have been published more recently by Money and Born (1951).

12

L. F. MARTIN

The tenderness of starch jellies can be maintained for longer storage times by addition of small amounts of surface active compounds t h a t delay aging of the gel which increases its firmness. Naturally occurring traces of stearic acid have this effect as shown by Hamer (1947) who found that complete defatting increased the rate of aging of gels of corn and wheat starches. The effect of polyoxyethylene stearates in retarding this change in wheat starch gels was studied quantitatively by Lord (1950).

110

90 % u)

3

E

E

+ 0

.2

70

0

w

2 50

30

0

1

2

3

4

5

6

Storage time, months

FIG.1. Effect of polyoxyethylene stearate, in 0.5% concentration in delaying the aging (toughening) of starch jellies. (Martin et al., 1953a.) Curves 1 and 2 for jellies of 10% starch, 3 and 4 for jellies of 12% starch; 1and 3 show penetrometer readings with addedpolyoxyethylene stearate, 2 and 4 readings for controls without added emulsifier.

Results obtained by Martin et al. (1953a) for starch gum candies containing 0.5 % ’ additions of polyoxyethylene stearate based on the weight of starch used are shown in Fig. 1. After 6 months storage the jellies made with the emulsifier were as tender, by penetrometer measurement, as the regular controls that had been stored for about 2 months. Monoglycerides produce similar results, but both types of emulsifiers introduce the practical disadvantage of making the jellies too tender initially t o be removed from the starch molds within 48 hr., which is essential in commercial production a t modern equipment capacities. b . Rancidity. Vegetable and animal fats are important ingredients of caramels, fudges, some types of nougat, butterscotch, chocolate, and

CANDY MANUFACTURE

13

butter creams. Martin et al. (1951a, 1952c) have investigated the stability of fats incorporated in various typical candies during storage at 30" C. (86" F.) with respect t o oxidation and hydrolysis. It was established t h a t fat could be extracted from the candies without significant alteration of peroxide or free fatty acid contents t h a t had been determined prior t o its incorporation into standard formulas. This made i t possible t o subject butter and other fats t o storage tests, with and without added stabilizers, under the exact conditions of exposure t o air and moisture prevailing in normal candies. Rates of deterioration and the effectiveness of antioxidants or other stabilizers may differ under these conditions from those that would be observed in experiments carried out with isolated fats at higher temperatures (Mayberry, 1949). Extensive studies of the stabilization of the fats themselves are reported in the literature. All of the fats tested in candies hydrolyzed slowly, yielding measurable amounts of free fatty acids in a few weeks. Hydrolysis continued thereafter at increasing rates during the remainder of the storage periods of 4 t o 6 months. The number of experiments conducted was insufficient t o indicate possible relationship of the rate of hydrolysis t o the nature of the fat and moisture content of the candies. A few preliminary tests of candies containing butter or coconut fat gave results showing that the reaction may be retarded somewhat by addition of glycerol in amounts about equal t o t h a t of the f a t present, but no effective means of delaying hydrolysis sufficiently t o be of practical value has been found (Martin et al., 195313). Oxidative rancidity, as measured by the amounts of peroxide formed, was not detected in processed vegetable fats extracted from these candies until they had been stored for 6 t o 12 weeks at 30" C. (86" F.). Oxidation of this type of fat proceeded so slowly that i t was of no practical significance at the end of the 24 weeks during which the tests were continued. Animal fats proved t o be much less resistant t o oxidation. Martin et al. (1951b) found t h a t butter incorporated in cream fondant was stable during a short induction period of only 2 t o 4 weeks, after which peroxides were formed rapidly. The onset of oxidation could be delayed for a t least 5 months by adding the prescribed amounts of any of the widely used antioxidants such as NDGA (nordihydroguaiaretic acid), BHA (butylated hydroxyanisole), or propyl gallate t o the butter prior t o mixing it in the candy batch. These synthetic or chemically purified stabilizers may not be employed under existing Pure Food and Drug Act regulations prohibiting the use of non-nutritive ingredients in confectionery; however, inactive, dried yeast or concentrates prepared from oat flour were found t o be equally effective. The antioxidant properties of oat flour had been reported by both Peters and Musher (1937) and Conn and Asnis (1937),

14

L. F. MARTIN

and similar uses of yeast had been described by Musher (1942). Concentrates prepared from extracts of oat flour were used t o stabilize fats by Musher (1944). The vitamins of the B complex contained in both oats and yeast do not possess antioxidant properties according t o Gyorgy and Tomarelli (1943), and the nature of the specific compounds responsible for stability imparted t o fats by these products is unknown. Being natural, nutritive food products, dried brewer’s yeast or oat concentrates may be added t o candy in amounts equal t o about 3% of the total batch weight, which provides sufficient antioxidants t o protect the butter or other animal fat present. c . Chocolate Bloom. Chocolate does not become rancid very readily because of the inherent stability of cocoa butter and the low moisture content of coatings or solid chocolate. If it is used t o flavor fudges or similar candies containing moisture, Sjostedt and Schetty (1946) found the liberation of free fatty acid9 in storage t o be proportional t o the percentage of water present. A form of deterioration peculiar t o chocolate is the graying or discoloration of its surface accompanied by loss of gloss, described as “bloom.” Flavor may not be affected perceptibly, but the appearance of bloomed chocolates is so unattractive as t o render them unsaleable. Sugar bloom is a white or light gray coating of sucrose crystals, readily distinguishable by their birefringence in microscopic examination. As it usually results from condensation of moisture on the candy, i t can be prevented b y proper packaging and storage t o avoid exposure t o high humidity or sudden changes of temperature. Fat bloom is more unsightly and of far greater economic importance than sugar bloom. The discoloration in this case is known t o be formed of surface crystals of fat that has exuded from the chocolate, and is observed in different forms under widely varying conditions. Improper tempering or storage a t high temperatures increase the incidence of bloom, but the fundamental causes of the phenomenon are still obscure. Present knowledge of this problem remains essentially that summarized two decades ago by Whymper (1933a). Recent information on the nature of fat bloom has been disclosed largely in patents covering proposed methods for its prevention. Most of these methods involve modification of the fat intended t o convert it t o more stable forms. Cook and Light (1940) proposed elaidizing the fat by treatment of chocolate or the separated cocoa butter with sulfur dioxide or oxides of nitrogen in order t o isomerize the oleic acid present t o the higher-melting, elaidic trans acid. Extracted cocoa fat can be transformed similarly by the action of nitrous and nitric oxides in a mixture of acetone, amyl acetate, and a small amount of water according t o Eipper (1948). Attempts t o prevent fat bloom by adding surface-active

CANDY MANUFACTURE

13

materials to improve and stabilize the degree of dispersion of fat throughout the chocolate have met with but limited success. An early suggestion of this approach appeared in a patent on the use of oleodistearin or, preferably, an emulsifier produced by oxidation and polymerization of cocoa butter (Clayton et al., 1937). It was soon followed by a patent describing nonblooming chocolate produced by incorporating esters of sorbitol (Eipper, 1938). Stabilization against bloom by small additions of the aminoethyl esters, or carbobenzoxyaminoethyl esters of diacylglycerophosphoric acid has been claimed by Rose (1948a,b). None of these methods is known t o be in practical, commercial use. Various other additives or modifications of the fat have been tried, but all have the disadvantage of affecting flavor or texture or both adversely (Clay, 1953). Recent work on this problem has been summarized by Neville et al. (1950) in reporting the degree of success attained by their experiments with polyoxyethylene sorbitan stearate and sorbitan stearate in concentrations of about 0.5%. These authors point out the need for a more reliable and rapid test for bloom as a measure of the degree of stability imparted by these surface-active agents. They also believe that the complete solution of the problem may require modification of the fat itself by selective hydrogenation, isomerization, or fractionation. Several patents have been issued on this application of various modifications or combinations of fatty acid esters of sorbitan and polyoxyethylene as bloom inhibitors (Mayberry, 1951; Cross, 1952, 1953). An exception to the general conviction that bloom can be prevented by altering the condition of the fat is a process proposed by Rubens (1944) requiring the separate grinding of dry, fat-free cocoa solids t o reduce their particle size, the finest dust being recombined with fat. 111.

PROPERTIES AND

REACTIONS OF

S U G A R S I N CANDYMAKING

Candies made solely of sugars with small additions of flavoring arid coloring substances may be supposed to be the simplest from a superficial consideration of their formulas. This simplicity is only apparent, as finished products of this class, such as hard candy, may be quite complex chemically. High temperatures are necessary to produce them with sufficiently low final moisture contents. These conditions are favorable for dehydration and reversion of all of the sugars present in addition t o inversion of sucrose, and at least a portion of the simple sugars undergo profound changes in structure. The occurrence of such reactions accounts for the fact that the candy can be produced as a supercooled glass, as the presence of glucose or invert sugar alone would not prevent crystallization indefinitely. Controlled crystallization is of more importance than

16

L. F. MARTIN

chemical reactions of the sugars in creams and other all-sugar types made by cooking a t lower temperatures. T o understand the problems of hard candy manufacture it is necessary t o consider in some detail the present knowledge of the chemistry of individual sugars used and their reactions in concentrated solutions a t elevated temperatures.

1. Efect of Heat upon Sugars

a. Dry Sugars. Extensive investigations of sugar decomposition reactions have succeeded in establishing only the initial steps and intermediates with certainty. The products obtained by heating “dry ” sugars are not entirely distinct from those formed in extremely concentrated solutions a t comparable temperatures a s water is liberated in early stages of the process. Numerous studies of caramelisation and other colorforming reactions of sugars have been summarized by Zerban (1947). The simple, stepwise dehydration postulated by Gelis (1857, 1859) from results of his early experiments on sucrose has been shown t o be a n inadequate explanation of the behavior of this sugar. His “ caramelan ” and ‘‘ caramelin” were not definite compounds, and the structure of the first product formed by loss of one molecule of water, which he termed 1 1 caramelen,” is more complex than G&s supposed. Pictet and his coworkers (Pictet and Adrianoff 1924; Pictet and Stricker, 1924) employed vacuum t o remove the water formed with better control of the rate of heating and succeeded in isolating the initial products of sucrose dehydration. They determined the structures of these compounds as completely as was possible a t the time of their work. Glucose and levulosan, the anhydride of fructose, are formed first without loss of water in the following reaction : CIZHZZOII 4 C6H1206 Sucrose

+C~HIOO~

Glucose

Levulosan

The levulosan is identical with the anhydride previously obtained by Pictet and Reilly (1921) on heating dry levulose. I n the next step of sucrose decomposition, water is eliminated from the glucose which Pictet and Castan (1920) had shown t o form an anhydride in the reaction:

+

C6H1206 4 C6Hl0O6 HzO Glucose Glucosan

Pictet and Adrianoff (1924) found that the glucosan and levulosan next react t o form isosaccharosan, the anhydride of sucrose. C6HloOs Giucosan

+ C6H1006

+

Levulosan

C~2HzoOlo Isosaccharosan

CANDY MANUFACTURE

17

This is the product of the first step of dehydration which Gelis termed (‘caramelen,” but which he was unable t o purify sufficiently for determination of its exact composition and structure. Further changes are complicated by the tendency of glucosan t o polymerize t o dilevoglucosan and higher polymers (Pictet and Ross, 1922), whereas levulosan is even more readily converted t o diheterolevulosans by dimerization (Pictet and Chavan, 1926). The importance of these sugar anhydrides, or reversion products, in candy cooking will be appreciated from the fact that high yields of diheterolevulosans are obtained by simply refluxing an 80% solution of fructose, as demonstrated later by Wolfrom and Blair (1948). b. Concentrated Sugar Solutions. It is not possible t o limit chemical changes on heating either dry sugars or their solutions t o the relatively simple, initial stages of reaction. Subsequent reactions proceed simultaneously with the consecutive steps of dehydration. Dark-colored polymerization products are formed by further loss of water from the sugar anhydrides (Wolfrom et al., 1951) together with simple end products of decomposition such as formaldehyde and hydroxymethylfurfural (Joszt and Molinski, 1935; Wolfrom and Blair, 1948). A key t o the transformations of sugars in solution under a wide range of conditions is provided by the rearrangement reactions of glucose and fructose discovered and first studied in detail by Lobry deBruyn and van Ekenstein (1895, 1896). These transformations are involved also when sucrose alone is used in candymaking, as tartaric acid or similar “doctors” aregenerally added t o provide sufficient acidity for its inversion t o the simpler hexoses. From any of these 3 sugars, or combinations of them, a system is established in which some glucose, fructose, and mannose are present and tend t o establish equilibrium with one another. Alkalis are particularly effective catalysts for the transformation of glucose into fructose and mannose, but the Lobry deBruyn rearrangement takes place throughout a wide range of pH. It has been found t o occur a t p H 6.4-6.6 in the presence of phosphates (Englis and Hanahan, 1945) as well as in solutions of either glucose or fructose buffered a t p H 7.5 and heated t o 90” C. (194” F.) (Schneider and Erlemann, 1951). Mathews and Jackson (1933) determined that fructose is most stable at p H 3.3, and Singh et al. (1948) found glucose t o have maximum stability in the same range at about p H 3.0. The rearrangement and interconversion of the simple sugars are invariably accompanied by side reactions leading t o the formation of sugar anhydrides and sugar acids, together with products of further degradation very similar t o those obtained by heating dry sugars. I n strongly acid solutions these side reactions predominate, but they occur to some extent under any conditions. Pictet and Chavan (1926) prepared fructose anhydrides by the action of cold, concentrated hydrochloric acid

18

L. F. MARTIN

on the sugar, and Wolfrom and Blair (1948) proved by chromatographic methods of purification that the products formed are diheterolevulosans. The rearrangement of sugars and the formation of by-products proceed t o various extents depending upon conditions of concentration, temperature, pH, and the catalysts present such as various bases used in the alkaline range. The effects of variations in all of these conditions upon the transformation of glucose in the Lobry deBruyn reaction have been investigated most thoroughly by Gottfried and Benjamin (1952). Their results show that the maximum yield of fructose attainable is 2175, accompanied by the formation of 8.5 % of unfermentable sugar anhydrides or reversion products, and 3 % of sugar or simpler organic acids, estimated as saccharic acid. At high temperatures and concentrations more glucose was transformed, largely into the side reaction products. It is interesting that the same reactions proceed slowly a t low temperatures under very mild conditions. Earlier results reported by Wolfrom and Lewis (1928) for the very slow conversion of glucose in 1 M solution by 0.035 N calcium hydroxide a t 35" C. (94.5' F.) are predictable from the data and constants of Gottfried and Benjamin, obtained from concentrated solutions at their boiling points. c. Importance I n Candy Manufacture. Consideration of the reactions described in the preceding sections makes it apparent t h a t even simple hard candies are much more complex than mere supercooled solutions of partially inverted sugar. The physical-chemical principles governing inversion as applied in candymaking have been treated comprehensively and precisely by Heiss et al. (1953), who point out the difficulty of exact control of this comparatively simple reaction. Inversion is but one of several possible initial steps in the sequence of reactions leading t o the formation of anhydrides, sugar acids, reversion, and caramelizatiori products, all of which may be derived from a single starting material, sucrose. Moreover, a minor proportion of all-sugar candies are made of granulated sugar alone, substantial proportions of invert or corn sirups being used in the bulk of the production. Corn sirup itself is a complex mixture of glucose with maltose, dextrins, sugar anhydrides, and polymers which have not been resolved completely. Less than one-fourth of the solids in the most widely used grade of corn sirup, with a dextrose eyuivalent of 42%, are actually glucose. This will be seen in Table 111, which gives approximate compositions of four different grades as compiled by Meeker (1950). Chromatographic methods are required for the separation of the noriglucose fraction into its constituents. This was accomplished in the case of a specially prepared hydrolyzate of the amylose fraction of starch by Dimler et al. (1952). Their results illustrate the complexity of starch hydrolyzates in general. The greater part of this fraction con-

19

CANDY MANUFACTURE

TABLEI11 Approximate Compositions and Properties of Corn Sirupsa ~

Types of Sirups

Composition Moisture Total solids Dextrose Maltose Higher sugars Dextrins Ash Properties Density, “Be. Dextrose equivalent Approx. relative viscosity at 100” F., poises PI3

Acid-enzyme High-acid Medium conversion conversion Regular conversion (%) (%I (%) (%) Total Solids Total Solids Total Solids Total Solids

19.7 80.3 18.0 17.0 16.0 30.0 0.3

19.0 81.0 22.0 26.0 21.0 21.0 20.0 11.0 37.0 23.0 0.37 0.3 43a 42

160 4.7-5.0

-

32.0 26.0 13.5 28.0 0.37 43” 52

104 4.7-5 .O

18.5 81.5 30.5 28.0 13.0 10.0 0.3

-

37.5 34.0 16.0 12.0 0.37

18.5 81.5 33.0 4 0 . 5 23.0 28.0 6.5 8.0 19.0 23.0 0.3 0.37

43” 63

43” 60

60 4.7-5.0

80 4.7-5.0

” From Meeker (1950).

sists of trioses, tetroses, and higher polymers of glucose, somewhat less than half of it being accounted for as maltose hydrate. Maltose, like sucrose, was shown by Pictet and Marfort (1923) t o yield an anhydride, maltosan, that may be formed when it is heated t o the temperatures required for hard candy production. Variations in the quality of granulated sugar for candymaking purposes can be explained by the effect of traces of catalytic substances on the reactions of sucrose a t elevated temperatures. The empirical “candy test” developed for use in the sugar industry has been widely used by candymakers t o determine the “strength” of sugars. This property has been defined as the resistance of the sugar t o inversion under standardized conditions of cooking t o finished hard candy (Ambler, 1927). The shorter the time required for the test candy t o crystallize, the stronger the sugar. As granulated sugar is a highly refined and exceptionally pure product, variations in quality must be caused by extremely small traces of impurities. It has been noted that a “weak” sugar may be strengthened by addition of as little as 0.001 % of soda (Ambler, 1927). Dehydration and caramelization of sugar are more sensitive t o changes in p H than inversion in the range of this test. Pucherna (1950) studied the effects of various catalysts upon the formation of color in an empirical caramelization test.

20

L. F. MARTIS

His procedure was improved by Kalyanasundaram and Rao (1951) who carried out the test by heating sugar in anhydrous glycerine. They investigated the effects of addition of 0.1 % of various substances on both color formation and sugar decomposition. Sodium chloride had least effect in destroying sugar and developing color, although it produced a high percentage of inversion. I n contrast, potassium chloride caused very little inversion but Catalyzed the destruction of 4.2% of the total sugar. Water added at a concentration of 0.1% caused both inversion and destruction of sugar. As would be expected, ammonium sulfate and chloride not only inverted the sucrose almost completely, but caused destruction of substantial percentages of the reducing sugars formed with the maximum development of color. IV. MODIFICATION OF SUGARPROPERTIES BY MINORINGREDIENTS Nearest in simplicity of formulation t o hard candies and other allsugar types are those in which the required properties are developed by incorporating small percentages of ingredients other than sugars. Reference t o Table I1 shows t h a t nougats, marshmallows, and pectin jellies are the principal examples of this class. Taffies, kisses, and molasses types in which flavor and color are the result of reactions of the sugars catalyzed by traces of nonsugar ingredients should be included in this category. Creams are not included because the optional small additions of starch or albumin occasionally used are not essential for the control of crystallization upon which their properties depend. Butter is used in butter creams primarily for flavor, rather than for modification of texture, and is usually added under conditions that minimize reaction with the sugar. The principal minor ingredients employed t o modify the properties of sugar are egg albumin, gelatin, and pectin, with less general current use of agar, tragacanth, and similar natural gums. Nougats are cooked a t high temperatures so that chemical reactions of the sugars are as important in their preparation as is the modification of texture by aeration with whipped egg albumin or other protein whipping agents. Physical properties are more important than chemical changes in the production of marshmallows and pectin jellies that are processed a t much lower temperatures. Their textures depend upon the formation of gels of sugars with the added gelatin or pectin under suitable conditions t o provide sufficient body with maximum tenderness. These candies must be produced with high moisture contents that must be retained in storage t o prevent deterioration of texture. 1. Protein Whipping Agents

a. Egg Albumin. The function of protein whipping agents in candy production is t o provide lightness of texture and low density in all-sugar

CANDY MANUFACTURE

21

formulations that, without such modification, would be essentially hard candies. The procedure generally followed is to prepare a portion of the batch of sugar sirup separately by whipping with albumin to produce frapp6 or nougat cream. From 10% t o 15% of this preparation is added with whipping to the balance of the cooked sugar sirup after it has cooled to a suitable temperature. Maximum foam volume and stability are the important requirements of proteins employed for this purpose. Some studies have been made of the whipping capacity of egg albumin prepared under different conditions, but there are no published results of research

PH

FIG.2. Dependence upon p H of the foaming of solutions of pure (crystallized) egg albumin, (1) dissolved in pure water, and (2) dissolved in 0.001 N KOH initially. (Thuman et al., 1949.)

on the optimum properties required for candy manufacture. The dependence of whipping quality upon p H has been determined for both fresh and frozen egg whites by Bailey (1935). Her results show th a t freezing and storage in the frozen state have a negligible effect upon the volume or stability of the whipped material. Egg albumin purified by crystallization was used by Thuman et al. (1949) in their precise study of foam volumes produced by bubbling under various conditions. The results which they obtained a t different values of p H are shown in Fig. 2 . Pure albumin dissolved in water yields maximum foam at p H 4.0,

22

L. F. MARTIN

slightly on the acid side of its isoelectric point, but no foam in the range from p H 7-11 adjusted by adding potassium hydroxide, as shown by curve 1 (solid). Curve 2 (dashed) is a plot of the foam volumes determined when the albumin was first dissolved in alkali, 0.001 N KOH, in which case foams are produced in the alkaline range as well as a t about p H 4 (the maximum production). Alkaline earth salts were found t o increase foaming in the alkaline range but caused a decrease in acid solutions. Dried egg albumin is used in practically all candymaking applications of the protein, as it can be stored and weighed conveniently. Drying has little effect on the foam-forming characteristics, freshly dried albumin being almost equivalent t o whole egg white in this respect; however, it tends t o develop color and off-flavor with loss of whipping power during storage unless it is properly prepared. Stewart and Kline (1941) discovered that small amounts of glucose normally present in egg white are the principal cause of this deterioration in storage. As little as 0.02% of glucose, or of other reducing sugars substituted for it, were shown t o produce rapid coloration, a decrease in solubility, and loss of whipping capacity. Practical methods were developed for eliminating the glucose prior t o drying. This is accomplished by natural or spontaneous fermentation, by inoculation with specific fermenting organisms, or more effectively by fermentation with yeast (Ayres and Stewart, 1947; Stewart and Kline, 1948). Research on the development of this process also has added t o knowledge of the control of temperature and p H in the drying operation necessary t o produce dried albumin of the best whipping quality (Kline and Stewart, 1948). The practical superiority of fermenfed and dried egg white has been established by Carlin and Ayres (1951) by comparison with unfermented material in angel cake baking. The fermented product retained 9501, of its effectiveness for this purpose for 2 weeks, and 90% for 12 weeks in storage a t 40" C. (104" F.), whereas the untreated egg white lost from 50% t o 66 % of its cake-volume producing quality under these conditions. Similar practical storage tests of the improved whipping agent for candy frappe production have not been carried out, but obviously would be of value in this application of dried albumin. b . Soy Protein. Effective whipping agents have been developed from soy flour following the determination of the essential conditions for their preparation by Monaghan-Watts (1937). F a t must be extracted completely by petroleum ether or other hydrocarbon solvents, as alcohol extraction has an adverse effect upon whipping quality. The product can be deflavored by heating for twenty minutes a t 130" C. (266' F.) under vacuum. It produced foam volumes equal t o those obtained with egg white whipped under the same standardized conditions. Maximum

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23

volumes were observed just above and below the isoelectric point of the protein a t p H 4.1, which is approximately the same as that of egg albumin. Addition of salt in concentrations up t o 2% increased the whipping capacity of this protein by as much as 20%. Perri and Hazel (1947) determined the stabilized foam volumes produced by bubbling nitrogen through solutions of soy protein at different p H values in the range from 2.0 t o 12.0. These measurements were made by a n adaptation (Perri and Hazel, 1946) of the method and apparatus developed by Clark and Ross

PH

FIG.3. Dependence upon pH of the stabilized foam volumes produced by bubbling nitrogen through solutions of soy protein. (Perri and Hazel, 1947.)

(1940) with the results shown in Fig. 3. The similarity of the behavior of soy protein t o that of egg albumin with respect t o p H will be evident from comparison of these results with curve 1 of Fig. 2. Maximum foamforming capacity in the practical p H range is observed a t values near the isoelectric points of both proteins. The application of commercial soy protein whipping agents t o production of frapp6 for use in nougats and other candies has been described by Butler (1942). Turner (1946) states that this protein does not coagulate on heating in the same manner as egg albumin in order t o maintain the body of nougat candies, but that it dissolves more readily and produces maximum volumes of frappe with the limited amount of water in concentrated sugar sirups. Soy protein may be used in frappe in proportions

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L. F. MARTIN

of 2 % t o 3 % of the weight of the sugars, as with egg albumin, but modified methods of preparation are recommended t o keep the moisture content below 18%. Unless used promptly, such frapp6s shrink in volume during storage t o a much greater extent than those produced with egg albumin (Clay, 1953). c. Milk Protein. Preparation of a whipping agent from the protein of skim milk or whey has been described by Leviton (1938) who employed extraction with aqueous alcohol t o obtain a soluble, undenatured product. Oberg et al. (1951) treated skim milk with acid, followed b y digestion with trypsin a t p H 8.0 and 50" C. (122" F.) t o obtain material that produced stable whips. Their properties have not been studied as extensively as those of egg albumin or soy protein, and they have been recommended generally only as a partial replacement for egg albumin in frapp6s. 9. Gelatin Gelatin is used principally for the production of marshmallows, ranging from the soft, standard type t o harder-grained candies molded in various novelty shapes. Both kinds are usually molded in starch that must be dried sufficiently t o form a durable outer skin while absorbing moisture from the candies t o produce proper gel formation. Soft regular marshmallows must retain the relatively high moisture content shown in Table I1 t o have good storage quality. Measurements of gel strength with the instrument devised by Bloom (1925) are the generally accepted basis for grading and standardizing the quality of gelatin. The procedure for making the determinations has been described by Richardson (1923). Bronson (1951) considers gelatin of 225 Bloom most suitable for making regular marshmallow, and lower strengths of 75 t o 125 Bloom best for producing grained novelties, but i t has been demonstrated by Gorfinkle (1953) and others that the final texture of the candy depends on other properties as well as Bloom. Whipping power, which is not related t o gel strength, is deemed t o be equally important by Clay (1953). Steigmann (1944) investigated gel setting times and the viscosity of gelatin solutions as possible measures of quality. The utility of these measurements, in which observed differences were magnified by protein dispersing agents such as urea and Teepol, remains t o be established by their correlation with the gel strength and whipping properties that are essential in candy production. The strength and stability of gels formed by gelatin depend upon its molecular size and the distribution of acid and basic groups in its structure. Friedman et al. (1939) attempted t o establish the molecular weight o€ gelatin by determinations of the acid and base combining capacities. More accurate estimates were obtained by Mosiman and Signer (1944)

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25

from data on the sedimentation equilibrium in the ultracentrifuge, which show that there are two distinct protein fractions with molecular weights of approximately 16,000 and 89,000. The effect of p H upon the disaggregation, or average molecular size of the larger protein molecules, was studied by Friedman and Klemm (1939) by measurement of their diffusion rates in solution. If ion effects were suppressed by potassium chloride in concentrations of 0.1 N , there was little change in molecular

Concentrattor molar

FIG.4. Variation of the setting time of gelatin gels with increasing concentrations of sucrose and leviilose. (Curves plotted from data of Friedman and Shearer, 1939.)

size between ~ € 1 2 . 0and 6.4. The variation of setting time of a gelatin gel caused by low concentrations of sugars was investigated by Friedman and Shearer (1939) and is of particular interest with respect t o use of the protein in candies. Their results, shorvn in Fig. 4, extend only to 0.2 JL solutions containing approximately 6.75 % sucrose and 3.6 % levulose, but it would be desirable t o determine whether the trend of decreasing setting times holds for concentrated solutions of these and other sugars. Invert sugar and glucose might be expected t o act like levulose in bringing about much more rapid setting than granulated sugar a t equal concentrations, at least within the range covered by these experiments. The addition of sodium hexametaphosphate or other complex phosphates in concentrations of 0.5% t o 5.0% is claimed by Grettie (1940) t o improve the whipping properties of gelatin in marshmallow compositions without decreasing their viscosity or gel strength. Collins (1940) patented the use

26

L. F. MARTIN

of sodium bitartrate, tartrate, lactate, acetate, or citrate in concentrations sufficient t o develop a final p H of 3.0 t o 4.7 t o produce smoother, fastersetting gels. 3. Pectin

Pectin is a valuable by-product of fruit processing as well as an essential ingredient of a variety of food products, including candies. Although it is found almost universally in plants of all species, suitable grades for use in the production of sugar jellies are obtained commercially from only two sources, apple pomace from cider presses and citrus peel remaining after juice extraction for canning. Innumerable investigations have been carried out on the theoretical as well as practical aspects of the varied uses of pectin. An excellent comprehensive treatise on the subject is now available (Kertesz, 1951a), and Baker (1948) has summarized some of the major recent developments in pectin chemistry more concisely. Most of this research has dealt with problems in the production of fruit jellies having lower solids contents and weaker gel structures than are required for pectin jelly candies. Some fundamental studies have included the range of high-solids concentrations. Recent research has established the details of the structure of pectic substances completely enough t o provide explanations of their behavior in terms of chemical constitution. Although the data obtained in many practical investigations of the low-solids gels may be extrapolated as a guide for the use of pectin in candy, there is urgent need of similarly thorough research on gels of high-sugar content. The principal value of this brief review of the subject will be t o indicate some of the lines that such research might. follow. a. Chemical Nature of Pectin. The source of pectin is an insoluble plant constituent, protopectin, which can be solubilized and extracted only by treatment with acids, alkalis, or enzymes. Some hydrolysis is necessary to convert it t o soluble forms and this inevitably alters the product in two respects: the size and homogeneity of the polymer are reduced, and some of the methoxyl groups are removed. Evidence summarized by Percival (1950a) has established the structure of unaltered, ideal pectin t o be completely methylated, branched chain polymers of galacturonic acid as first suggested by Morel1 et al. (1934). Such an ideal pectin should contain 16.3% methoxyl, but it has not been isolated as yet and all of the substances actually studied and used are pectinic acids. The general term pectin (or pectins) designates those water-soluble pectinic acids of varying methyl ester content and degree of neutralization which are capable of forming gels with sugar and acid under suitable conditions (Kertesz, 1951b). Complete demethylation produces pectic acid. The unit linear chains have been shown by Jansen et al. (1949) t o be composed of a

CANDY MANUFACTURE

27

minimum of about 32 galacturonic acid groups, and these units are joined by branching t o form polymers of very high molecular weights. Average molecular sizes estimated by various methods range from 16,000 t o 50,000 according t o Svedberg and Gralhn (1938), and were found t o be above 100,000for some pectin preparations studied by Owens et al. (1946). The foregoing very brief description of the structure of pectinic acids provides some explanation of the variability of commercial pectins. The polymer sizes or molecular weights are averages determined for particular pectin preparations which may be mixtures of polygalacturonide esters of widely varying sizes. This heterogeneity and the fact t h a t different fractions of the material may differ also in degree of demethylation, account for variations in gel-forming properties (Speiser and Eddy, 1946). This has been explained in detail by Kertesz (1951~).I n addition, significant amounts of araban and galactan are so closely associated with the pectin that they were believed t o be essential units of its structure until evidence t o the contrary was obtained by Hirst and Jones (1939). They may be present in amounts ranging from 5% t o as much as 70% of the weight of total pectic substance (Kertesz, 1951d), and are termed “ballast.” The practical use of pectin has been established and simplified only by long and continuing efforts t o perfect methods of standardization and grading (Wilson, 1926; Baker and Woodmansee, 1941), without which it would be impossible t o depend upon the performance of commercial preparations. h. Applzcations i n Candy Production. As pectin jelly candies necessarily have a high-sugar content, they are produced with regular, highmethoxyl pectin, generally of 150 grade. One pound of such pectin will suffice t o produce jelly of prescribed strength from 150 lb. of sugar under standardized conditions. Details of the various methods for determining jelly grade are described by Kertesz (1951e). Such high-methoxyl products are all demethylated t o a considerable extent during extraction, ranging from a minimum of about 7 % t o slightly over 11% methoxyl content. Variation of the degree of esterification within this range and differences in the size and homogeneity of the polygalacturonide molecules affect the gel-forming properties. Production of slow-setting pectins by patented acid treatment procedures is believed by Baker and Goodwin (1944) t o depend primarily upon reduction of methoxyl content, although all high-methoxyl pectins are not necessarily quick-setting and those of high jelly grade tend t o set slowly. Slow-set types that start t o gel a t about 54” C. (130” F.) in 65% sugar solution are recommended for ease of manipulation and development of better body in cast jellies. If the pectin contains more than about 7 % methoxyl, it will not produce a jelly unless sugar is present in concentrations of at least 40%,

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L. F. MARTIN

or preferably higher. This distinguishes the low-methoxyl pectins from ,, the regular, less demethylated products that have been standardized for sugar jelly production. Myers and Baker (1934) first showed that pectins containing as little as 3% methoxyl can produce useful gels if they have been demethylated under conditions that do not cause extensive depolymerization. Speiser et al. (1947) have presented evidence t h a t the gel in this case is of a different type, being formed by ionic bonding in the presence of calcium ions through the preponderance of free carboxyl groups. With ester groups predominating in the high-methoxyl pectins, gel formation depends upon hydrogen bonding for which the hydroxyl groups of sugar are essential. Although the high-sugar concentrations of candy are more than ample t o form regular pectin jellies by hydrogen bonding, a n interesting range of properties can be developed by various modifications of the low-methoxyl products with appropriate adjustment of concentrations, pH, and calcium ions used. Demethylation has been effected by acid (Baker and Goodwin, 1941), by alkali (McDowell, 1951), or by the enzyme, pectinesterase (Willaman et al., 1944), either during extraction or by treatment of isolated high-methoxyl pectin. The different methods yield products having dissimilar gel-forming characteristics, thus affording a wide range of possible variations of jellies. Owens and Maclay (1946) have investigated the properties of 65 % sugar gels formed by pectins ranging from 10.8% methoxyl t o as low as 4.5% methoxyl. The low-methoxyl products used were variously demethylated by acid, alkali, or the enzyme. Gel strengths were measured by a rigidometer based upon the principles of the B.A.R. jelly tester (Lampitt and Money, 1936, 1939; Campbell, 1938) and, although they were less than the strengths desired for candies, the results are of interest in indicating the possibilities of using the modified products in high-sugar content jellies. The.limiting p H for gel formation decreased from p H 4.5 for the undemethylated pectins t o p H 2.8 for those of lowest methoxyl content. Below the limiting values, no maxima were observed and gel strength for either type of pectin remained constant down t o p H 1.4. Increased sensitivity t o the presence of calcium was observed as the methoxyl content decreased, as reported by others (Hills et al., 1942; Baker and Goodwin, 1944). An especially interesting observation in this study was the dependence of gel strength upon the molecular weight of the high-methoxyl pectins of approximately the same methoxyl contents. The enzyme-demethylated product produced weaker gels than those demethylated by acid or alkali under comparable conditions. The application of regular grades of pectin in candymaking has been described by Cruess (1946), and Cruess et al. (1949) have published a number of formulas for candies made from fruit juices, employing 150

29

CANDY MANUFACTURE

grade pectin. If alkali-demethylated, low-methoxyl pectin is used, satisfactory jellies can be made with larger proportions of corn sirup substituted for sugar according to Hall and Fahs (1946); and Martin et al. (1952b) described the preparation of candies containing large percentages of honey with a similar type of pectin. Demethylated pectins form gels of high as well as low-sugar content according t o Angermeier (1953), but the former have not been studied as extensively as the low-solid gels for which these pectins are uniquely suited. Kertesz (1951f) states that, “It is also clear that the commercial value of low-ester pectins must be defined in a different manner than the old-fashioned high-ester types.” The gel-forming power of low-ester products has not been standardized in terms of grade, and acid, alkali, or enzyme de-esterified pectins cannot be used interchangeably in jelly formulas. Poulsen (1953) has presented evidence that standard, candy-type jellies made with regular pectin are superior to similarly formulated low-methoxyl pectin jellies in texture and resistance to sweating.

4. Natural Gums The gel-forming substances classified as natural gums are now used t o a negligible extent in candy production. Only scattered references t o their use in any types of candy appear in a recent reference book on these gums by Mantell (1945), and no research on candy applications of gum arabic or tragacanth has been published. Gum drops, originally produced with gum arabic, are currently made with either starch or pectin. The use of gum tragacanth as a binding agent in lozenge paste has been superseded by gelatin or dextrin (Clay, 1953). Combination of gum tragacanth with lactates improves its dispersion in a product described by Buchanan (1945) for use in confectionery.

v. MAJORINGREDIENTS

OTHER

THAN

SUGARS

Many important types of candies derive their characteristic properties from nonsugar ingredients that are used in such substantial proportions that they not only act as modifiers of the sugars, but impart their own properties to the confections. The principal examples are fudges, caramels, and starch jellies. As shown in Table 11, the milk solids, fats, and starch constitute from at least 5% to a maximum of 35% of the finished batch weights of these candies. Cocoa solids and cocoa butter are major ingredients of chocolate goods, imparting the flavor and desirable texture. Cooking procedures and temperatures may be regulated primarily t o achieve optimum physical blending or, in some cases, they may be such as t o cause extensive chemical reaction and modification of the ingredients, Nuts, peanuts, and fruits that are incorporated physically in candies

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L. F. MARTIN

are in this category. Pertinent results of research on each of these diverse food materials are summarized in the following sections. 1. Milk Products

Milk introduces proteins, fat, mineral salts, and a n additional sugar, lactose, into the composition of candies in which it is used. Concentrates such a s sweetened condensed or evaporated milk are generally employed rather than whole milk. Skim milk and nonfat milk concentrates or solids find wide application. The dry milk powders are used extensively in fudge and chocolate, and suitable grades, properly reconstituted, can be used satisfactorily in making caramels. Typical caramel flavor is developed by the proper reaction of milk and sugars. Additional butter is incorporated in limited quantities of these candies in lieu of the vegetable fats used for their large-scale production. The use of butter t o develop the characteristic flavor of butter creams has already been noted, and its reactions with sugars in cooking a t higher temperatures t o a low final moisture content produces butterscotch. Milk solids products of various kinds are constituents of milk chocolate candies and coatings. Numerous reactions in addition t o those of the sugars previously described in section I11 become possible in complex formulations involving constituents of milk. The predominant chemical changes are those resulting from ‘ I browning” reactions of the milk proteins with sugars. Attention was first directed t o such phenomena by the work of Maillard (1912, 1913), and they have been the subject of increasingly extensive investigation since that time. The mechanism of the interaction of natural products containing aldehyde and amino groups is not definitely established, although it has been studied in experiments far too numerous t o be described here. The most significant results of this research and their interpretation will be found in comprehensive recent reviews such as that by Danehy and Pigman (1951). The “browning” reactions in milk itself are particularly important in the production and storage of dry milk products, and they have been investigated very thoroughly in recent years. A pertinent study of the development of color in lactose solutions was carried out b y Webb (1935) with the conclusion that lactose caramelization alone does not account for the formation of color in evaporated milk. Phosphate in the buffer used in some of these experiments had a specific color-producing effect. Addition of amino compounds led t o formation of more color than that attributable t o caramelization of the sugar. From a later study of the effect of lactose concentration, pH, and other variables upon the formation of color in casein-lactose solutions, Kass and Palmer (1940) held t o the conclusion that lactose caramelization is responsible for changes observed on heating milk. More

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recent evidence from determinations of the decrease in the amounts of certain amino acids (Patton et al., 1948) and of amino groups (Mohammed et al., 1949) of proteins upon heating with glucose supports the view that browning is a protein-sugar reaction. Lea (1948) showed that the amino groups of milk proteins are lost in a 1: 1 ratio t o the disappearance of reducing sugar in the course of the reaction. Tarassuk (1947) has summarized the results of studies of the proteinsugar reaction bearing upon the practical problem of alteration of color and flavor in production of evaporated milk. He added evidence that the presence of oxygen at sterilizing temperatures is a n important factor in accelerating the adverse effects of further decomposition of the initial condensation products. Oxygen is consumed and carbon dioxide is formed in these later stages of the Maillard reaction. Similar changes occur, although more slowly, in the “ d r y ” state t o which Lea (1948) and his co-workers have directed their attention (Henry et al., 1948; Lea and Hannan, 1949, 1950). Knowledge of the chemistry of “browning” is of fundamental importance in several respects for the production of candies cmtaining any form of milk solids. It affects the quality of condensed and dried milk products employed as ingredients. Conditions must be regulated t o limit the extent t o which this reaction takes place in the production of fudge or milk chocolate, in which i t is desirable t o retain unaltered milk flavor. Improved caramel flavors can be developed by taking advantage of conditions t h a t have been found t o accelerate reaction of the milk proteins with sugars, or t o alter the course of the browning reaction. Storage life of the candies is affected by changes resulting from the slow interaction of the ingredients when at a lowmoisture content, which has been the subject of investigation by the Cambridge (England) group under Lea. Pertinent results have been reported by Lewis and Lea (1950) in their study of the loss of amino nitrogen of casein in its reactions with glucose, fructose, maltose, and lactose a t low temperatures and a low-moisture content. This change proceeded most rapidly with glucose, a t lower and approximately equal rates with either maltose or lactose, and most slowly with fructose. Candymaking would profit by the extension of such research t o the effects of excess proportions of the sugars used upon the course of the browning reaction. Butter is used for its flavor in some fudges and caramels, and in butter creams. It is incorporated in such candies under conditions t h a t minimize reaction or alteration. Means of retarding the development of oxidative rancidity have been described in section 11, 3. Richmond (1953) reports t h a t the keeping quality is generally known t o be improved markedly by adding the butter t o the batch at the highest possible temperature that

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L. F. MARTIN

will not impair its flavor, probably by inactivation of the enzymes. Martin et al. (1951b) have reported an experiment in which freshly churned butter was found t o produce inherently longer lasting candies than butter of apparently equal quality that had been stored under refrigeration for a month. Similar observations had been made by Holm and Greenbank (1923) on the keeping quality of butterfat in milk powders. The fat was more stable in powders prepared after holding the milk for only 12 hr. than in those produced from milk held for 1 day. These authors consider the deterioration of butterfat to be primarily the result of enzyme action. Butter is stabilized by cooking t o the high temperatures and final concentrations required t o develop the flavor of butterscotch candy, but no research has been reported on the chemical changes occurring under these conditions. 2. Fats Other than

Dairy Butter and Cocoa Butter

Large quantities of processed vegetable fats are consumed in candy manufacture, principally in fudges, caramels, and nougats, with smaller quantities being required for a variety of other candies, such as creams and jellies. Formulas for fudge and nougat call for amounts u p t o 5% of the batch weight and as much as 15 % is used in some caramels ; however, nougat and caramel may be made without vegetable fat. The larger proportions of fat effectively control or prevent the crystallization of sugar and impart plasticity and smoothness of texture required in the highest quality products. An extremely wide range of grades with different melting points, plasticities, and other properties suitable for various candyproduction requirements are available, and modifications meeting virtually any specifications are possible. Even a cursory review of research so extensive as that which has been applied t o the technology of fats is beyond the scope of this article, and recent comprehensive texts such as that by Bailey (1951) should be consulted for important details. Four of the principal classes of fats listed by th a t author are important in candy production. They are: (1) vegetable butters, typified by cocoa butter, which will be considered in relation t o chocolate; (2) lauric fats, such as coconut, palm, and babassu that contain 40-50% lauric acid; (3) oleic-linolenic fats represented by peanut and cottonseed, with less than 20% saturated acids; and (4) linolenic fats, of which soy is the principal example that finds use in candy. Cocoa butter has been established as the ideal candymaking fa t because of its availability from cocoa and chocolate production, and its unique plastic properties with a narrow melting range below body temperature. Such research as has been done on other fats with candy applications as its objective has dealt with their modification t o simulate

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33

the properties of cocoa butter. Hydrogenation of low-melting, lauric-type coconut f a t provides products melting in the same range as cocoa fat. Removal of the low-melting glycerides by fractionation of coconut fat a t 22’ C. (72’ F.) produces vegetable stearines suitable for candy production. Selective hydrogenation has been employed by Ziels and Schmidt (1949) to convert peanut fat into high iso-oleic forms having properties similar t o those of cocoa butter. Various types of fat may be used in caramel production, but Clay (1953) reports that the refractive index is important in affecting the translucency of the finished candy, high refractive indices giving a more desirable creamy appearance t o caramels or toffees. 3. Starch

There are two distinct ways in which starch is used in the production of candies-as a major ingredient of starch gums or jellies, and as the molding medium for the large variety of candies that are shaped by casting in starch. Different properties are required for these two applications, and manufacturers furnish special grades for each purpose. Corn starch of suitable regular or modified grades is used almost exclusively because of its availability and low cost. Results of research applicable t o jelly production and t o molding will be considered separately, and it will be evident that the unique properties of corn starch are better adapted t o candymaking than the corresponding qualities of other starches such as wheat, rice, tapioca, or cassava. a. Starch Jellies. These candies are made by forming rigid starch gels in which high concentrations of sugars happen t o be present. The sugars alter the gel-forming properties of the starch which is a major ingredient that must be present in amounts of a t least 7 t o 12% or more. This distinguishes the starch jellies from those made with pectin which, in relatively minor proportions, serves t o modify the properties of sugar and t o form gels in which the sugar is the principal structural material. The qualitative requirements of starches and processing techniques for gum candy production have been described by Kooreman (1952) who states that the ideal starch for this purpose would be one that “boils as thin as water” at the concentration necessary t o produce proper gel strength of the finished product. The nearest practical approach t o this unattainable ideal is t o use modified, thin boiling starches in the range of 40 t o 70 nominal fluidity. The viscosity of the gelatinized starch dispersion and the strength of the gel formed on cooling after concentration t o between 76 and 80 % solids are the most important properties of starches governing the quality of the candies produced in this way. Kooreman points out that it is essential to gelatinize the starch as completely as possible, and

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L. F. MARTIN

that p H 5-6 produces maximum viscosities in cooking that will result in stronger gels and better “body” or firmness of the finished candy. Starch gums are generally molded by casting in dry starch which assists the setting of the gel by reducing t h e final moisture content t o between 15% and about 18% as a maximum. The importance of rapid setting t o a sufficiently firm body for handling in continuous production, coupled with adequate moisture retention t o maintain the desired tenderness in storage, has already been explained in section 11, 3. The fundamental chemistry of the structure of the amylose and amylopectin constituents of starch as they are now known has been summarized most concisely and clearly by Percival (1950b). Amylose is the long-chain polymer of glucose that constitutes approximately 20 % of corn starch, whereas the balance consists of the branched-chain, amylopectin polymer. Some understanding of the nature of these two fractions is essential for interpretation of the results of practical experiments on the formation of gels under conditions employed in producing starch gum candies. Interesting and useful data on starch gel formation were reported by Seck and Fisher (1940a,b), although some of their theoretical interpretations have not been substantiated. The ability t o form useful gels, defined as both “form-elastic ” and “volume-elastic,” was proportional t o the degree of hydration of the starch. The degree of swelling was proportional to the viscosity of starch dispersions, but these properties were not related t o gel formation. Their conclusion that seed starches, such as corn or wheat, are capable of forming gels whereas root starches are not is not valid, at least for modified starches. Clay (1953) reports that high-quality gum drops are produced advantageously with tapioca and other root starches modified by the chlorination process described by Fuller (1943). Seck and Fisher did find that starches modified by oxidation formed stronger gels. The deduction from this observation that the amylopectin fraction was principally responsible for the formation of rigid gels is a t variance with measurements by Hamer (1947), which show that gel strengths of various starches are roughly proportional t o their amylose contents as reported by Bates et al. (1943). Hamer’s (1947) results are particularly applicable t o the production of starch jelly candies. He developed and standardized a method for determining the breaking strength of jellies prepared under reproducible conditions from a variety of starches. Corn and wheat starches formed the strongest gels in concentrations of 10 t o 15%, the practical range for candy production. Arrowroot starch also formed strong gels a t these concentrations. Other starches formed gels of comparable strength only when used in much larger amounts. At 12% concentrations, completely defatted corn and wheat starches formed gels with two t o three times the

CANDY MANUFACTURE

35

breaking strengths of those prepared from the same starches before extraction t o remove traces of fat or adsorbed fatty acids. Hamer also determined the effect of increased cooking times u p t o a maximum of 30 min. in producing stronger gels. Similar techniques were used by Bechtel (1950) t o measure both the rigidity and breaking strength in a detailed study of modified as well a s unaltered corn starch gels. Some of his data were obtained on starches in the range of fluidities used in candy production. Increasing degrees of acid modification reduced the consistency of the cooked paste proportionately more than the strength of the finished gel. At a fluidity of 40 the consistency was only about 10% of that of pastes of unmodified starch, whereas the corresponding gel was 30% as rigid and retained more than 2001, of the breaking strength. Extensively altered starch of 60 fluidity formed increasingly firm gels as the cooking temperatures were increased from 88" C. (190.4" F.) t o the maximum of 96" C. (205" F.). Gels of unmodified starch exhibited maximum rigidity and breaking strength when formed by cooking a t 94" C. (201" F.). The behavior of an intermediate grade of 30 fluidity was less consistent, showing a slight maximum in breaking strength a t 94" C. (201" F.), but minimum rigidity at this temperature. Changes in the properties of the gels on aging were included in this investigation by Bechtel. He found that those prepared from unmodified starch increased only about 20% in both breaking strength and rigidity in the period between 4 and 24 hr. after they were formed. Slight modification by acid altered this behavior markedly. Starches of 10 t o 20 fluidity formed gels whose breaking strength and rigidity increased as much as 90% in the same period of aging. I n view of the increased gel strengths reported by Hamer (1947) for defatted corn and wheat starches, this difference may be partly the result of removal of some of the adsorbed fatty acid during the treatment of the starch with mineral acid for modification. Starch normally contains about 0.8% free fatty acid which Schoch (1942) found t o be removed only by repeated extraction with lyophilic solvents such as dioxane or methanol. Favor and Johnston (1947) reported that this impurity prevented the increase in firmness of starch gels on aging. Although Lord (1950) questioned the soundness of this conclusion, he did find that addition of 1% stearic acid t o defatted wheat starch raised the temperature required for complete gelatinization from 76" C. (169" F.) t o 80" C. (176" F.). The lowering of gelatinizing temperatures and formation of clearer and firmer gels has been reported also by Caesar (1944). Maximum gel strength and firmness are not necessarily the optimum properties required for the manufacture of starch gum candies. Rapid initial setting t o jellies that, are sufficiently firm t o be handled mechani-

36

L. F. MARTIN

cally is essential, but subsequent increase of rigidity and strength causes toughening and loss of desirable quality. Proper balance of the grade or quality of starch, of time and temperature of cooking, and of the concentration of starch in relation to the sugars used is essential for the production of tender, high-quality jellies th at will have adequate storage life. Further research is needed on the quantitative effects of high concentrations of sugars upon the measurable properties of the gels. Certain inferences may be drawn from the results of investigations of pure starch gels, but this work is of value principally as an indication of lines of experimentation to be followed in determining the properties of starch over the range of conditions and effective concentrations used in candymaking. Scientifically sound and accurate methods will have t o be developed for measuring the strengths and rigidities of the high-sugar content jellies, as only roughly reproducible, empirical data are provided by penetrometer determinations of tenderness now used for routine control of quality (Martin et al., 1952a). b. Starch Molding. Unmodified corn starch possesses the required physical properties for retaining mold impressions and, if suitably dried, of absorbing moisture from the candies cast in it. It has the additional advantages of being edible, and low in cost as it may be sifted and reused repeatedly. Many different types of candy are shaped by depositing in starch and much ingenuity has been applied to the mechanization of this operation in ((mogul,” continuous production equipment. Hard or crystallized creams as well as those in which invertase is used t o develop fluidity do not require drying of the starch as they become sufficiently solid by cooling and crystallization. The starch is dried somewhat below its normal moisture content of l O - l Z % for starch jellies and t o a greater extent for pectin jelly production. The extreme case is that of marshmallow, for which starch must be dried t o about 6 % moisture content to absorb sufficient moisture t o effect proper setting of the gelatin. Addition of very small amounts of mineral oil is widely used to improve the definition and stability of mold impressions. Material that has been reused a number of times acquires superior molding properties t o those of fresh starch, which is usually added in small amounts t o replace losses. This conditioning is attributed by Liebig (1953) t o a polishing action that makes the granules more spherical. Starch has the disadvantage of being susceptible t o contamination and it also presents an explosion hazard when dried, although only one serious starch dust explosion in a candy plant has been reported. Martin et al. (1949) experimented with various U.S.P. food grades of calcium carbonate in attempts t o overcome these disadvantages. This inert material was suitable for molding hard creams on a laboratory scale, but could

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37

not be used for jellies or marshmallow as it has no moisture absorbing properties. Steel (1949) proposed the use of mixtures of carbonate and starch that proved satisfactory for all types of cast candies, except that in proportions of 50% or more of the carbonate the mold impressions were unstable. As Hartmann et al. (1950) found that a t least 60% calcium carbonate is required in mixtures with starch t o prevent spark ignition, and as much as 90% t o prevent ignition at heated surfaces, their use does not provide a practical solution of the problem. Hartmann and Nagy (1949) listed 38 major industrial starch dust explosions during the past 50 years as evidence of the urgent need for practical means of eliminating this hazard in other uses of starch as well as in candy production.

4. Cocoa and Chocolate Candies consisting entirely of chocolate in its different flavor modifications and a wide variety of forms, including those in which nutmeats are incorporated, constitute one of the major types of confectionery. Large amounts of chocolate are used in the coatings applied t o candies of almost every other type, and substantial quantities of cocoa powder and liquor chocolate are used t o flavor fudges, creams, hard candies, and many other items. The total consumption of products of the cocoa bean can be estimated from the amounts of these materials given in Table I t o have been approximately 350,000,000 lb. in 1950. This is substantially greater than the usage of any other candy ingredients except sugar and corn sirup. The unique properties and importance of chocolate have made it the subject of specialized study, and comprehensive reference books such as those of Whymper (1921) and of Jensen (1931) should be consulted for details of its production and uses. Whole nibs of cocoa beans, after fermentation, decortication, and roasting, are ground t o produce a basic material that is neither cocoa nor chocolate. Cocoa is a product of lower fat content obtained by pressing the ground beans t o remove a large proportion of the cocoa butter. Ample quantities of cocoa butter are usually available from this operation t o increase the fat content of ground nibs that are combined with sugar t o produce chocolate. Milk solids as well as sugar are added t o milk chocolate types. Refining and adjustment of the fat content converts the ground nib material into the bitter liquor chocolate employed as flavoring and as the basis of various chocolate compositions. a. Composition of Cocoa Products. Cocoa beans vary in composition, as would be expected, with differences in varieties and the localities in which they are grown. The manner in which they are fermented and the extent of decortication and separation of shell material introduce further variations. Pressing may be carried out t o remove more or less of the fat,

38

L. F. MARTIN

and roasting conditions are adjusted to develop different flavor qualities. It is not surprising that the few fairly complete analyses of cocoa beans or products processed from them that have been reported give somewhat widely divergent results. Examples of such analyses are assembled in Table IV. The composition of beans when freshly harvested and at TABLE IV Partial Composition of Cocoa Beans and Nibs Whole Beansa Cotyledonsa Cotyledonsa Commercialb Fresh Unfermented Fermented Beans (%) (%) (dry) (dry) Moisture Fat Total nitrogen Protein nitrogen Protein Total carbohydrates (sugar, starch, cellulose, pectins, gums, etc.) Sugar (glucose) Pectins Cellulose Pentosans Gums Fiber Tannins Pigments Theobrom in e Caffeine Ash 0

b

33.0 37.0 -

-

3.65 53.05 2.28 1.50

2.13 54.68 2.16 1.34

6.0

12.4

-

1.8 2.1

-

1.2 0.3 2.2

6.23 26.69 -

-

18.34

-

-

0.30 2.25 1.92 1.27 0.38 2.09 2.24 5.30 1.71 0.08 2.63

0.10 4.11 1.90 1.21 1.84 2.13 1.99 4.16 1.42 0.07 2.74

26.32

-

4.48 -

1.15 0.16 5.49

Condensed from Knapp (1937). Anonymous (1938).

different stages of preparation (as determined by Knapp, 1937) is compared with analytical data on average commercial beans and processed chocolate (Anonymous, 1938). As the fat content is of primary importance next to flavor, the fact that the West African beans analyzed by Knapp had about double the fat content of the commercial product of unstated origin and method of preparation is particularly noteworthy. A larger percentage of shell included in the commercial sample analyzed may account for part of the difference and is suggested by the higher values for ash, non-nitrogenous residue, and for cellulose which was separately determined in this case. Adequate data are not available to establish the typical composition or

39

CANDY MANUFACTURE

the range of variations to be expected for cocoa products. With the exception of a few major constituents such as the cocoa butter, starch, theobromine, and caffeine, individual chemical compounds present have not been identified and determined separately, and no complete investigations of composition have been reported. The characteristic pigments, cocoa red or cocoa purple as they are designated by Knapp (1937), have been classified as anthocyanins by Forsyth and Rombouts (1952). Fincke (1932) extracted fat-soluble fractions which he considered more important than the tannins or volatile constituents such as diacetyl in giving chocolate its desirable flavor and aroma. Three tannin fractions were separated by Aasted (1941), who speculated as t o the role of their oxidation in flavor development. Cocoa butter is an important industrial fat in applications other than candymaking, and its composition has been determined by Lea (1929), by Hilditch and Stainsby (1936), and most recently by Meara (1949). Results of the two latest investigations are in good agreement, as shown in the following tabulation: Composition of Cocoa Fato

Molecular percentages

Fatty acids

Weight per cent (Hilditch and Stainsby)

Palmitic Stearic Oleic Linoleic

24.4 35.4 38.1 2.1

Glycerides Palmitostearin Oleodipalmitin Oleopalmitostearin Oleodistearin Palmitodiolein Stearodiolein Triolein

(Hilditch and Stainsby) (Meara) 2.5 6.5 51.9 18.4 8.4 12.0 -

2.6 3.7 57.0 22.2 7.4 5.8 1.1

" Iodine number: 36.7.

Distribution of the acids is less random than in the glycerides of most other fats. The high molecular percentages of the individual glycerides provides the chemical homogeneity that accounts for the narrow melting range and other important properties of the fat. Its behavior on crystallization governs the changes produced in physical properties of chocolate by tempering. Vaeck (1950) distinguished three fractions by thermal analysis of cocoa butter. Further investigation by this author (Vaeck, 1951a,b) produced evidence for an unstable polymorphic form obtainable by rapid cooling which was transformed into a modification melting a t 20" C. (68" F.). Storage a t this temperature for a month resulted in complete transformation t o a stable form melting at 33.7-35.7" C. (92.7-96.3" F.). There is evidence for the presence of a larger number of

40

L. F. MARTIN

individually crystallizable, component glycerides, most of which exhibit polymorphism. I n his study of the composition of cocoa fat, Meara (1949) separated a t least eleven fractions by crystallization and determined-the melting ranges of stable and unstable modifications. b. Chocolate Viscosity. The importance of viscosity in molding, and particularly in the application of coatings, has been noted in the general discussion of physical properties. Viscosity of melted chocolate depends largely upon the properties of cocoa butter, but Stanley (1941) notes that it is affected by the previous processing history of the material, by the nature and particle size distribution of other cocoa constituents or added solids, and by the presence of moisture, air, and lecithin. For effective molding or coating work, chocolate must be used a t temperatures sufficiently near t o that of solidification, a t which it is not fluid enough t o be handled efficiently unless it is modified. Lecithin is the most widely used additive for reducing and stabilizing the viscosity of chocolate. Earlier research on the problem of altering or controlling this physical property has been summarized by Whymper (1933a). Results of more recent investigations appear in patents covering proposed treatments or additives devised t o reduce the viscosity of molten chocolate. Thurman (1937) described the addition of phosphatides from soap stock a t about the same time that foreign patents (Hansa-Muhle, 1937) claimed the effectiveness of similar phosphatides as viscosity reducers. Lanolin, or similar unsaturated fats were phosphorylated and polymerized by treatment with phosphorus pentoxide t o produce materials claimed t o be effective for this purpose by both Ross and Rowe Inc. (1939), and by Jordan (1941). Cocoa butter itself, or the fatty acids derived from it, were used by Jordan (1940) in preparing synthetically phosphorylated materials for addition t o chocolate. Sulfonated as well as phosphorylated diglycerides are claimed by Harris (1939) t o produce the required reduction in viscosity. These are typical examples of the numerous patent disclosures of empirical solutions of the problem, which is evidently complex and not yet understood on a scientific basis. Both the apparatus and procedure for determining the viscosity of chocolate have been studied with the object of standardization or improvement. Modifications of the MacMichael (1915) viscosimeter were developed by Kampf and Schrenk (1929) and by Eckstein (1937) specifically for use with chocolate. These are based upon the principle of measuring the torsion produced in a standardized wire by rotation of a cylinder immersed in the chocolate. A thorough study of the conformity of observations on chocolate samples t o the theoretical equation expressing the torsion as a function of dimensions of the wire and bob, and of the size and speed of rotation of the cup containing the sample, was reported

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41

by Stanley (1941). He determined factors for converting measurements made with various wire and bob sizes t o MacMichael degrees based upon the recommended standard use of a No. 27 wire with a bob 2 cm. in diameter, immersed t o a depth of 4 cm. in the melted chocolate a t 38' C. (looo F.) in a 7-cm. cup rotating at 20 r.p.m. It is thus possible, within the limits of accuracy of the conversion factors, t o cover the extreme range from less than 50' MacMichael for ice cream coatings t o the viscosities of heavy-bodied chocolate approaching 2000' MacMichael by employing bobs of either 1-cm. or 2-cm. diameter with 3 wire sizes, Nos. 26, 27, and 30. Zenlea (1938) also recommended standardization using the No. 27 wire with conditions the same as above, but a No. 26 wire with a 3-cm. immersion of the 2-cm. diameter bob and a speed of rotation of 15 r.p.m. was preferred by a committee of the American Association of Candy Technologists (Kempf, 1949a) as representative of practice in the industry. These conditions were used by Mitchell (1951) in a statistical study of the reproducibility of the method applied by two different operators t o numerous samples of a uniform lot of chocolate. The range of determinations was k l l ' from the value of 178.5" MacMichael which was found to be the viscosity of the samples; the standard deviation was 3.61'. A method capable of giving more accurate and reproducible results is much to be desired. The validity of converting measurements with other wire and bob sizes t o MacMichael viscosities using prescribed standard conditions has been questioned by Freundlich (1937, 1939). He advocates determination of chocolate viscosity a t 35" C. (95' F.) by using heavier wires a t a speed of rotation of 35 r.p.m. Variations observed in his measurements may arise partly from the rapid heating t o 40.5-46.1' C. (105-115" F.) employed in melting the samples, as Stanley (1941) has shown the effect of previous heat treatment upon the results of viscosity determinations and stresses the importance of duplicating conditions exactly, preferably by slow heating t o the same final temperature t o melt the samples. Viscosities determined a t a lower temperature, just above the melting point of the chocolate, have the merit of being more representative of its behavior in practical use, and Freundlich (1937) applied his procedure t o obtain a measure of the body, or "covering power" of coatings. Similar information was obtained by Stanley (1941) from curves of viscosities at various rotational speeds, measured at the standard temperature of 37.8" C. (100' F.). Extrapolation of such curves t o zero speed of rotation gave values for the yield stress which, expressed a s a percentage of viscosity, provides a measure of the body of the material. This work included experiments on the effect of lecithin upon both the viscosity and body of both cocoa fat and chocolate.

42

L. F. MARTIN

The pure fat behaves like an ideal liquid and has zero body when measured in this manner. Chernenko et al. (1951) used a cone penetrometer t o measure changes in plasticity of solid chocolate produced during treatment or on storage. There was an increase with time of the plastic modulus measured in this way. Their results indicate that small amounts of lecithin reduce the plastic modulus only temporarily. Clay (1953) reports that the most promising innovation, based upon a new principle of measuring viscosity, is the application t o chocolate of the Ultraviscoson (Roth and Rich, 1953), although Stanley (1941) believes t h a t it requires further development for practical use and general adoption. c. Conching and Tempering. Development of the velvety texture and final modification of the flavor of chocolate are accomplished by heating a t carefully regulated temperatures for periods of 36 t o 72 hr. or longer in machines called "conches." The name refers t o the shell-like shape of the container in which the chocolate is kept in motion during the process. Conching temperatures may be anywhere between 110 and 210" F. (43.3 and 98.8' C.) depending upon the initial quality of the chocolate and the intensity of flavor modification desired. No studies have been reported of the chemical changes that occur during this operation. When it is completed t o the satisfaction of the operator, the untempered product may be molded for storage or run directly t o tempering kettles or machines for the final step of processing prior t o its use in molding or coating finished candies. The purpose of tempering is t o develop desirable physical properties in the chocolate by promoting crystallization of the fat in very small crystals of the most stable modifications. Proper tempering prevents the development of internal stresses t h a t would result from unnecessary crystallization after cooling and thus definitely reduces the tendency t o bloom. The crystalline forms of cocoa fat obtained by effective processing in this step also possess the important characteristic of providing for sufficient contraction on setting t o release the candies from molds. Whether carried out by hand in ordinary kettles or in automatic tempering machines, the steps of the process are the same. The chocolate is heated under constant agitation t o 46-49' C. (115-120' F.) t o melt all crystals of fat. The temperature is then reduced gradually t o 29-30' C. (84-86" F.) t o recrystallize the component having the highest melting point together with some of the next highest melting glyceride fraction. As crystallization proceeds the material becomes a thick paste of extremely small fat crystals nearing solidification. When seeded in this manner with stable crystalline forms of the fat, chocolate retains its temper while reheated for use a t 31.7-32.2" C. (89-90" F.). Alternatively, the melted chocolate may be cooled only t o this temperature range at

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43

which i t is used, seeded with 10% of well-tempered chocolate, and stirred until the entire batch is tempered.l The tempering process has been described in detail by Kempf (1949a,b), and discussed most recently by Koch (1952). T h a t the postulated explanations of the results obtained, based upon various interpretations of the underlying phenomena, are controversial is evident in the criticism of the latter author’s deductions by Whymper (1952a). There is general agreement upon the primary importance of proper crystallization of the cocoa butter, which has long been recognized as the major purpose of tempering. No such agreement has been reached upon the utility of methods proposed for determination of the effectiveness of the process, or the degree of temper obtained under different conditions. Cooling curves determined b y plotting temperatures at various times of cooling under standard conditions were used b y Pichard (1923) as a means of detecting adulteration of cocoa butter. This method was applied t o chocolate by Jensen (1931) t o determine its temper. Pichard (1932) also extended his method t o chocolate products as well as cocoa butter. Its application t o control of the tempering process was later developed more completely by Pichard (1937). All of the earlier attempts t o adapt such cooling curves t o the practical control of the process have been summarized by Whymper (195213). Additional modifications have been proposed recently by Easton et al. (1951), and by Meyers and Graham (1952). Criticism of the latter workers’ procedure b y Whymper (195%) emphasizes the need for a new approach t o the development of a practical method for measuring and controlling temper. Utility of the test procedure advocated by Easton and his co-workers has been questioned by Clay (1953). The possibility of using ultrasonic vibrations t o determine the temper of chocolate has been suggested by Mickevicz (1951). I n its present state of development, the method based upon this principle may be suitable for measurernent of viscosity (Roth and Rich, 1953), but this property has no simple or direct relation t o the state of crystallization and temper of chocolate. Better tempering was claimed t o result as an incidental benefit of the method of conching in stages patented by Aasted (1939). The primary purpose of this conching procedure is t o enhance the development of flavor, and its effect upon subsequent crystallizat,ion of fat during tempering must be secondary. d . High Melting Modifications of Chocolate. The natural melting range of cocoa butter below body temperature provides one of the most desirable qualities of chocolate, but it makes storage and distribution difficult during summer months. High-melting-chocolate is essential for the proIndebtedness to Mr. Clifford Clay for these details of conching and tempering is gratefully acknowledged.

44

L. F. MARTIN

duction of candy items for use in military rations. Numerous methods have been proposed to overcome the impairment of quality that results from substitution of the cocoa butter by hardened fats or t o raise the melting point of cocoa fat in conformity with identity standards. Penn (1941) hydrogenated chocolate liquor as well as added fat with Raney nickel, increasing the melting point of the fat component naturally present. A process similar t o the winterizing of vegetable oils was applied by Carver (1943) to cocoa butter to obtain the higher melting glyceride fractions for use in making heat-stable chocolate. The same end was sought, in addition t o stability against bloom formation, in the patents of Cook and Light (1940) and of Eipper (1948) on elaidizing the oleic acid present in cocoa butter. Increasing or modifying the sugar content has been described as another method of formulating high-melting chocolate candies. Bars made with dextrose, lactose, or other sugars with similar properties are claimed by Sarotti (1943) t o be more resistant t o melting t.han those of chocolate made ent,irely with sucrose. A higher sucrose content can be employed if the sugar is dispersed in the chocolate in the form of extremely fine crystals like those in fondant, according t o McGee (1948). As extra-fine grades of sugar are generally employed in chocolate manufacture and are very thoroughly dispersed during refining and tempering, the result of this proposed modification is primarily a reduction in the proportion of lom-melting cocoa butter in the product. Kempf and Hoben (1949) give formulas for milk chocolate compositions th a t are sufficiently stable t o heat for summer distribution. These contain, in approximately 100 lb.: chocolate liquor of 52% f a t content, 14.75 lb.; sugar, 49.75 lb.; dry skim milk, 12.25 lb.; cocoa butter, 23.25 lb.; water, 2.5 lb., and vanilla flavoring. Emulsifiers of the type th at have been patented as preventives of f a t bloom (Eipper, 1938; Mayberry, 1951; Cross, 1953) have been used also for improvement of the palatability of chocolate made with high melting, noncocoa fats to withstand the extreme temperatures t o which ration candies must be subjected. Although the fats used melt above body temperature, the emulsifiers are reported t o minimize the unpleasant, waxy texture sensation which makes such fats objectionable. Alikonis and Farrell (1951) recommend the use of 1%of a mixture of equal parts of sorbitan monostearate and polyoxyethylene sorbitan monostearate for this purpose in chocolate bars and coatings to meet ration specifications. High lauric acid fats are used, and coconut oil hydrogenated to a melting point of 43.3-45.6' C. (110-114° F.) is preferred as the replacement for cocoa butter. The coatings are of the cocoa type, containing only 7.5% to a maximum of about 18% of cocoa powder, with coconut fat replacing

CANDY MANUFACTURE

45

92-94 % of the cocoa butter content of regular chocolate coating. Approximately S-9% of cocoa butter is retained in the chocolate bar for which these authors give the following composition: sugar, 50%; chocolate liquor, 17%; whole milk solids, 16%; hydrogenated coconut fat (110114" F.), 16%; and 1% of the mixture of emulsifiers. These modified and stabilized formulations are permissible for chocolate required for manufacturing military ration candies, but they do not conform t o the standards of identity for chocolate or regulations prohibiting the use of non-nutritive substances in confectionery for the civilian market. 5. Nutmeats and Fruits

The principal method of using nutmeats and fruits in candies is by incorporating forms in the finished batches just prior t o molding or forming the desired pieces. Diced or whole fruits may be added as a step in molding items such as cordial fruit candies or cordial maraschino cherries. The cream centers of this type are initially firm enough t o be chocolate coated mechanically or by hand, but contain sufficient invertase t o produce the required amount of sugar sirup by inversion after they have been coated. Peanut butter, almond paste or marzipan, jams, and similar materials that may be cooked into the candy or dispersed throughout it are used in smaller quantities. Reference t o Table I shows that peanuts are most important, comprising more than half the volume and about 40% of the total value of all ingredients in this group utilized in 1950. Coconut and almonds are the other major items, accounting for most of the remainder of the total consumption of fruits and nutmeats combined. Comparison of the figures for 1950 with those for 1947 indicates the extent t o which utilization of individual products may be altered by fluctuating prices or availability. Very little research has been conducted specifically upon candymaking uses of these products, and details of their production and processing for general food use would be superfluous t o this review. Useful summaries of existing information on the composition and chemistry of each of the fruits and nuts used in candy production will be found in the comprehensive treatise by Jacobs (1951). Reference will be made only t o particularly suggestive or pertinent investigations of problems of processing and storage of the individual ingredients of this class. a. Peanuts. Different types of peanuts have been found t o possess significantly different storage qualities. Development of rancidity was more rapid in Spanish peanuts than in Virginia or Runner types stored under the same conditions in the extensive study carried out by Pickett and Holley (1951). Organoleptic methods were used by these workers t o determine the rates of deterioration. The same relative stabilities are

46

L. F. MARTIN

exhibited by the oils extracted from the peanuts. In a study of oils extracted under comparable conditions from 16 different varieties, Fore et al. (1953) found that those from the Spanish types were less stable to oxidation determined by chemical tests. Flavor can be preserved for much longer storage times by refrigeration according to Woodroof et al. (1949), who recommend the use of activated carbon t o minimize absorption of odors. Cecil and Woodroof (1951a) tested a variety of antioxidants that were effective in protecting flavors of peanuts and other nutmeats in storage. Owen (1950) claimed that 1-2% of salt, containing 5 g. per lb. of ascorbic acid, was effective in stabilizing the flavor of peanuts to which it was applied. An antioxidant salt preparation containing propyl gallate, citric or other acids, and polyhydric alcohols, was patented by Hall and Sair (1950) for this and similar food applications. Reznikova (1941) states that antioxidants, not specified in the available report of her work, were ineffective, but that peanut flavor could be stabilized by alcohol, sugar sirup, or oatmeal. Higher alkyl or alkylene esters of polyhydric alcohols were applied to various nutmeats including peanuts t o prevent separation of oil by Neal et al. (1949). Sugar-amine browning reactions were the most important changes observed by Pickett and Holley (1951) in their extensive study of roasting. The recorded information on the composition of peanuts has been reviewed critically and summarized in a comprehensive bibliography by Guthrie et al. (1949). The earliest study of their composition by Payen and Henry (1825) noted similarities to that of almonds. Total unsaturated acids, oleic and linoleic, were found in significantly lower percentages in Spanish than in Virginia-type peanut oils by Jamieson et al. (1921), an observation at variance with the later determined greater susceptibility of the Spanish peanuts t o development of rancidity. Mean results of more recent analyses of a large number of samples by Stansbury et al. (1944) showed the oil of Spanish type peanuts to have a somewhat higher iodine number than the Runner or Virginia. Effectiveness of low-temperature storage was demonstrated by the preservation of samples for analysis for 2 years in sealed cans a t 1' C. (33.8' F.) without appreciable changes in total nitrogen, oil content, free fatty acids, or iodine number of the oils, as reported by Stansbury and Guthrie (1947). Changes in the extracted oil detectable by spectrophotometric methods were found by Pons et al. (1948) t o be much less at 1' C. (33.8' F.) than at 27" C. (80.6" F.) after 4 years storage. b. Almonds. The composition of almonds has received relatively little attention and no studies of changes in storage have been reported. The very early analyses by Payen and Henry (1825) have been supplemented by Pavlenko's (1940) determinations of the 15 to 18-fold increase in

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benzaldehyde content and other changes on ripening, and by comparison of the compositions of domestic and imported almonds initiated by Hart (1930) and completed by Pitman (1930). These investigators developed a shearing test for the objective measurement of texture. They also obtained some data on the composition of different varieties in relation t o seasonal changes and locality of production. The average oil content of whole kernels was 51-54%, a maximum of 59% being found in almonds of the Drake variety grown in the Sacramento Valley. Drake almonds gave lower shear test values than varieties of lower oil content. The oil is highly unsaturated according t o Heiduschka and Weisemann (1930) who reported the fatty acid composition t o be oleic, 77.0%, linoleic, 19.9%, and palmitic acid only 3.1%. Almonds are known t o be rich in a variety of enzymes and arc a source of emulsin which hydrolyzes glycosides. c. Pecans. Changes affecting the quality of pecans after they have been incorporated in candies have not been investigated, but their handling and storage has been studied extensively in order t o provide nutmeats of the highest quality for confectionery and other uses. The work of Brison (1937, 1945), extending over a 10-year period, established the importance of prompt refrigeration and storage a t low temperatures. Pecans stored a t - 15" C. (5" F.) did not become rancid in 2 years, whereas those held just below 0" C. (32" F.) developed rancidity in 11 months. Brison also determined certain chemical changes that occur during storage, but found that neither the phloroglucinol test nor the free fatty acid content parallels the development of rancidity determined by organoleptic methods. His results show that storage of pecans a t 1.7" C. (35" F.) and 90-92% relative humidity as recommended by Baker (1938) is not adequate for long holding times. Coating with a 40% sugar sirup was found by Godkin et al. (1951) t o improve the keeping quality of pecans. Cecil and Woodroof (1951b) showed that pecans are stabilized by antioxidants applied by addition t o the roasting oil or, in the case of salted nuts, by admixture with the salt. A combination of butylated hydroxyanisole, propyl gallate, and citric acid in total concentrations of 0.02% of the oil or 0.2% of the salt used, increased stability in the Shaal oven test 133 %. Interesting experiments were conducted by McGlamery and Hood (1951) on heat treatments intended t o inactivate the enzymes responsible for changes t h a t produce rancid flavors. Either hot air circulation, or immersion in oil after rapid air heating, was used t o bring the internal temperature of the nutmeats t o 80" C. (176" F.) for periods of 1 t o 12 min., followed by rapid cooling. Subsequent storage and organoleptic tests of treated and untreated pecans indicated beneficial results of this treatment applied a t any time u p t o 4 months after harvest. d . Coconut. Despite its importance as a flavor-imparting ingredient

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and the considerable quantities of coconut used in cream or in shredded forms in candymaking, no research on this application of the material has been reported in the scientific literature. The oil, which has been studied thoroughly, has applications in candy previously discussed, but only the carbohydrates of dried copra have been investigated by Caray (1924). Glycerol is used t o improve moisture retention of much of the dried coconut used in candymaking. Recent patents describe the use of purified, carbonyl-free propylene glycol and butylene glycol as humectants (Kaufmann and deMaya, 1952), and the addition of sorbitol for the same purpose (Welker et al., 1953). A rapid method of analyzing copra, applicable t o determining the content and quality of oil in coconut, has been developed by Pinto and Enas (1949). e. Other Nutmeats. Relatively small quantities of brazil nuts, cashews, filberts, walnuts, and hazelnuts are utilized in candy manufacture. If these present any special problems in storage or preparation for use as candy ingredients, or in candy formulation, no published research has dealt with the subject. Available data on their composition, relating principally t o the oils and proteins, have been assembled by Jacobs (1951). f. Fruits. The not inconsiderable volume of fruits, jams, and fruit products used by the confectionery industry is made u p of a large variety of ingredients too numerous to be considered individually. A summary of published information on the composition, storage, processing, and preservation of quality of these products will be found in the treatise by Jacobs (1951). Fundamental research on enzymatic browning, one of the most important factors in deterioration of stored or processed fruit quality, has been reviewed b y Joslyn (1951). Quantities of dried fruits are used in candy production, and whole fruits are often subjected t o heat and desiccation by incorporation in candies. For this reason, the changes produced by chemical, or nonenzymatic browning are important ;progress in research on this problem has been summarized by Stadtman (1948). Kirchner (1949) assembled and reviewed available information on the composition and chemical properties of fruit flavors. Concentrated natural fruit essences recovered by a process developed by Milleville (1948, 1950) and described by Milleville and Eskew (1944) have been used in pectin jellies by Martin et al. (1946). Conditions for making high quality glace fruits were studied by Tressler (1942), and Pentzer et al. (1942) patented an apparatus and method for their production.

VI. PRODUCTION METHODS More attention has been given t o mechanization than t o the fundamental processes of candy production. The voluminous patent literature on the subject is beyond the scope of this review, and deals almost exclu-

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sively with mechanical improvements in carrying out the same basic operations that have been employed traditionally. Even the so-called "continuous" vacuum cookers widely used for mass production of hard candy are actually high speed, intermittent batch cookers. This is evident in the description of one of the most modern installations for hard candy production by Ziemba (1950). Very recent innovations have been described in the development of truly continuous processes and equipment for cooking various types of candy. These provide the advantage of rapid, more uniform, and precisely controlled heating, in addition to the saving of time and labor. Most of these continuous flow methods, employing machines such as the "Votator " and "Turba-film" evaporators, are in the experimental stage, although commercial installations have been reported in a few cases. The shorter heating times made possible in such equipment alter the extent of reactions that are important in producing desired qualities. Limited reaction of the ingredients is advantageous in producing hard candy or marshmallow, but is ineffective in developing flavor of caramel which requires longer cooking times. A better understanding of the chemical changes occurring in any particular cooking process, and their dependence upon temperature and time relationships, will be essential in working out the most effective applications of these novel methods of candy manufacture. I n truly continuous cooking only a small volume of the material is heated for a short time t o the maximum temperature required t o convert it t o the final candy composition. Dissolving of ingredients and precooking in batch kettles are usually necessary, but the most critical stages of concentration and reaction are effected within seconds in a continuous stream of material flowing t o the forming or molding operations. An example is the process and apparatus recently patented by Leach (1953) to produce hard candy. A reservoir of sirup, precooked t o approximately 132" C. (270" F.), feeds the charge through a heating tube of small volume in which the temperature of the sirup is raised rapidly t o 149" C. (300" F.) or higher. A specially designed valve ejects quantities of the cooked material into die-molds on a continuously moving chain. I n the starch jelly process described by Bolanowski et al. (1952), the prepared batch is adjusted t o the moisture content desired in the finished candy t o be molded in starch, and preheated t o 76.7-82.2' C. (170-180" F.). The starch gel is formed by rapid heating t o 140" C. (285" F.) in a Votator unit. A second unit cools the gel promptly t o the temperature suitable for continuous depositing in starch molds. Among advantages claimed for this process are uniform cooking and elimination of the long holding period in hot rooms t o dry the candies t o the proper moisture content. Premixed ingredients for caramel are rapidly cooked and concentrated

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by evaporation of water in a process employing the Turba-film evaporator of Swiss design (Anonymous, 1952). Mechanical rotary blades produce the turbulence and maintain the film conditions for rapid heating of the viscous material. I n this case, additional cooking is required to develop satisfactory caramel flavor. Alikonis (1953) has described an ingenious method of cooking and aerating marshmallow continuously by a n air jet th a t directs the cooked fluid against a target, after which it passes through a porous alumina dispersing sleeve. The same author (Alikonis, 1949a,b) has patented a one-step process with continuous features for manufacturing chocolate liquor, as described in more detail by Slater (1952). I n a different line, Visenjou (1952) has developed a n inclined tube apparatus t o roast nuts in oil continuously and has applied it t o the processing of cashew nuts. The radically different conditions of processing necessitated or made possible by continuous operation of the cooking steps in candy manufacture, the possibilities of which are just beginning t o be explored, may be the next major development in the evolution of the industry. Wide adoption of such methods for making all types of candies would follow the trend in modern food processing toward more uniform conditions and results, automatically and precisely controlled without dependence upon human judgment and with further reduction of labor costs. Next t o the investigation and understanding of the chemical or physical changes in combining various ingredients t o produce candies, means of accurately controlling the conditions upon which such changes depend are of greatest importance in manufacturing candies of high and uniform quality. These two lines of development should go hand in hand as the development and perfect:on of new processing methods will depend upon the progress of research in discovering the fundamental nature of the processes involved and establishing the conditions for obtaining optimum results.

VII. SUMMARY If this review has outlined clearly the wide diversity and complexity of the fundamental problems of candy manufacture and the almost limitless opportunities for further research in this field, it will have served its primary purpose. Scientific principles have been applied too seldom t o investigation of particular candymaking processes. With few exceptions, past attempts t o apply such principles have been based upon oversimplifications of the problems involved. The real complexity of some of these problems has become apparent only recently as a result of progress in research on sugaxs, starches, proteins, fats, and other ingredients. Theoretical as well as experimental methods had to be developed before many of the difficulties inherent in studying the chemical and

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physical processes of candy production could be overcome. Efforts of earlier workers were undoubtedly limited by the inadequacy of available methods. There was reason t o believe t h a t the only sure guide t o progress was the accumulated experience and “know-how ” of the candymaker. Fortunately, these restrictions no longer apply, as the detailed knowledge of chemical structures, reactions, and properties of the natural products used t o make candies has expanded rapidly in recent years. Precise, effective methods are being devised or improved continually and applied t o increase this store of knowledge. The urgent need is to extend fundamental investigations with the tools of modern science into the specific areas of candymaking conditions and requirements. We have noted the extent t o which this has been done, but have cited many more examples of‘ obvious applications of newer research methods or recently acquired knowledge that remain t o be explored. It is fair t o state that the candy industry has made continual, significant technological progress, but it is also accurate t o observe that this progress has been one-sided. Knowledge of the products themselves and of the basic processes of manufacture has not kept pace with developments in mechanization, production engineering, and merchandising. The undesirable consequences of such unbalanced progress are clear t o leaders of the industry, and Adelson (1953) has stated that: (‘In the final analysis, we have got t o have good candy in order t o sell it. . . . I n the past we may have given too little attention t o the candymaking end of our industry.” Attention t o the candymaking end calls for a greatly expanded, well-coordinated research program t o be carried on within and by the industry to bring it fully abreast of other food-processing enterprises. Such research can make rapid strides with the scientific knowledge and means now available. Any programs of research, whether carried on by individual firms or supported cooperatively by the entire industry, must produce tangible improvements t h a t can he translated into increased profits. This requires concentration on major problems with emphasis on practical solutions, but i t also calls for sound, fundamental investigation of the basic principles of candy production. Future research should include proportionately more inquiries of a fundamental nature than has been the case in past work summarized in this review. Approximately, but simply defined, practical or applied research deals with “how” processes operate, whereas fundamental research seeks t o discover ( ( w h y ” they operate as they do: the results of applied research usually are limited t o the solution of one particular problem, whereas those of fundamental research invariably are applicable t o the solution of many problems. T o illustrate this with reference t o candy, merely determining how variations of the proportions

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of sugars and of times and temperatures of cooking affect the tendency of hard candy t o become sticky would be of less practical value than discovering why such candies are hygroscopic a t all. Intelligent research in this field, including an effective balance of fundamental and applied studies, will not attempt t o supplant the a r t or experience of the candymaker that has brought many types of candy t o a high state of perfection. Nevertheless, the possibility of further improvement can be ascertained only by acquiring a better understanding of the chemical and physical changes involved in production methods now in use. Opportunities for immediate and almost certainly profitable investigation are afforded by a variety of problems related t o storage quality in order t o lengthen the shelf-life of candies of every type. Section 11, 3 was devoted t o a summary of efforts t o correct deficiencies in storage quality that have been the cause of substantial losses throughout the industry. Winger (1952) has estimated allowances for returned goods ranging from 0.45% t o as much as 0.81% of total sales during recent years. These figures were based upon reports of some 170 firms representative of every line of candy manufacture, and represent total losses b y all manufacturers amounting t o between $4,500,000 and $8,000,000 annually when applied t o the total wholesale value of approximately $1,000,000,000 of candy production. This is only the direct, tangible cost of goods t h a t become unsaleable. Less complete deterioration impairs the quality of large amounts of candy t o a n extent sufficient t o cause even greater loss from reduced sales volume, although this indirect loss cannot be estimated accurately. Improvement in texture stability has been most significant, and some progress has been made in retarding the oxidative deterioration of fat in butter creams and similar candies. Further improvement of the keeping qualities of marshmallows, jellies, and gums can be brought about by attention t o the fundamental properties of the gelatin, pectin, and starch used in their production. Each of these modifiers of the sugars behaves differently in forming gels that will retain desirable properties. Extensive practical experimentation has made negligible progress in solving the problems of chocolate bloom, or the hygroscopicity of hard candy. Modernization of candy production has been accomplished largely by mechanizing materials handling, and operations such as molding, forming, depositing, coating, and packaging. Revolutionary advances in production techniques are promised by the high-speed, strictly continuous cooking methods that are being introduced on a pilot plant or limited commercial scale. Such methods require far more precise control than the slower cooking in batch kettles under the constant surveillance of experienced candymakers. The conditions that have been found empirically

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t o give the best results in batch cooking provide a very imperfect guide for developing optimum conditiocs for the new procedure. It is essential t o obtain more precise, fundamental data on the chemical and physical chemical changes involved in order t o take full advantage of the efficiency and uniformity of operation of the new processes. The chemistry of the sugars, in the range of conditions used in candymaking, is of foremost importance. It is evident from the results of research described in section I11 t h a t candies may no longer be considered simply as solid solutions or dispersions of sugars that have undergone no changes more profound than inversion of sucrose. There are no recent reports of investigations of the behavior of sugars in candy production, and even the obvious applications of the latest discoveries in this field t o candy problems have been neglected. Major lines of future investigation should deal with the role of sugar anhydrides or reversion products and caramelization reactions in governing the properties of many import a n t types of candy. The effect of heat upon sugars in the highly concentrated solutions formed in candy cooking has not been studied in a manner comparable t o the investigations of chemical changes produced by heating the dry sugars or their dilute solutions. Results of the extensive work of van Hook and Bruno (1949) on the kinetics of sucrose crystallization in sugar manufacturing may be applied advantageously in developing more effective control of its crystallization in candies. The different modifications of sugar properties produced by albumin or other protein whipping agents, by gelatin, and by pectin will be understood better in the light of fundamental principles developed by the research described in section IV. It has been possible here t o include brief descriptions of only the most significant or pertinent work on each of these ingredients. Careful study of the background and details of the investigations cited will suggest many more extensions of this experimental work to candy applications than i t has been possible t o note in review. The estimates in Table I show that the quantity of milk products used in confectionery is exceeded only by those of cocoa and chocolate products and of the sugars. The reactions which the constituents of milk undergo in various candymaking processes are more complex than those of other ingredients, and no results of experimental work on this subject have been published. Changes produced by heating, drying, or storing the milk products themselves are beginning t o be understood only recently through the latest and most fundamental research on this important food. The ultimate objective of this work is t o devise processing methods or conditions that will minimize or prevent alteration of natural qualities. A uniquely interesting subject for study by the same fundamental methods would be the production of caramels in which reactions of the

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constituents under controlled conditions are necessary t o develop the desired flavors. Results of such a study would not only provide a basis for improving caramel-cooking procedures, but would serve t o complement knowledge gained from research on stabilizing unaltered milk flavor for application in making other types of milk candies. Important properties for candymaking applications have been determined in considerable detail for fats and starches. Significant recent progress has been made in stabilizing animal fats, particularly butter, against oxidative rancidity, but the problem of hydrolysis of both vegetable and animal fats in candies having a high-moisture content remains for future investigation. Recent experimental work on the nature of starch gels and factors affecting their properties is especially useful in providing practical guides for starch gum candy production. Quantitative methods of measurement make it possible t o determine how gel strength and rigidity are affected by variations of the fluidity of the starch and the conditions under which it is cooked in making jellies. These methods should be applied t o more thorough investigation of starch gels formed in the presence of high concentrations of sugars. Changes that occur on aging are particularly important for maintaining quality of the candies during storage. Cocoa and chocolate have been studied more extensively for their specific applications in candymaking than have any of the other ingredients of candies. Knowledge of the constitution and properties of cocoa butter is more complete than that of the nonfat fractions of these materials, but their compositions have been determined as accurately as the older methods of analysis permitted. Major problems are still unsolved, notably the causes and prevention of chocolate bloom. Little is known of the chemical changes responsible for flavor development in conching. Useful contributions may be expected from research employing the latest precise, effective methods t o investigate the composition of chocolate and the phenomena of both conching and tempering. Cocoa products have not been analyzed by chromatographic or similar modern techniques for separation and identification of the individual chemical constituents of complex natural substances. A systematic study of the composition of cocoa nibs and of the materials obtained in successive stages of processing into various types or grades of chocolate mill be essential t o provide data for a rational, scientific approach t o the development of better processing methods and improved products. The background and methods required for such a study are now available. Measured in terms of expenditures, the confectionery industry has applied a smaller percentage of its gross income of almost $1,000,000,000 per year t o research than any industry of comparable size. Its progress

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has been achieved t o a great extent by the application of research performed by the industries supplying its principal raw materials, including chocolate, t o the improvement (for particular uses) in candy production. There is evidence that this complacency is yielding t o an awareness of the possibilities of research within the industry on specific candy production problems. The variety of these problems, and the complexity of some of those suggested by this review, afford manifold opportunities for fundanierital investigation by the most modern scientific techniques. TO the extent that the industry avails itself of these opportunities, it will continue t o progress and keep pace with the development of scientific food technology. ACKNOWLEDGMENTS This outline of problems in candy manufacture and the present status of research applicable to their solution would not have been possible without the continual advice and encouragement of many friends and associates in the candy industry. It is a sincere pleasure to acknowledge indebtedness to Mr. Philip P. Gott, President, and members of the Research Committee, progressive leaders of the confectionery industry, for an understanding of these problems acquired during a decade of cooperation in research on a modest scale between the National Confectioners’ Association and the Department of Agriculture. I am particularly grateful to Mr. Clifford Clay, of Stephen F. Whitman & Son, for his invaluable review of most of the manuscript and generous information on many topics besides his authoritative treatment of chocolate tempering and problems in chocolate candy production.

REFERENCES Aasted, Iainedmuch fatty substances. According t o Haines (1937) Cobbett and Graham Smith (personal communication) did not approve of Desoubry and Porcher’s techniques, but did find that when guinea pigs were fed wet cabbage, bacteria entered into the organs and muscle and the animals had diarrhea. I n another early study Nocard (1895) concluded that the serum of fasting horses was almost always sterile whereas that collected from horses shortly after they had been fed often contained bacteria. Ficker (1905) starved rabbits, dogs, cats, and mice from 3 t o 17 days and then fed them with infected food which contained a n easily identified organism. Four hours after the animals had ingested the contaminated food, they were sacrificed. The bacteria were found in the blood and organs. Ficker concluded that prolonged starvation decreased the resistance of the gut t o invasion. Griffith (1911), using monkeys, pigs, goats, dogs, and cats, showed that tubercle bacilli were able t o pass through the mucous membrane of the alimentary tract and t o reach the adjacent lymphatic glands within a few hours of their ingestion. Only a small proportion of the bacilli ingested were able t o penetrate the mucosa, the rest passing out with the feces. Of those t h a t did penetrate, the majority mere arrested in the glands but some, after a period the duration of which appears t o vary with the animal used, were carried into the lungs or even beyond; the shortest period of time between their ingestion and their demonstration in the lungs was 4 days. Hulphers (1934) described several organisms found in the lungs of slaughtered hogs. He indicated that most of the microorganisms appear t o be soil flora. Tarozzi (1906a,b) and Canfora (1908) injected spores of Clostridizim tetani into animals. After a considerable length of time (up t o 55 days) a leg or bone of each animal was broken or injured and the animals were observed for tetanus which usually occurred following this treatment, showing the longevity of spores in the normal body. Jensen and Hess (1941) conducted extensive studies t o support their contention that the

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muscle tissue and bone marrow of living hogs are generally sterile but that many kinds of bacteria can be isolated from these sites after the animals have been slaughtered. A study conducted by Adamson (1949), although made with human cadavers, has direct bearing on the subject of tissue invasion. His study revealed that a large proportion of 804 lymph nodes removed from human corpses contained a variety of bacteria, with coliforms, micrococci, and streptococci predominating. He suggested that the bacteria found in the nodes might have gained access to the body through skin abrasions, became localized in the nodes and survived there for some time. Lepevetsky et al. (1953) obtained samples from 11 chucks and 12 rounds of beef from 23 cattle. The prescapular lymph node, the humerus, and muscle tissue bordering that bone were removed from each check while the popliteal lymph node, the femur, and neighboring muscle tissue were taken from each round. Bacteria were isolated from 15 of 23 lymph nodes, from 3 of 23 marrow samples, and from 2 of 23 muscle samples. The numbers recovered from infected lymph nodes ranged from 80-764,000 per gram whereas only 2 samples each for bone marrow and muscle tissue showed a sufficient number of organisms to be counted. Bacteria isolated from the various samples represented the following 12 genera : Aerobacter, Alcaligenes, Bacteroides, Clostridium, Corynebacterium, Escherichia, Flavobacterium, Micrococcus, Proteus, Pseudomonas, Serratia, and Streptococcus. (Of the 93 organisms isolated, 31 were classified in the genus Streptococcus, 18 in Escherichia, 10 in Aerobacter, and 8 in Pseudomonas.) I n comparing their results with those of Jensen and Hess (1941) and of Adamson (1949), the Ohio workers considered that the morphology of the flora studied in their investigation had similarities t o those from human lymph nodes and that the muscle tissue and bone marrow had more freedom from bacteria than t h a t which was removed from the slaughtered hogs as reported by Jensen and Hess. Further, VC'eiser et al. (1954) considered that the results confirmed observations on sour rounds that the spoilage obtained appears t o propagate from lymph nodes.

4. Effects of Fatigue o n Spoilage It has been long recognized that fatigue exerts a pronounced effect upon the defensive mechanisms of animals. Charrin and Roger (1890) found that exercise in a revolving drum increased the susceptibility of rats to anthrax and quarter evil (blackleg). Ficker (1905) did not believe that fatigue alone facilitated the passage of bacteria into the tissues from the gut of animals but that fatigue in conjunction with starvation promoted their entrance from the intestines. He postulated that decreased

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gastric secretion, diminished peristalsis, and acclerated flow of blood and lymph during muscular exercise, aided the penetration of bacteria, and Ficker suggested that this would explain the observation that the flesh of animals slaughtered after having been driven long distances decomposes very quickly. I n his opinion, fatigue helped t o facilitate the passage of organisms through the walls of the gastrointestinal tract more rapidly than would have been the case from hunger alone. Boycott and Price-Jones (1926), working with considerable numbers of rats, found t h a t fatigue usually led t o a lowering of the rectal temperature by 2.8 t o 8.2" C. (5 t o 15" F.) and, occasionally, by more than 11.1" C. (20" FA).They used as a n infecting organism, Gaertner's bacillus (S'almonella enteritidis), a natural parasite of the rat. These workers concluded that, after feeding, S. enteritidis found its way into the spleen of both normal and fatigued rats; however, fatigue increased illness and mortality. Boycott and Price-Jones did not believe that the permeability of the intestine t o ingress of microorganisms was dependent upon fatigue, a s indicated by Charrin and Roger and by Ficker, since almost all of the controls (only 1 of 28 of which was ill) as well as the survivors (14 of 27) of the fatigued animals had S.enteritidis in their spleens when they were killed at the end of the experiment. However, they felt that their obserrations clearly demonstrated that, under certain circumstances, infection was promoted by fatigue. Meat packers have learned that, in order t o maintain high quality in meat, it is necessary t o delay the slaughter of animals when they are fatigued, hungry, or thirsty; also, it has been long known that animals killed after they had become exhausted during a hunt, in fighting, or when struggling violently during slaughter undergo rigor mortis early and putrefy rapidly (Jensen, 1915). Various explanations have been offered to account for this phenomenon. One has been previously stated in this paper (i.e., that fatigue decreased the resistance of the gut t o invasion with subsequent entry of the organisms in the deep tissues); a second is that blood is retained in the vessels when the animal goes into shock. For example, Morrison and Hooker (1915) found that the outflow of blood from the perfused organs of a shocked animal was less than that from similar organs under normal conditions. Further discussion relating t o the efficiency of bleeding in the shocked or dying animal is given in a later section of this article. A third factor to be considered is the modification of globulin. Antibodies are thought t o be modified serum globulins. It may be of interest t o note that Burrows (1949; pp. 307-8) states that "there is a feeling on the part of a number of competent investigators that the serum proteins including globulin do not exist as such in the body

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but are artifacts arising as a consequence of laboratory manipulation.” Still another factor involving fatigue as a causal agent for putrefaction refers t o the ultimate’ p H and keeping quality of the carcass. The relation of ante-mortem condition to post-mortem changes in glycogen and lactic acid content of muscles was reviewed by Bate-Smith (1948). According to Bate-Smith (1938b, 1939) the pH of muscle steadily falls from the moment that circulation stops; the precise pH of the muscle immediately prior to that time depends on the recent history of muscular activity. If the muscles have been completely inert and well supplied with oxygen before death, the lactic acid content of the tissue is very low. If the animal is given time to rest, the lost muscle glycogen is restored according t o Callow (1936) and Callow and Boa2 (1937). The p H of muscle with minimal lactic acid is about pH 7.4 (Bate-Smith, 1 9 3 8 ~ ) . Since the glycogen content is approximately 10/0,the lactic acid formed is about 1.1% and the pH reached in full rigor about p H 5.6 (Bate-Smith, 1948). However, if the muscle is exercised (fatigued) shortly before the animal is slaughtered, much of its glycogen reserve has been converted to intermediates of the glycolytic cycle and may be lost during slaughter (Bate-Smith, 1936, 1938a). I n fact, Best et al. (1926) have shown that it is possible t o eliminate glycogen from the muscles by inducing death by convulsion through the injection of insulin. Callow (1939) found that the glycogen reserves of the hog were easily depleted and believed th at feeding was necessary to restore muscle glycogen after exercise. In other experiments reported by Callow (1939), hogs were shipped one mile by truck and then walked a quarter of a mile before slaughter. The ultimate pH in the psoas muscle of a group of animals which was rested overnight after this treatment, but not fed, averaged pH 5.80 as against pH 5.79 for the unrested group; whereas a group that was both rested and fed had an average pH of pH 5.58 as against pH 5.87 for the unrested control group. Earlier, Callow (1936) pointed out t ha t “resting” must, in fact, be resting in order to be effective. He said t ha t pigs tend t o fight when strange groups are mixed together in resting pens and, under such circumstances, recovery of glycogen does not take place. Since the quantity of acid present in the dead muscular tissue depends principally upon the quantity of glycogen a t the moment of death, the final acidity of the meat derived from animals depleted of glycogen is less 1 The p H after all residual glycogen has been converted. Although i t is generally assumed that the ultimate p H of muscle is the result of the post-mortem accumulation of lactic acid arising from glycogen, there are many other normal muscular constituents t h a t are acidic in nature.

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than t h a t for animals having adequate reserve glycogen for normal activity. Callow (1949) indicated that the following ranges in ultimate p H were obtained for beef, lamb, and pork during the course of his investigations: beef p H 5.1-6.2; lamb p H 5.4-6.7; pork p H 5.3-6.9. The importance of this relationship was illustrated by Ingram (1948, 1949) who presented data showing that a slight acidification of the medium markedly reduced the rate of growth of a wide variety of bacterja. Also, Bate-Smith (1948) points out t h a t “if meat has reached its ultimate pH with a n excess of glycogen still remaining, the growth of microorganisms may not cause an alkaline shift, because production of base will only result in further breakdown of glycogen to lactic acid and i t will not be until the glycogen is completely exhausted that the p H can begin t o rise. I n other words, from the point of view of resistance t o bacterial growth there cannot be too much glycogen in the muscles.” Callow (1935) also attributed t o fatigue the difference in electrical resistance between the muscles of hogs butchered on the farm and those slaughtered in packing plants. Subsequently, he (1936, 1938) demonstrated a correlation between a high resistance of flesh and its degree of acidity. Banfield (1935) and Callow (1935, 1936, 1938) regard this relationship as due t o swelling of the fibers, with a subsequent narrowing of the channels through which ions can move freely. Callow (1937) used the terms “open” and “closed” t o differentiate structure of meats; open structure is considered t o have low p H values, moist feel, firm texture, and pale color; closed structure is associated with a sticky dry feel, flabby texture, and darker color. Bate-Smith (1933) considers the stickiness with the high pH due not only t o the swelling of the fibers but also due t o the dissolution of myosin. Ingram (1948, 1949) demonstrated that the rate of growth of bacteria can be greatly diminished by a fall of 0.1 pH unit and, therefore, intimated that growth becomes progressively less probable as the p H falls below pH 6.0. Dry, salt-cured hams made from fatigued pigs are thus likely t o be tainted, and the incidence of taint will diminish as the pig is rested and fed (Callow, 1937). Moreover, it is not only the growth of the anaerobic bacteria which cause tainting in hams that is affected by the state of the live pig, but also the growth of contaminating bacteria on the surface of the meat. Madsen (1943) found that the formation of slime due t o the growth of aerobic bacteria on the surface of sides of bacon is retarded and the storage life prolonged by feeding and resting pigs before slaughter. Thus the growth of aerobic as well as of anaerobic bacteria on and in the flesh, post-mortem, are affected by the state of the animal a t the time of slaughter.

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IV. SLAUGHTER 1 . Killing

With the exception of the animals slaughtered for the Kosher trade, the method of killing cattle and calves differs from t h a t used for hogs and sheep. As a consequence of these differences in operation, the microbiological changes taking place in the tissues during the dying period may vary with the type of animal being considered. Haines (1937) lists several methods of slaughter:

(1) Simple bleeding after sticking in the thorax or cutting the throat. (2) Bleeding after previous mutilation of the medulla, e.g. pithing. (3) Bleeding after previous stunning with a blow from a hammer, killing mask, or captive-bolt. (4) Bleeding after stunning electrically. S o t in Haines’s list, but a commonly used method on the farm is killing the animal by shooting. Also, an immobilizing procedure has been introduced recently (Murphy, 1950) which may revolutionize the method of handling animals preparatory t o bleeding them. Ordinarily, cattle and calves are stunned by a blow from a hammer t o the animal’s head. With cattle, several animals are crowded into knocking pens (the number varies among plants and also with the size of the animals) t o prevent excessive movement. After all of the animals have been stunned, a shackling chain is put around each animal’s two hind legs and it is hoisted t o a n overhead rail for bleeding. Depending upon the number of animals and the proficiency of the knocker, several minutes may elapse from the time that the first animal is knocked down and the last animal is in position t o be bled. There is opportunity during this period for the hide t o become impregnated with filth from the walls and floor of the killing pen. A slit is cut through the hide down the middle line of the throat and into this a knife is inserted. The arteries and veins of the throat are then severed by a slight sidewise cut. (In most cases stunned cattle cease breathing quickly.) Although stunning suspends the heart action so that bleeding is largely mechanical under ideal conditions, some of the animals recover partial consciousness during these operations. Under such circumstances, the microbiological implications discussed later for the hog also apply. I n England cattle are often killed by the use of a captive bolt pistol. Although this method may have some humanitarian values, hair and hide, blood and bone, and the bolt as well are embedded in the brain. The con-

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taminative load of bacteria introduced with these agents quickly attack the damaged tissues rendering them unfit for food usage in a short time. Hogs and sheep are driven into shackling pens where they are shackled by one of the hind legs and hoisted on t o a n inclined rail. Immediately after the animal is suspended, i t is “stuck” by a quick thrust of a sharppointed, two-edged knife into the throat severing the large blood vessels leading to the head. Often the animal struggles violently on the shackle rail and great care must be exercised by the sticker t o prevent injury t o himself or t o the flesh of the animal. After sticking, the blood quickly drains from the carcass. In England pigs are stunned electrically before shackling, for humane reasons, but this method has not been received very favorably on this continent because changes induced in the lungs and pleura make veterinary inspection difficult (Gibbons, 1953). According t o Haines (1937) there are numerous reports in the literature attempting t o show that one method gives more complete bleeding than another; he criticizes most of these as involving inadequate numbers of animals. Jensen and Hess (1941) criticize the concept t h a t faulty exsanguination is responsible for increased bacterial spoilage, and cite, as evidence, many tests made over a period of 4 years wherein hogs were bled from large and from small incisions and dropped into scalding vats immediately or 15 min. after sticking. They stated that no conclusive evidence was presented t o show the superiority of one method of bleeding over another. Recently a process whereby carbon dioxide is administered in controlled quantities in tunnels through which the animals are moved, has been developed for immobilizing hogs (Slater, 1952). The method was also suggested for cattle, sheep, and poultry (Anonymous, 1953). Sufficient gas concentration is inhaled by the animal t o asphyxiate it during most of the shackling and sticking period. Although the technique was developed primarily t o reduce injury t o the workmen and t o the flesh of the animal, it has several values of microbiological importance as well. For example, immobilizing (1) diminishes animal fatigue; ( 2 ) reduces the amount of dirt on the animals and workers; (3) assures more accurate and sanitary sticking; and (4) allows animal respiration and thereby improves bleeding. 6. The Stick-Knife

Sticking of life animals in this country is restricted t o hogs and sheep; cattle and calves are stunned by a blow to the head before bleeding. However, with any animal in which the heart is still beating, changes cannot be considered post-mortem. For this reason, it is best t o consider

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the physiological changes and the immediate microbiological aspects which attend them as being agonal. As previously mentioned, Jensen and Hess (1941) found the skin of a hog t o be heavily contaminated. They considered the use of the stickknife a very important avenue for the introduction of these microorganisms into the tissue. Jensen and Hess did not feel th a t the tissue of the live animals were contaminated but that the bacteria are introduced during slaughter and while the animal was dying, they are circulated by way of the blood stream. They suggest the following sequence of events after a n animal is stuck: The stick-knife passes through the skin severing the jugular vein and sometimes the carotid artery; then arterial and venous blood sweeps over the contaminating blade, returns to the heart and is pumped throughout the arterial system. According t o Jensen and Hess (1941) the heart of a hog may beat from 2 t o 9 min. after the stick wound is made. Visible movement is detectable for at least 40 sec. after the animal has been stuck. I n addition, some of the hogs contract their heads in the direction of their forelegs and in this manner withhold a good deal of the blood flow for 5 to 15 sec. by constriction and hematoma, allowing more arterial and venous blood from this area t o reach the heart and eventually t o be circulated in the arteries. Also the severed or pierced vessels may be under reduced pressure owing to the labored breathing resulting from oxygen starvation during exsanguination. The flow of the pooled blood and blood within the vein passes toward the heart. These workers also point out th at the stick-knife ordinarily is wet and heavily contaminated with spores. They theorized that, on the basis of the amount of serum or liquid blood in a 250 Ib. hog, a knife blade introducing only 50,000 bacteria could infect the circulating blood with over a dozen bacteria per milliliter since phagocytosis would not necessarily eliminate many bacteria in the short span of life after sticking. I n a series of biopsy studies which Jensen and Hess (1941) made of blood, bone, bone marrow, and muscle tissue, with one exception (an animal which harbored Hemophilus sp. in all tissues while alive and also after dressing), none of the hogs showed the presence of microorganisms in the tissue or blood. After the surgical fields were closed, sutured, and heavily covered with celloidin, the animals were immediately taken to the killing floor, hoisted, stuck and bled, washed, dehaired, butchered, and dressed. Post-mortem findings indicated that bacteria entered the animal during these manipulations. Jensen and Hess considered th a t fewer bacteria were t o be found in the blood of the heart of sterilely stuck hogs than in those t hat had been septically stuck. I n both cases, they found considerable numbers of aerobes and anaerobes in the incoming blood up to the time that the heart stopped beating. They considered that the

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sterile sticking method and long bleeding mere not adequate t o prevent contamination of blood. They concluded that the presence of bacteria in the blood, muscle, and bone marrow of these aseptically bled hogs indicates that the organisms enter during the dying period. Jensen and Hess (1911) also placed pure cultures of bacteria on the blade of the knife just prior t o sticking and then used this instrument t o sever the vessels in the hog’s neck; they found that the specific bacteria introduced could be isolated from the marrow of the tibia and other long bones. It should be mentioned that the largest percentage of blood remaining in a slaughtered animal is found in the muscle. The stick-knife contamination and the sequence of events following the severing of the neck vessels should favor the entry of coliform and other bacteria into the blood and marrow. However, some factor prevents their reaching the marrow or surviving therein if they do reach it. It has been long known (Adami, 1899) that undiluted, fresh mammalian blood is bactericidal. Boyer (1926) called attention t o “the absence of the Bacillus coli (Escherichia coli) group of organisms from . . . hams (although) members of the group . . . are almost invariably found on the surfaces of the carcasses which are exposed during killing floor operations. Their absence is of special significance in that it goes far t o eliminate the possibility that the organisms present in the hams gain access during killing floor operations.” Jensen and Hess (1941) thought that bactericidal action of the blood was responsible for the absence of coliforms from heart blood or from ham. I n order t o test this theory arid t o find a clue to the presence of only certain bacterial species in the bone marrows, they conducted the following tests. Blood was withdrawn aseptically from the tail of a live hog and was allowed t o drip directly into sterile flasks containing glass beads t o defibrinate the blood. Various freshly isolated strains of bacteria were added t o this blood t o give a level of about 50,000 viable cells per ml. and subcultures were made a t short intervals up to 24 hr. It was found t h a t the suspensions of Staphylococcus aureus (Micrococcus pyogenes var. aureus), and E. coli often were sterile after 2 t o 5 hr. whereas the bacteria which Jensen and Hess termed the ham-souring types, such as certain strains of Serratia, Achromobacter, Clostridium putrefaciens of McBryde (1911) (C. lentoputrescens), and Pseudomonas, were more or less resistant t o the bactericidal effects of hog blood. 3. Scald Tank

Use of a scald tank is restricted t o hogs since the hides of the other meat animals except for those of calves and “hothouse” lambs are removed by skinning. These latter are dressed with their skins or pelts remaining on the carcass t o prevent excessive dehydration of the meat.

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Ordinarily the water in the scald tank is maintained a t a temperature of 62.8 t o 65.6" C. (145 t o 150" F.) and contains soap and sal soda t o aid in the loosening of hair and scurf from the body of the hog. The numbers of organisms that are contributed t o this water during the process of a day's operation are almost astronomic. If one considers the bits of soil, fecal material, blood, and other foreign material that may be on the feet, hair, and hides of these animals, it is easy t o realize that millions and even billions of bacteria may be present in every milliliter of the water after the first few carcasses have traversed the scald tank. It has been pointed out by a number of workers (Empey and Scott, 1939a,b; Empey and Vickery, 1933; Mallman et aE., 1940; Haines, 1937), that the flora which ultimately develops on the meat come principally from soil organisms associated with the hoofs and hides. Unfortunately, no studies are available t o indicate the role of the scald tank as a cross-contaminating medium. Depending somewhat on the speed of movement of the process line, hogs move through the tank in approximately 3 t o 5 min. Most of the carcasses float; occasionally, however, one will sink t o the bottom and remain there for some time before i t is rescued. The Meat Inspection Division of the Bureau of Animal Industry specifies that a hog shall hang on the bleeding rail for a t least 6 min. This interval has been required in order that the animal be lifeless before being dipped into the scald tank. However, Jensen and Hess (19$1) observed that the hearts of hogs stuck by using the large incision method (5-in. slits) continued to beat for 6 t o 9 min. Under such conditions, and with the sticking and scalding limitations listed, it is easily possible for the circulatory system of a n occasional animal t o be functioning while i t is in the scald tank. Also, it is doubtful t h a t the temperature of the scald water is sufficiently high t o destroy spores or even all of the vegetative cells. However, the heat is sufficient t o destroy many of the psychrophilic bacteria. Few studies are available which relate t o the presence of bacteria in the circulatory or respiratory tissues of animals immediately after slaughter. However, government regulations forbid the use of hog lungs for edible purposes owing t o the amount of tank water which enters during scalding (White and Patrick, 1916). Spray (1922) reported that, in several instances, microorganisms were recovered from apparently normal lungs of pigs slaughtered in a commercial packing plant. Jensen and Hess (1941) found that the lungs of hogs, stuck in the ordinary manner and sent through the scald tank and dehairing machine with the wound open, were highly contaminated. I n addition, they found t h a t when a sterile technique was used in sticking, followed by scalding in a hot spray cabinet, samplings of the washings and swabs taken from the lungs showed no

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more than a few colonies. The bacteria which were identified were representatives of the genera Bacillus, Achromobacter, Micrococcus, Pseudomonas, Clostridum, Pasteurella. Alcaligenes, and a miscellany. Although Jensen and Hess reported that substitution of a continuous spray in steaming cabinets instead of the scalding bath does not aid materially in reducing the bacteria in the carcass, they do not present data substantiating this statement. Recently , a t least in one commercial installation, use of the scalding tank has been replaced by a steaming cabinet. Ti. POST-MORTEM 1. Skinning

Ordinarily cattle and sheep are skinned before they are eviscerated. With cattle, the usual procedure is t o perform part of the operation while the carcass is resting on the floor of the eviscerating room or ‘(bed” on its back or side. There are some recent innovations in this dressing procedure wherein the animals are skinned in the same manner as that used for hogs, i.e., suspended from a rail. Recently, a device has been placed in operation which functions mechanically t o partially separate the hide from carcass of cattle. Also, in some installations the hide is removed by applying pressure from air hammers instead of by the use of knives. With sheep, the skin is removed while the animal is suspended from the rail. The pelt is partly removed by cutting and by pressing the fist between the pelt and the thin white membrane covering the flesh. Empey and Scott (1939a) found that the transfer of microorganisms from the hide t o the underlying tissues begins with the first stage of skinning. They found populations ranging between 10,000 and 100,000 per sq. em. of the superficial tissues of the carcass. Numbers of organisms in the tissues were highest in the region below the initial incision through the hide and lowest in the areas that were furthest removed from this region. The population of bacteria on knives used for incising pieces of hide of various lengths were found t o range between 80,000 and 40 million organisms per blade. Empey and Scott state that, in addition t o the numbers that may eventually be transferred during separation of hide from carcass, there are some that are planted directly by the blade when it touches the exposed tissues during the initial incision through the hides. Further transfer was found t o occur when parts of the hide that had been separated again came in contact with the carcass while the skinner was changing position. Experiments showed that when such areas of direct contact were made, contamination equal t o one-third that of a n equal area of hide was transferred. Additional organisms were transplanted to the carcass from the hands, arms, legs, and clothing of workmen. How-

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ever, Empey and Scott were unable t o estimate the number of organisms tha t were planted in this manner. The clothing of the workmen who were engaged in removing the hide and which came in contact with the hair were found t o acquire a high level of contamination. I n the presence of moisture, blood, and tissues, these organisms found the substrate suitable for multiplication or survival and often reach levels as high as 3 billion per gram or 30 million per sq. cm. in the material that could be scraped from the clothing of workmen who had participated in the skinning of approximately 100 carcasses during a 6-hr. period. The population acquired by the hand of a workman during the handling of the hair on about 100 sq. cm. of hide reached 2 million. I n addition to what may be regarded as a permanent microflora of the skin and hair, the hide and hoofs of the animal carry varying amounts of soil. Soil was found to contribute t o the contamination of the carcass in about the same manner as did the hide, but Empey and Scott (1939a) state t ha t it is not possible to accurately determine the number of organisms originating from this source. Populations of microorganisms vary considerably in different soils. According t o Sarles et al. (1951) as few as 1000 or as many as 10 billion bacteria may be found in a gram of soil; the usual range was stated to be from 1-10 million. Then too, the hoofs and lower parts of the legs usually carry a considerable amount of soil and, if the workman engaged in handling the feet subsequently touches other areas of the carcass, soil provides a high proportion of the contamination. 2. Dehairing, Xhaving, and W a x Dipping

As with scalding, the dehairing, shaving, and wax dipping operations are restricted t o hogs. After the animal has been properly scalded to loosen the hair and scurf, it is lifted from the scald tank b y a hook placed under the tendons of one of the hind legs. Then the hog passes through a dehairing machine where the hair is beaten or scraped from the carcass. Although this device “polishes” the hog by removing most of the hair and scurf, from a sanitary viewpoint, bacteria or spores may be pounded or scratched into the skin of the animal. Following dehairing, the carcass passes through a hot water shower and then parts not properly dehaired are shaved with sharp knives. The hog may or may not be singed t o remove the remaining hair and to change the coloration of the skin. If this processing step is used, the head and two front feet generally receive most of the heat. The final processing steps before the animal is sent t o the chill room

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are those involving shaving and wax dipping. Shaving is done with sharp knives which remove the remaining hairs on the surface of the carcass. I n many plants in the United States the animal is then immersed in a vat of moulten wax which adheres t o the carcass. Then the hog is withdrawn and the wax quickly congeals, enmeshing the remaining hair and practically all of the hair roots. After the wax is congealed, i t is pulled from the carcass. The adhering hair and scurf are separated by screening and the wax remelted and reused. There is no information available concerning the introduction of bacteria during the dehairing operation or the removal of these organisms later in the shaving and the wax-dipping processes. Also, no data could be found which related to the destruction of bacteria or spores in the molten wax. 3. Evisceration

I n general, all meat animals are eviscerated in approximately the same manner. However, there are some rather important differences in the techniques that are involved in removing the viscera from these several animals. The animals arrive on the evisceration floor, shackled by their hind feet and hanging head down. The hog is opened from tail t o throat by the use of a knife and a cleaver. An electric saw sometimes is used to help divide the belly side along the median line. The bung is loosened and the entire viscera is removed in one continuous operation. The fat is pulled and then the hog is split through the center of the spinal column. The hams are faced by removing fat and skin from the flank and cushion sides of the ham. The carcass is again sprayed and sent t o the cooler. In cattle, the breast bone and aitchbone are sawed through exposing thoracic, abdominal, and pelvic cavities. The esophagus, bung, and bladder of beefs are tied to prevent regurgitation or contamination of the carcass with fecal matter or urine. The bung is loosened and dropped within the pelvic cavity permitting the viscera t o fall free. After evisceration the carcass is split in half usually with the aid of an electric saw, and the sides washed and scrubbed with hot water and covered with wet muslin sheets a t the time the carcass is sent to the chilling room. These cloths usually are moistened in hot brine or hot water before they are draped closely over the surface of the carcass. Pressure is applied in the draping of the shroud in order to give the carcass a smoother appearance and more desirable conformation. Shrouding also is credited with preventing some loss of moisture and with bleaching of the fat. However, Jensen (19$5) states that microbial growth is induced a t the interface of textile and water film and, to a smaller extent, on the beef. He also mentions that pins used t o fasten the shroud t o the carcass may a t times cause

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the formation of dark, discolored spots or even areas several inches in width and depth. I n sheep, the body is opened by splitting the breast bone and, after loosening the bung, the viscera is removed without dividing the aitchbone. Following this operation a spreadstick is placed inside the thoracic cavity t o permit air circulation when the carcass is sent t o the chill room. The feet and lower leg of cattle and sheep are not saved for edible purposes; owing to their unclean condition, i t is almost impossible to remove them so as to prevent contamination. An extensive study of the microbial contamination encountered in a beef packing plant was made by Empey and Scott (1939a,b). Although they accredited a considerable inoculum of organisms on the carcass as having come from the soil and dirt on the hide and hair and from the skinner, they reported th at freshly voided feces or intestinal content which accrued before the completion of skinning and the accidental puncture of various sections of the gastrointestinal tract by knives by the workmen were responsible for some of the contamination. Principal sources of microorganisms on the eviscerating floor include contamination that is airborne, from the water used in washing and rinsing the carcass, from cloths and brushes used for wiping tools or carcasses, from bandsaw races and other tools such as knives, saws, cleavers, and hooks, and from the hands of the workmen. I n many abattoirs and packing plants, the sticking and scalding area for hogs, and the knocking and bleeding enclosure for cattle and sheep, are separated from the eviscerating and cutting floors. Although these operational steps have been separated principally for aesthetic reasons, they also serve to reduce airborne contamination on the eviscerating floor which otherwise would accumulate from dust and dirt derived from the milling and thrashing of the animals while they are on the killing rail or in the knocking pens. Empey and Scott (1939a,b) presented data t o show the microbial deposits from air of slaughter floors; they found a n immediate plate count of about 30 bacteria and about 2 molds per sq. cm. per hr. Haines (1933a) surveyed the bacterial flora in two types of slaughter houses : the small privately-owned killing shed and the large modern processing plant. He found the numbers of organisms in the former were proportional t o the number of animals being slaughtered and the climatic conditions. This did not hold when the small killing plant was compared with the larger, more modern, abattoir. Haines stated th at although each animal is to be regarded as a potential source of a given load of bacteria which will be scattered during handling, the degree of infection was less owing to the better system of ventilation in the large abattoir.

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Also, in the two types of processing plants studied by Haines (1933a)) analysis of the types of flora present in air samplings showed striking differences which he attributed t o the diverse methods of handling the blood, inedible viscera, and hides. I n the more modern abattoirs in England where chutes and drains were provided t o carry away offal, there were only 9% of the flora which were “intestinal’) types compared with 19% for the other type of slaughterhouse. Haines reported the following types in air; Staphylococci, Micrococci, Bacillus, Pseudomonas, Achromobacter, Flavobacteria, Azotobacter, Proteus, Aerobacter, Coliforms, and molds. I n modern American packing houses elaborate conveying and processing devices are employed for handling viscera after they have been removed from the animal. Generally, stainless steel trays are used t o catch the internal organs as they are removed or drop from the carcass. Pans and carcasses move together in sets in the processing line until both the animal and its viscera have passed post-mortem inspection. Trays are emptied into appropriate chutes, rinsed, and, after cleaning, ret]urned to service. During eviscerating operations, workmen often use their hands t o make incisions, t o wash or brush inner surfaces, and t o remove blood and other contamination. Also, the veterinary inspector may touch edible parts of the animal during the course of his inspection of the head, heart, liver, lungs, kidneys, stomach, intestines, membranes of the thorax and abdomen, various groups of lymph glands, internal and external surfaces of the body, and exposed bones. From time t o time, bacterial transfer from one part of the animal t o another is a n expected consequence of these manipulations. Also, during such contact, the workman or inspector may transplant part of the flora that is on his hands t o the meat. Horwood and Minch (1951) examined 34 hand-washing samples which they had obtained from 22 food handling establishments. They isolated large numbers of bacteria from the hands of food handlers; also, Escherichia coli, hemolytic staphylococci and streptococci, and aerobic spore forming bacteria were frequently recovered. Horwood and Minch concluded that (1) food handlers frequently bring the hands in contact with food when the us? of a n implement is indicated and ( 2 ) hands are frequently soiled with the discharges from the nose and mouth and in other ways. After the animal has passed inspection, the carcass is sent t o the chilling room but the edible viscera and offal receive additional handling. Ordinarily, the brain, heart, liver, and kidneys (not removed from sheep but sold with the carcass) are saved. I n addition, the stomach of hogs and the rumen and reticulum of sheep and cattle are often processed and used

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in the manufacture of tripe. The intestines of all three of the meat animals commonly are processed and used for sausage casings. Before animal st,omachs or intestines can be utilized for food purposes i t is necessary that they be turned or cut open t o permit the ejection of their contents and a thorough washing of their interiors. Also, before intestines are satisfactory for sausage casings, fat must be removed from the outer surface and the intestines turned with the inside out and run through the sliming machine. Inedible components of the viscera, together with material flushed from the gut of the animal, are conveyed into appropriate chutes for disposal as tankage or fertilizer. Water, when applied with force directly from the t a p or hose, does not of itself contribute many microorganisms. The wash water dislodges much of the blood, tissue debris, hair, and scurf t h a t accumulates on the carcass during the skinning and dehairing operations. I n all probability i t also reduces the total amount of contamination even though it more uniformly redistributes through other tissues bacterial populations from local areas where their numbers may be large. Similarly, microorganisms may be transferred t o carcasses from the hands and clothing of the operator or splashed on the animal from the walls and floor of the washing stall. Haines (1933a) reported t h a t when walls were covered with a rough plaster, material which splashed on them was readily retained and absorbed. He also pointed out that walls of this type were often covered with mold growth and that carcasses brushing against such walls became contaminated. Empey and Scott (1939a) cautioned against the use of plaster or wood. Rooms in most modern meat packing plants have smooth concrete floors, tile walls and steel girders, which are easily washed down. Empey and Scott (1939a) exposed sterile cloths t o the fine mist produced from the contact of water with floor. They estimated the extent of contamination dislodged from the slaughter floor t o range between 1000 and 4000 organisms per sq. cm. for a 2-sec. exposure. Numbers of microorganisms found in water collected in utensils or for the washing of tools depend upon the previous history of the water and of that for the utensil or tool. If water is recycled or reused, its microbial content increases rapidly. Also the blood, grease, and bits of tissue introduced into the water when the tool is washed or immersed provide nutrients t o support the growth of microorganisms. Haines (1933a) reported some extremely high counts (2-25 million per ml.) for water that was used in swabbing carcasses under rather poor conditions and concluded that such practice served merely t o inoculate the flesh instead of t o clean it. The chief types of bacteria associated with this water were : Aerobacter cloacae, Staphylococcus epidermis (Micro-

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coccus tardior), Micrococcus candicans, Escherichia communior (coli var.), Proteus vulgaris, Micrococcus luteus, Pseudomonas, and Micrococcus perjavus. On the other hand, under cleaner working conditions and with water changed at frequent intervals, the water used for swabbing carcasses contained 1500 organisms per ml. principally of the following types: Xarcina lutea, Escherichia communior (coli var.) , Flavobacterium, and Achromobacter. Water used for the washing of cloths and brushes employed in wiping or cleaning parts of the carcass was found by Empey and Scott (1939a) to become heavily populated with organisms. The loads reached an equilibrium soon after the beginning of the day’s operation. Numbers per milliliter of fluid ranged between 20,000 and 350,000 with a median of 140,000, These workers found, also, that the populations on the cloths ranged bet\%-een-20,000 and 100,000 per sq. cm. They considered that the question regarding whether or not organisms were transferred from cloth to carcass or were removed by wiping depended on the relative populations of both cloth and tissues prior to the operation. Material adhering t o the blades of saws, knives, cleavers, and to the platform of conveyor tables often is contaminated and mechanically inoculates the newly exposed cut flesh of the animal with organisms derived from air, water, handlers, or miscellaneous sources. Portable and stationary electric saws are receiving increased usage in modern packing plants. The blades move at high speed and the blade-guards quickly accumulate tissue materials which are removed only periodically. Large loads of microorganisms inhabit this material and, from time to time, are caught on the revolving blade to inoculate new tissue.

4. Chill Room Present practice in the handling of meat is to send the carcass to the chill room as quickly after evisceration as possible. Several reasons have been advanced for the adoption of this procedure in order t o prolong the storage life of meat. Among these are: (1) to free it of body heat, (2) to firm the flesh, (3) to delay undesirable bacterial and chemical changes, and (4) to prevent shrinkage. This procedure has not always been in vogue as may be witnessed by the comments of Moran and Smith (1929) decrying the general practice of the trade at that time of allowing the animal to remain at room temperature after slaughter until the animal heat had dissipated. As they pointed out, and as others have indicated since (Jensen, 1945),it is imperative to chill the carcass quickly in order t o avoid early spoilage of the meat.

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Haines (1937) stated that, even under the best conditions of chilling, the temperature in the region of the 0s pubis of beef will not drop below 5" C. (41" F.) in less than 48 hr. I n most packing plants in the United States the old method of allowing beef t o hang for several days or weeks in the packing plant is no longer used. Although just 10 years ago beef commonly was held about 5 days at 2 t o 3" C. (36 t o 38" F.) after slaughter before it was removed for final disposition (Jensen, 1944), now the ordinary holding period is reduced to 2 t o 3 days. For pork and mutton the usual chilling time is overnight. I n America, no specific schedule for aging is set other than the time given in the chill room, in transit, and in the distributing house. The use of ultraviolet light in the chill room also serves to shorten the aging time since it permits the maintenance of a higher room temperature in order t o obtain more rapid meat tenderization while inhibiting microbial growth (Christensen, 1940; McIntosh et al., 1942; Ewell, 1943; Sotola et al., 1943; Volz et al., 1949). The air and walls of the chilling room are subjected t o contamination from workmen and materials with which they might come in contact. Further, organisms can be transferred t o the carcass by the circulating air and by the feet of the workers; areas of the animal th a t are near the floor are especially receptive. Richardson et al. (1954) state th at cooling rooms in meat packing plants undergo a rigid sanitary clean-up a t the end of the day. During this clean-up, where sawdust is used, it is stirred up so th a t the air becomes permeated with mold spores which remain suspended for many hours. They considered th at this practice provided a source of contamination of the carcasses for the following day. Fresh sawdust is used in chill rooms and elsewhere in packing plants t o absorb blood and fluid from beef, pork and sheep carcasses as it drips t o the floor. Sawdust is cheap, absorbs moisture readily, is a good insulator, and is readily available in most localities. According t o Empey and Scott (1939a), the number of organisms deposited from the air was invariably higher in those rooms in which sawdust was used. They reported that fresh sawdust used in chill rooms contained, on the average, 3 million organisms per gram; of these about 1 % were considered to be psychrophilic bacteria. Sawdust, after exposure on floors of chill rooms, showed evidence of increased moisture content and of blood stains. Microbial populations of chill-room sawdust were predominately bacterial, with counts ranging between 60 and 450 million low-temperature types per gram of moist material. Also, the extent of microbial contamination deposited in the air of the chill room indicated counts of about 8-1100 bacteria, 10-100 yeasts, and 2-250 molds per

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sq. em. of agar surface per day. Percentages of psychrophilic bacteria were 29 % a t 20 days and 43 % a t 40 days of incubation. Air samplings made in a beef cooler by Richardson et al. (1954), using a n electrostatic sampling device, indicated mold spore contamination ranging from 882 t o 4500 spores per plate exposed for 2 min. in afternoon tests. Three t o ten men were working in the area (8 f t . X 40 ft. X 150 ft.). Richardson et al. considered that the activities of these individuals kept the mold spores suspended and in motion. Empey and Scott (1939a) found that plaster walls in processing rooms usually were cleaner than were wooden walls. Modern chilling rooms in this country utilize compressed corkboard as a n insulation material, these establishments report that this type of insulation remains in excellent condition for many years (Anonymous, 1952). Several investigators (Talayrach, 1901; Klein, 1908; hIcBryde, 1911; Massee, 1912; Monvoisin, 1918; Bidault, 1921; Brooks and Kidd, 1921; Brooks and Hansford, 1923; Haines, 1931, 1933a,b; Moran et al., 1932; Empey and Vickery, 1933; Gorovitz-Wlassova and Grinberg, 1933; Empey and Scott, 1939a; Mallmann et al., 1940; Kirsch et al., 1952; Ayres, 1954) have identified organisms isolated from fresh meat held a t chill room temperatures (Tables I and 11). The taxonomic distribution of the various bacteria, molds, or yeasts isolated indicates that the following genera are represented : (bacteria) Pseudomonas, Azotobacter type, Chromobacterium, Micrococcus, Gaffkya, Sarcina, Diplococcus, Xtreptococcus, Lactobacillus, Achromobacter, Flauobacterium, Xerratia, Proteus, Bacterium, Bacillus, Clostridium, Diphtherozds, Streptomyces, Actinomyces; (molds) Rhixopus, Phycomyces, Mucor, Thamnidium, Geotrichum, Monilia, Aspergillus, Penicillium, Xporotrichum, Botrytis, Verticillium Torula, Cladosporium, Alternaria, Stysanus; (yeasts) Rhodotorula, Wardomyccs, Saccharomyces. Although a rather heterogeneous flora has been found on chilled meats by various workers, many of the organisms recovered may be transient or adventitious. The majority of these were not identified with specific defects in the stored meat; only a few of them were associated with spoilage. Various explanations have been offered for this restricted residential bacterial load on the surface of the intact carcass. One opinion held among workers in the meat industry seems t o be that some reduction in humidity facilitates preservation of all types of meat. For example, Jensen (1945) states that certain mold growths are troublesome on cold-storage meats if humidities of coolers are not controlled. However, there is evidence in the literature (Scott, 1936; Ogilvy and Ayres, 1951b) which indicates that the influence of relative humidity has little effect in delaying microbial

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TABLEI Genera of Bacteria Viable on Refrigerated Beef

Genera represented Pseudomonas Azotobacter type Chromobacterium Micrococcus Gaffkya Sarcina Diplococcus Streptococcus Lactobacillus Achromobacter Flavobacterium Serratia Proteus Bacterium Bacillus Clostridium Diphtheroids Streptomyces Actinomyces

Empey Gorovitzand Wlassova Empey Mall- Jensen Haines, Vickand and mann and Kirsch 1931, ery, Grinberg, Scott, et al., Hess, et al., Ayres, 1933a,b 1933 1933 1939a,b 1940 1941 1952 1954

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growth on meat surfaces. Certainly in a dry atmosphere, the diffusion of water from the interior of meat helps t o maintain a higher moisture content near the surface than is indicated by the relative humidity of the surrounding air. Haines (193313) offered a possible explanation for the growth on uncut surfaces being limited: namely, t h a t the carcass surface of the whole or quartered animal is covered by a layer of fat and connective tissue and has poor nutrient qualities for most organisms. Also, many of the organisms coming in contact with the meat are mesophiles which grow poorly a t low temperatures (Haines, 1934) and, when the animal is chilled, die before conditions in and on the meat are again favorable for their growth (Ayres, 1951). If the carcass is properly and quickly chilled, putrefaction is minimized inasmuch as many of the organisms responsible for this type of decomposition are inhibited. The principal types of changes of refrigerated fresh red meats are four in number. These are: (1) off-odor and slime, (2) ham souring or bone stink or taint, (3) black spot, whiskers, and other mold discoloration, and (4) fat rancidity.

T m i , L I1 Genera of Molds and Yt,nsts ViaLlc. on Refrigcrated Meats

Genera represented

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Massee, 1912

Monvoisin, 1918

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Molds Rhizopus Phycomyces Mucor Thamnidium Geotrichum Monitia Aspergillus Penicillium Sporotrichum Botr ytis Verticil tium Torula. Cladosporium Alternaria Stysanus Yeasts Rhodotorula Wardomyces Saccharomyces

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a Most mycologists use the name Torula t o refer to certain of the Dematiaciae (Skinner et al., 1947) and, in particular, the genus Cladosporium. Brooks and Ransford (1923) did not consider the organism that they had isolated to be a species of Cladosporium. b AIso inclndes Dernatiaeium and Hormodendrum. Described a pink yeast.

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a. Off-odor and Slime. Haines (193313) considered that the cut flesh is subjected t o an increase in numbers of microorganisms even while i t is stored a t refrigeration temperatures. Ingram (1949) pointed out that the low-temperature group of bacteria is very greatly affected by humidity, tending t o form a slime more readily on cut surfaces than on skin, connective tissue, or fat. Microorganisms appear first in damp pockets, such as folds between the foreleg and breast of a carcass, and their spread is greatly promoted by the condensation which occurs when a cold carcass is exposed t o warm, damp air. I n 1901, Glage isolated slime-forming bacteria, which he called "Aromobakterien," from the surfaces of meat stored a t low temperature and high humidity. He considered t h a t there were 7 species, one of which predominated. The bacteria were oval t o rod shaped with rounded ends and occurred occasionally in chains. They were motile, aerobic, liquefied gelatin slowly, and turned litmus milk alkaline; they grew well a t 2" C. (35.6" F.) but poorly at 37" C. (98.8" F.) ; the optimum temperature was thought t o be a t 10 t o 12" C. (50 t o 53.8" F.). On fresh meat t,hese organisms produced a gray coating which later became yellow. Glage noted that a characteristic aromatic odor, which he considered rather pleasant in the early stages, accompanied the growth of these organisms. As they grew, the surface of the meat became covered with tiny drop-like colonies which increased in size and finally coalesced t o form a slimy coating. Haines ( I 933b) thought that Glage's " Aromobakterien" were identical with, or closely related to, the organisms that he considered responsible for the characteristic "slime" which appears on meat surfaces when stored a t low temperatures. Haines considered that, ('with the exception of a certain number of organisms of the Pseudomonas group and a few Proteus, the bacteria growing on lean meat stored in the range 4-0" C. (40-32" F.) almost all belong t o the Achromobacter group." At the same time, but independently, Empey and Vickery (1933) observed that 95 % of the initial flora of beef, capable of growth a t - 1" C. (30.2" F.), consisted of members of the genus Achromobacter, the remainder were species of Pseudomonas and Micrococcus. During storage the relative numbers of Achromobacter and Pseudomonas increased while those of Micrococcus decreased. Later Empey and Scott (1939a) found that less than 1%of the microbial populations growing on the surfaces of beef at 20" C. (68" F.) were viable a t - 1" C. (30.2" F.) and, although bacteria represented 97 % of the contamination acquired by beef surfaces a t the higher temperature, yeasts and molds made up a greater share of the population a t -1" C. (30.2" F.). These workers considered the four principal genera of lowtemperature bacteria comprising the initial flora of the carcass t o be: Achromobacter 90 % ; Micrococcus, 7 %; Flavobacterium 3 % ; and Pseudomonas, less than 1%.

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More recent reports (Jensen, 1944; Ayres, 1951; Kirsch et al., 1952) have indicated that species of Pseudomonas have a relatively greater importance in causing off-odor and slime than that assigned them by the earlier workers. However, Ayres et al. (1950) pointed out that, owing t o changes in classification made in the most recent edition of Bergey’s Manual (Breed et al., 1918), a number of types of organisms which were previously reported as being Achromobacter, no doubt would be classified as members of the genus Pseudomonas in the present schema since the organisms in question were reported t o have polar flagellation. Later. Kirsch et al. (1952) came t o this same conclusion. b. H a m Souring, Bone S t i n k , or Bone Taint. Ham souring, bone stink, and bone taint are terms used in the industry t o indicate a putrefactive spoilage of particular importance in the deep tissue of large thick pieces such as the hind quarters of pork and beef. The joint fluid and bone marrow, as well as the flesh of hams from dressed hog carcasses, seldom were sterile as early as 45 min. after slaughter (Boyer, 1926). I n 1908, Klein reported a n anaerobic nonspore-forming bacillus from ‘(miscured hams ” which he called Bacillus foedans (Eubacterium foedans). Three years later, McBryde (1911) reported a n entirely different type of organism, Bacillus putrefaciens (C. putrefaciens), t o be the etiologic agent involved. Boyer (1926) isolated both aerobic and anaerobic bacteria; among the spore-forming anaerobes which he identified were Bacillus putrefaciens (Clostridium putrefaciens)) B. histolyticus (C. histolyticum) , B. sporogenes (C. sporogenes), B. tertius (C. tertium), and a n unidentified organism resembling B. oedematicus (C. novyi) . Tucker (1929) implicated C. putriJcum (C. lentoputrescens), whereas Moran and Smith (1929) showed that C. sporogenes could cause ham souring. The unidentified bacterium isolated by Boyer (1926) resembling R. oedematicus (C. novyi), is probably the same as that described by Haines and Scott (1940) and associated with bone taint of beef. Haines (1937) indicated that there were at least two types of bone taint: (1) true “souring,” a n anaerobic production of a volatile, evilsmelling fatty acid or related compound, and ( 2 ) true putrefaction or “green bone.” A (‘sewage-like” odor was considered t o emanate from the synovial fluid from an example of the first type. A large variety of proteolytic anaerobes were implicated as being responsible for the second (Howarth, 1917; Savage, 1918). Jensen and Hess (1941) catalogued various types of ham souring and asserted t h a t salt-tolerant bacteria which grow a t 0 t o 3.3” C. (32 t o 38’ F.) in bone marrow can cause any kind of souring. They named these bacterial genera t o be : Achromobacter, Bacillus, Pseudomonas, Proteus group, Serratia, Clostridium, Micrococcus, streptobacilli, and a miscellaneous group.

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Moran and Smith (1929) pointed out that the experience of the trade has shown that bone taint can be reduced if the animals are well rested and watered and if, after slaughter, the beef sides are cooled as rapidly as possible. c. Black Spot, Whiskers, and Other Mold Discolorations. Talayrach (1901) was one of the first workers to identify molds responsible for the contamination of chilled meats. A few years later Massee (1912) found that discolored black spots on chilled beef and mutton shipped to England from Argentina were due to the fungus threads of Cladosporium herbarum which penetrated the superficial layers of the meat. Brooks and Kidd (1921) made a detailed study of the “black spot” fungus and concluded that, except for its disagreeable appearance, the meat was firm and safe for human consumption. The infection was considered to be entirely superficial. They associated the organism Cladosporium herbarum with the defect and reported that, in addition to beef and mutton, lamb, veal, and rabbit meats were also affected. The mold was shown to be able to grow and produce black spots on meat kept at a temperature several degrees below the freezing point. In addition t o Cladosporium, Brooks and Kidd (1921) mentioned several other molds which could develop on chilled meats. Later, Brooks and Hansford (1923) described the morphology of several strains of molds able to grow on meats and, in certain instances, determined their minimum and maximum temperature requirements for germination and sporulation. In chill rooms having high humidities, the mycelia of molds of the family Mucoraceae growing on the surfaces of meats produce “whiskers.” Rhizopus, Mucor, and Thamnidium commonly are the genera incriminated (Brooks and Kidd, 1921; Brooks and Hansford, 1923; Bidault, 1921; Jensen, 1945). Tomkins and Smith (1932) refer to a sticky condition on the surface of meat caused by extensive growth of molds prior t o their development of aerial hyphae. This condition is not t o be confused with bacterial sliming. I n this country, discolorations caused by the growth of molds on chilled meats are less common than formerly owing to the fact that animal carcasses are seldom aged now for long periods of time in the packing plant. d . Fat Rancidity. Certain of the unpleasant tastes and odors in fat of stored beef were considered by Lea (1931) and by Haines (1933b) to be caused by microorganisms growing either in fatty tissue or in the adjacent muscle. Lea found that the fat of beef carcasses stored in still air a t 0’ C. (32” F.) was good after 25 days but somewhat tainted at 42 days; a tainted odor was present a t 15 days. Later (Lea, 1938) stated that tainted

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fat may contain several million bacteria per gram. Haines (193310) and Vickery (1936a,b) showed t h a t some of their strains of Achromobacter and Pseudomonas and all of the yeasts isolated were lipolytic. Coccoid forms (similar t o Alcaligenes viscosus except that they were almost spherical, nonmotile, and did not produce ropiness) commonly were encountered (Ayres et al., 1950; Ogilvy and Ayres, 1951a) on off-odor or slimy chicken. Although these organisms were comparatively inactive on artificial biochemical media, they showed pronounced ability t o hydrolyze fat and so may play some role in spoilage of meat. Of the Pseudomonas cultures isolated by Sulzbacher and McLean (1951) from fresh pork sausage, 70 % were lipase-forming organisms. 5. Cutting and Storage

After carcasses leave the chill room they are quartered, cut, and trimmed. The sources of contamination-knives, saws, conveyers, tables, air, water, workmen-are similar t o those which the meat came in contact with during evisceration. However, there are a t least three important differences which accentuate the bacteriological loads in the cutting room: (1) cut surfaces and juices support the growth of large bacterial populations, ( 2 ) the microorganisms which have become entrenched and have multiplied during evisceration and in the chill room are redistributed by cutting, and (3) much larger amounts of surface are exposed t o potential contaminants. T h a t some organisms do find cut flesh a satisfactory environment for abundant multiplication even though i t is kept a t refrigeration temperatures was forcefully illustrated by Weinzirl and Newton (1914, 1916). These workers proposed a bacteriological standard of 10 million organisms per gram for ground meats after observing that the earlier standard of not over 1 million suggested by Marxer in 1903 (Weinzirl and Newton, 1914) would result in the condemnation of most ground meats. This situation, according t o Tanner (1944) results from the fact that, since ground meats “may be made from meat scraps and handled carelessly, they are subject t o marked bacterial development because grinding thoroughly distributes bacteria, releases juices, and provides a much larger surface for the bacteria.” According t o Moran (1935) and Mallmann et al. (1940) most of the problems associated with the spoilage of meat are surface problems. Moran and Smith (1929) reported a negligible increase in numbers of organisms in the deep flesh of beef stored for 2 weeks at 5” C. (41” F.), which led Moran t o conclude t h a t spoilage by bacteria in the deeper parts of the flesh is unimportant compared with that a t the surface. This opinion was confirmed by Ayres and Adams (1953) who recovered

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about as many organisms from fresh beef by swabbing a square centimeter of surface with sterile moistened cotton as by mincing a gram of meat having a n equivalent amount of surface. They considered the swab method t o give more duplicable results and t o be easier t o use; difficulty was experienced in obtaining cuts of uniform depth and surface area. On the other hand, these workers obtained larger counts after storage when cut sections of tissue were analyzed. Earlier, Ayres et al. (1950) had pointed out t h a t bacteria may perfuse chicken flesh during storage. Various workers refer t o certain minimal concentrations of bacteria a t the time that incipient spoilage becomes apparent. Schmid (1931) associated the limit of saleability of beef with a bacterial count of 50 t o 100 million per sq. cm. of surface; when bacterial growth was in the advanced stage, meat was slimy t o the touch. Haines (1933b) and Empey and Vickery (1933) reported that the slime point was attained when the surface loads were 32 million and 50 million, respectively. The minimum number of organisms required for sliminess of beef was reported by Moran (1935) t o be 3 million. Although Lea (1931) reported that chilled beef in carcass form could be stored for 60 days a t 0" C. (32" F.), Haines demonstrated that small pieces of lean meat consistently developed "slimes" in from 8 t o 18 days at ' 0 C. (32" F.) even when the surrounding air had a comparatively low humidity. When Haines (1933a) analyzed the '(slime" which developed on meat from animals killed in a poor type slaughter house after storage for about a week in the range 5-0" C. (41-32" F.), he found loads of almost 28 billion organisms per sq. cm. The predominating organisms were Achromobacter (71 %), Pseudomonas (1 1 %), Proteus (1 1 %), and Actinomyces (7 %) . Haines and Smith (1933) presented data in graphical form showing the effect of the initial contamination on the time required for the development of slime on beef stored at 0" C. (32" F.). Ayres (1954) showed the effect of resident populations on the ultimate storage life of meats stored a t 0, 4.4 and 10" C. (32, 40, and 50" F.) (see Fig. 1). It can be noted t h a t the time intervals which elapse before slime develops a t 0" C. (32' F.) in the later study almost coincide with those reported by Haines and Smith. Also, it can be seen that the level of initial contamination has a marked influence on the ultimate storage life of the meat. Initial counts on meats cut or prepared in accordance with sound sanitary practice had relatively small numbers of bacteria when compared with similar items handled less carefully. Ayres (1951) reported that during the first day or two, bacterial counts for meats stored a t 4.4" C. (40" F.) show a n initial decline before microorganisms begin t o proliferate (see Fig. 2). Presumably, this temperature

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MICROBIOLOGY OF MEAT ANIMALS

may be unsuitable for survival or growth of organisms other than the spoilage types and, in these early hours, insufficient time has elapsed for the psychrophilic organisms t o replace losses of the adventitious flora. Figure 3 illustrates the relationship which exists between temperature of storage and the length of time elapsing before the appearance of slime

,

4

1

1

8

12

1 16

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KEEPING TIME ( D A Y S )

FIG.1. Relation of initial bacterial counts to time required for appearance of slime on fresh beef a t three temperatures (from Ayres, 1954).

on pieces of lean meat (Ayres, 1954). Note that the slope of the curve representing results of recent tests indicates that temperature differentials of only 1-2" C. (1.8-3.6" F.) below 7-8" C. (44.6-46.4' F.) result in significantly longer keeping times for the meat. Also, the improvement in the storage life of beef processed under present sanitary conditions over that of pieces of meat studied by Haines and Smith (1933) is readily seen if the curves for slime points in the two investigations are compared. The longer keeping times of meat sampled in recent trials over those

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JOHN C . AYRES

analyzed 20 to 25 years ago may be attributed to refinements now in use during the slaughtering and dressing procedure and t o the speed in handling the carcass and of getting cuts into the chill room and cooler under adequate refrigeration. I og--

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FIG.2. Effect of type of initial flora of frozen ground beef trimmings (I) and experimental fresh ground beef (11) on their storage lives. Percentage distribution of organisms b y types at different storage times (from Ayres, 1951).

Samplings made from the native surfacw of 37 matched sets of knuckle, inside round, and outside round of cutter and canner grade beef (Ayres and Adams, 1953) shipped to the Iowa State College laboratory from a Chicago meat packing plant indicated that aerobic loads usually ranged from 10,000 t o 1 million bacteria per sq. cm. or from 100,000 to 10 million per gram. For areas that had been sliced at the packing plant just before the meat was shipped, microbial populations ordinarily varied from 10,000 to 10 million per sq. cm. or per gram. It should be pointed out

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that the surface flora on the native areas was permitted t o develop from the time the animal was skinned whereas organisms on sliced portions may not have been introduced until after the quarter was dissected. The number of aerobes differed considerably among samplings. For example, two surface tests indicated the presence of less than 100 bacteria per sq. cm. whereas two others had 10 million. That i t is not only the size of the initial contamination which determines the time interval before incipient off-odor or slime points become

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apparent is shown in Fig. 2 . Two samples of ground beef, one prepared from trimmings and stored in the frozen state and the other made t o order from chunks of chuck and plate, had almost identical starting loads. Examination of 50 representative colonies isolated from subculture of the frozen ground trimmings (sample I) indicated 78% t o be of the typical spoilage (Achromobacter-Pseudomonas) type while only 18% of the isolates from the freshly ground meat (sample 11) was comprised of typical spoilage forms. Many gram-positive micrococci, sarcinae, and bacilli and short, gram-negative rods as well as other miscellaneous forms made up over 80% of the flora. The two types of comminuted meats were stored in cellophane bags a t 4.4" C. (40" F.) until, by organoleptic

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JOHN C. AYRES

evaluation, each was considered spoiled, The trimmings (sample I) spoiled in 3 days whereas the fresh ground beef (sample 11)kept an additional 3 days. At the time of spoilage, more than 98% of the Aora from both meats was comprised of typical spoilage forms. The load of cells growing anaerobically was consistently less than that of organisms growing in an atmospheric environment; the proportion of cells which would reproduce without free air represented 15-200/, of the total flora. From 81 samplings, 70% of the determinations showed from 1 to 10 aerobes to each anaerobe; there was an over-all ratio of aerobes to anaerobes of 6 :1. The number of cells or spores that survived a 20-min. heating period at 80" C. (176" F.) and which were still capable of reproducing when grown under atmospheric conditions, differed widely among samples. This group, tentatively classified as aerobic (or facultative) spores, may play a significant role in the spoilage of canned meats and merit considerable study in the future. It would not be advisable to consider that all of the surviving organisms were spores. Various workers (Sulzbacher and McLean, 1951; Ayres, 1951) have demonstrated the presence of species of Microbacterium in meats. Should certain heat-resistant species or strains of this genus be represented, the organisms probably could have survived the heat treatment given. Also, it is quite possible that heat tolerant micrococci, actinomycetes, and other vegetative cells can withstand 80" C. (176" F.) for 20 min. in a meat substrate. Since the technique used for detecting putrefactive anaerobic spores diff ered from the other methods for determining viable organisms, results are not directly comparable. Burke et al. (1950) made several determinations of anaerobic spore loads in pork trimmings and found numbers of spores to average less than 1 spore per gram of meat. Later, Steinkraus (1951) extended this work to include samplings from four commercial packing plants in Iowa and reported that 90% of the samples of fresh pork trimmings which he obtained contained less than 3 spores per gram. The maximum spore count found in any sample tested was 51 spores per gram. About the same number of spores were recovered from meat obtained from various plants. Ninety-nine separate samplings were made of packaged raw beef. I n no case did the putrefactive anaerobic spore count exceed 1.4 spores per gram; in only 11 samples were there more than 0.06 spores per gram. Screening tests of seventy-five cultures of putrefactive sporeforming organisms isolated from fresh pork trimmings and pork luncheon meat indicated 21 of the isolates to be obligate anaerobes (Steinkraus and Ayres, 1951). Species were tentatively identified t o be similar t o the following organisms in Bergey's Manual (6th edition) : Clostridium tetano-

MICROBIOLOGY OF MEAT ANIMALS

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morphum, C . novyi, C . carnis, C. paraputrijcum, C . tetani, C . histolyticum, and C. sporogenes. Also, an organism having biochemical characteristics similar t o Cameron’s Putrefactive Anaerobe No. 3679 was isolated.

VI. IMPROVEMENTS IN PROCESSING PRACTICES I . Desired Additional Studies

It seems paradoxical, but none the less true, that one of the values which derives from a review article is that, in evaluating the work that has been accomplished, attention is called t o a much larger task which still remains to be done. This is particularly true as it refers t o bacteriological studies which concern the slaughtering and dressing of meat animals. As a general example, it should be mentioned that very little information is available regarding the incidence and types of microorganisms found on pork, mutton, lamb, or veal. Inasmuch as the several animals are exposed to somewhat different environmental conditions, feeding habits, and slaughtering procedures, certain differences in amount and t>ypeof flora remaining on the carcass are to be expected. Many additional studies are required t o determine the role of the animal’s defensive mechanisms in preventing gross contamination of its musculature. Also, there is need for fundamental studies which concern the activity of hyaluronidase, collagenase, and capsular substance in aiding or limiting microbial invasion. Information is desired regarding numbers and types of organisms (1) entering the tissue a t the wound site, (2) coursing through the circulatory system through heart action, and ( 3 ) inhaled during final respiratory activity. There is considerable information concerning the effects of feeding but some of it is controversial. For example, Madsen (1943) mentioned that the intestines of sugar-fed pigs were more easily washed than were those of unfed animals but Gibbons (1953) reported that, in his studies, packing plant personnel found no difference. It would be of interest to know how the types of flora of the fed and unfed animals compared. What influence does acidity produced by metabolic activities of organisms during the degradation of the carbohydrate have on the deposition of the intestinal wall’s slime-layer ? Although Callow (1949) and Ingram (1949) have indicated that fatigue in hogs may render carcasses of these animals more susceptible to bacterial decomposition than they would be otherwise, these conclusions have been deduced or extrapolated from a knowledge that the glycogen reserves in fatigued animals are depleted and that slight acidification of media greatly reduce bacterial growth rates. Although there is good

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probability that the train of events which Ingram outlines does take place, more direct evidence is needed. Also, information concerning the difference in the microbiology of starved ruminant and nonruminant meat animals might be of value in determining the reason that cattle and sheep can be starved longer than can the hog without ill effects. British workers (Bate-Smith, 1937, 193%; Callow, 1949) have explained this phenomenon as being ‘‘expected because the pig is badly out of training, whereas both sheep and cattle usually get plenty of exercise.” No consideration has been given to the enormous reservoir of food in the rumen, t o the digestive lag for these animals, or t o the possibility that microorganisms in the rumen may serve as sources of nutrilites. The use of detergents and sanitizers for cleaning and aiding in mechanically removing of filth and microorganisms from the live animal is deserving of study. Although Empey and Scott (1939a)b) made a n exploratory study of the use of chlorine solutions under laboratory conditions, no further report could be found of its value when tried under field conditions. Since that time, “in-plant chlorination” has been used for many of the other food processing fields and its value either on the hide of the live animal or on the carcass after dressing should be determined. The use of antibiotics in meat processing has been suggested by Weiser et al. (1953, 1954) as a means of preventing deep spoilage, especially prior t o dressing out. I n several exploratory studies that they made regarding the effect of aureomycin, they found th a t the antibotic delayed spoilage of infused meats when they were compared with controls. However, they posed several questions which, among others, will need t o be answered before the use of antibiotics can be considered. For example, the consumer must be assured that the material added is not toxic even though it may be used for longer periods of time. Also, it will be necessary to know whether or not some organisms develop resistance t o the antibiotic upon its continued usage in a foodstuff. Then, too, questions arise as t o the manner and nature of modification of the flora in meats. There is urgent need for published information regarding the incidence of the several types of aerobic and anaerobic microorganisms found in and on meats. I n the past, some question has been raised regarding the value of such observation. Certainly mere counts and taxonomic cataloguing of species encountered on meat does not present sufficient evidence to incriminate or exonerate the product. However, studies which determine not only the numbers of microorganisms present but their contribution to its ultimate spoilage should prove of considerable worth. The construction of some of the pieces of equipment used for eviscerating or cutting should be made more satisfactory from a sanitary

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st,andpoint. For example, band saws now in use harbor much tissue debris. Reciprocating saws (e.g. jig saws) might be devised in such manner t h a t tissue debris does not accumulate and serve t o provide a n inoculum for successive pieces of meat. The design of all equipment should be such that easy and rapid cleaning is possible. More study is needed t o ascertain the significance of pathogens in meats. McCullough et al. (1951a) present evidence t o show that the exposure of packing plant personnel t o Brucella through the handling of hog carcasses presents a n important problem. Further, they (McCullough et al., 1951b) point out that, since Bang’s reactor cattle probably are sent for slaughter because they were no longer profitable t o the owner, such infections may have been of long-term durations. As a mat,ter of record, programs have been initiated which encourage the sale and slaughter of reacting animals. Since Brucella were recovered from numerous sites in a significant number of the animals examined (42 of 100 reactor cattle), these workers considered processing of such carcasses hazardous. Probably of even more grave consequence, is the possibility that meat of reactor animals may pass a clinical inspection and be considered safe for human consumption whereas in fact i t is not. 2 . Suggested Improvements

It is universally recognized that animals should not be fatigued, frightened, chilled, or overheated just prior t o slaughter. Immobilization by asphyxiating with carbon dioxide is a t present in use for hogs; its use for sheep and cattle is suggested as well. Before the animals regain consciousness and prior t o bleeding, the site where the stick-knife is t o enter should be sterilized by flooding or swabbing with a n approved germicidal solution (e.g. a quaternary ammonium compound). Also, the knife blade should be cleansed and sanitized by rinsing in water t o free it of blood, hair, and other debris and immersed in a sanitizing agent before sticking the next animal. After sticking, the practice of allowing the carcass to remain hanging on the rail, free of contact with other carcasses until bleeding is completed and all heart and respiratory action ceases, has much t o recommend it. (Six minutes generally is considered adequate.) It would seem that these operations could be expedited and simplified in such manner t h a t they would not interfere with the routine now in practice. Removal of hair or hide depends upon the type of animal considered. The microbial loads associated with dirty hides, hoofs, and hair provide ample witness t o the need for more adequate cleansing before such animals are killed. Force spraying over the whole area of the hide with a sanitizing agent would serve t o facilitate the removal of loosely adhering

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soil and filth. However, on some animals fecal material gets “caked on” the flanks and feet so tenaciously that it is almost impossible t o wash free unless a laborious and expensive soaking operation is undertaken. Often the animals are in this condition before they reach the packing plant; in many instances livestock accumulate filth owing t o poor bedding on the farm or in the feed lots. It would seem that one way to discourage such insanitary practices would be to downgrade animals if they have noticeable deposits of scurf. An alternative suggestion when there are only a few animals, would be to segregate these animals before they reach the killing floor and t o make further attempts t o clean them or t o handle them separately a t the end of the day’s run immediately before time for the “rlean up.” Cattle and sheep skinning should be done in such a manner that the workmen minimize touching of the exposed carcass with the hair or wool. The hide, after it is cut free, should be removed immediately from the skinning floor. Running water should be available and sanitizing of hands and tools should be enforced when accidental contact is made with dirty hair or wool. Since the hands of the workman are a very important sourcc of contamination, the skinner should be trained not to touch the carcass whenever such contact can be avoided. Prevention of contact with the musculature after handling the lower legs and feet is particularly important. Also, insofar as possible, evisceration of cattle and sheep should be handled separately from skinning or fisting. Wherever possible, mechanical devices such as hooks should be employed and sanitized between animals. Use of the scald tank t o loosen hair and scurf on the hog has little t o recommend i t from a sanitary point of view. Hot-water spraying of individual carcasses in steam cabinets is advocated a t as high a temperature as possible without scalding or reddening the skin. The bung should be plugged and the plug should remain with the viscera or be sterilized before reuse. If the hog is t o be wax dipped after scalding, dehairing, and shaving, the wax should not only be settled free of hair but should be strained or filtered and possibly heated thoroughly before reuse. I n many plants, scalding, dehairing, and waxing are completed before the hogs are permitted on the eviscerating floor. This practice should be required universally. The eviscerator should avoid alternate touching of the meat of the carcass and intestinal or fecal matter. If the esophagus, bladder, bung, or rumen are t o be tied, this should be done by machine or by gloved hand with sanitization between animals. It should be ascertained by examination that all personnel coming into direct contact with any portion of the

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animal which is t o be used for edible purposes are not carriers of any of the enteric pathogens; preferably, the workers should be licensed food handlers. Federal or plant inspectors should not be permitted t o handle or probe viscera and carcass interchangeably. During protracted examination of the organs of the intestinal tract, a ruptured or infected tissue is a n almost certain eventuality. When and if, such occurrences take place, superficial rinsing of the hands is but small safeguard against the contaminative load th a t is transferred to the inspector's hands. If no other method of examination is feasible, the inspectors should work in pairs-the one examining and handling the carcass, the other probing the viscera. Walls of eviscerating rooms should be constructed of glazed tile while floors should be made of smooth concrete or cemented brick or tile. Supporting structures should be of concrete or painted steel. All conduits, water, and heating pipes should be separated from the interior of these floors in order t o permit and expedite force spray washing. Adequate drainage facilities are requisite. Any points wherein off a1 might collect should be kept separate from the lines where meat is moving. Implements used in the eviscerating and cutting rooms should be sanitized whenever practical and possible during operation. Except for chopping or cutting blocks, tables, benches and conveying devices should be made of metal. Wood, when required, should be edge-grained maple or wood of similar porosity. I n any case, surfaces over which the meat must move should be kept as free as possible from bacterial infusion. Tissue debris should not accumulate on saw races. Where practical, ultraviolet irradiation should be employed t o reduce surface contamination and t o permit more rapid tenderization. Circulation of air in the chill room must be assured; there should be no dead pocket areas. Sawdust, when used, should have a n approved fungicide incorporated t o reduce contamination deriving from drippings and from feet of workmen. Use of antibiotics should be evaluated by the industry as a means for reducing contamination on meat surfaces. Finally, extrapolation of the information presented in this review indicates that the nature and number of bacteria found on meat cuts can be used, not only t o determine if the meat is processed under good sanitary conditions, but also to predict its expected storage life. Examination of Fig. 1 reveals that meat cuts with initial loads of 100 bacteria will keep only 5 t o 6 days a t 10" C. (50" F.) but a t 0" C. (32" F.) will not become slimy until stored for more than 14 days. On the other hand, if meats have high bacterial loads, their storage life is only slightly prolonged by the use of lower temperatures. The chill room cannot be regarded as a

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means of controlling the flora on the carcass of animals if prior handling of the carcass has been carelessly done. The significance of the kind and amount of Aora which resides on meat when it leaves the packing plant prompts the author of this article to insist that standards for bacterial numbers are needed both for wholesale and for retail meats. This recommendation is made with full realization that similar suggestions advocated for ground beef by Marxer (1903) (see Weinzirl and Newton, 1914) and by Weinzirl and Newton (1914,1915) failed to gain many proponents. As a matter of record, Hoffstadt (1924) and even Weinzirl (1924) concluded that there was no correlation between freshness of meat and its bacterial content. However, Haines (1937) criticized the views taken by Hoff stadt and Weinzirl since the organisms they isolated and studied (staphylococci and proteolytic anaerobes) do not grow a t temperatures below about 10" C. (50" F.). Workers (Schmid, 1931; Haines, 1933a; Empey and Vickery, 1933; Ayres et al., 1950), who have independently estimated populations on flesh of different animals at the time of slime formation, have obtained counts showing remarkably good agreement with regard to numbers, viz. 32-100 million bacteria per sq. cm., and two types of organisms predominating; Pseudomonas and Achromobacter. Also, since it has been demonstrated that most of the flora is found a t or near the surface, samplings can be performed quite simply through the use of (1) direct smears on glass slides, (2) spot plates, or (3) cotton swabs. Tolerance limits for loads of organisms on and in meats have not been established. However, little imagination is required to estimate the storage life of meats having initial loads of from 1-10 million bacteria per gram. Until such time as the industry has made a concerted study of the customary loads on and in meats, the following counts for wholesale cuts are considered by the author to be reasonable and are proposed for consideration: (1) aerobic population: 10,000 to 100,000 bacteria per sq. cm.; (2) anaerobic population: 5,000 to 50,000 bacteria per gram; (3) most probable number of cells (M.P.N.) surviving 20 min. of heating at 80" C. (176" F.) and growing aerobically: 10-100 per gram; (4) M.P.N. of cells surviving 20 min. of heating at 80" C. (176" F.) and growing anerobically: less than 1 spore per gram. REFERENCES Adami, J. G. 1899. On latent infection and subinfection, and on etiology of hemochromatosis and pernicious anemia. J . Am. Med. Assoc. 33, 1509. Adami, J. G., Abbott, M. E., and Nicholson, F. J. 1899. On the diploccoid form of the colon bacillus. J. Exptl. Med. 4, 349. Adamson, C. A. 1949.A bacteriological study of lymph nodes.Actu Med. Scand. 337,l.

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Amako, T. 1910. Untersuchungen uber das Conradi’sche Oelbad und den Bakteriengelialt der Organe gesunder Tiere. 2. Hyg. Infectionskrankh. 66, 166. Anonymous. 1952. What changed meat packing from a trade to an industry. Natl. Provisioner 126 (4), 197. Anonymous, 19L3. Progress cast in bronze. Food E n g . 26, 43. Arias, C., Burroughs, IV., Gerlaugh, P., and Bethke, R. 31. 1951. The influence of different amounts and sources of energy upon i n oitro urca utilization by rumcn microorganisms. J . A n i m a l Sci. 10, G83. Ayres, J. C. 1951. Some bacteriological aspects of spoilage of self-service meats. Iorcn State College J. Sci. 26, 31. Ayrcs, J. C. 1954. Manuscript in preparation. Ayres, J. C., and Adams, A. T. 1953. Occurrence and nature of bacteria in canned berf. Food Technol. 7, 318. Ayres, J. C., Ogilvy, W.S., aud Stewart, G. F. 1950. Post-mortem changes in storetl meats. I. Microorganisms associated with dewlopment of slinic on eviscrratcti cut-up poultry. Food Technol. 4, 199. Hanfield, F. H. 1935. The electrical resistance of pork and bacon. J . Soc. C h e m I n d . 64, 411T. Fate-Smith, E. C. 1933. Physiology of muscle protein. A n n . Kept. Food Invest. Board ( B r i t . ) , p. 19. Bate-Smith, E. C. 1936. The effect of fatigue on post-mortem changes i n muscle. Dept. Sci. I n d . Research Ann. Rept. Food Invest. Board (Brit.), p. 21. 13:rte-Smith, 13. C . 1937. The special metabolism of the pig. D e p t . Sci. 1nd. Kcwtrrrh ii7an. Rept. Food Invest. Board ( B r i t . ) , p. 22. Ihtc43rnith, 12. C. l938a. The buffering of muscle in rigor: ProttLin, phosphate, : i r i ( l .I. I’hysiol. (London) 92, 336. . C. 1938b. Physiology of rigor mortis. Dept. Sci. I n d . Research .1iin. Rept. Food Invest. Board ( B r i t . ) , p. 15. Bate-Smith, E. C. 1938~.The carbohydrate metabolism of slaughterhouse animals. Dcpt. Sci. I n d . Research dnn. Rept. Food Invest. Board (Brit.),p. 22. Bate-Smith, E. C. 1939. Changes in elasticity of mammalian muscle undergoing rigor mortis. J . Ph?ysiol. (London) 96, 176. Hate-Smith, E. C. 1948. The physiology and chemistry of rigor mortis, with special reference t o thr aging of beef. L4dvances in Food Research 1, 1. I1.090

86 87 90 90

.90 .91 .92 .93

Blue Value 248 257 153 117

From Olson and Harrington, unpublished.

fine particle-sized material, an increased package density, and a decreased Blue Value (reflecting a decrease in the degree of pastiness of the reconstituted product) were obtained. 2. Predrying Operaticns

Of economic importance in the manufacture of potato granules is the fact t ha t peeling, in the usual sense, is not an essential operation. All but

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a negligible portion of the peel fragments can be removed from the product by screening after the product is dried. Of advantage, in addition t o the elimination of peeling, is the potential increase in yield because screening removes a smaller portion of potato flesh than do the conventional peeling methods. On the other hand the peel may impart a flavor t o the product, resembling t h a t of baked potatoes, which may or may not, be desired. Further, the presence of skin on the potatoes may make more difficult the detection of bruised and spoiled tissue that should be removed by trimming before the potatoes are cooked. I n the absence of peeling, vigorous mechanical washing must be used t o insure removal of dirt. Peel may contribute adversely t o product color. I n the production of good mashed potatoes it is essential that the cooking time be appropriate t o the tubers used. Different raw materials require different cooking times. I n general, the higher the total solids content of potatoes, the shorter the time required for cooking. A properly cooked potato will break u p into a mealy mash when mixed without undue rupture of tissue cells. If potatoes are undercooked, the tissue cannot be satisfactorily mashed and one of two conditions will generally prevail. If the mashing action is mild, lumps of undercooked tissue remain in the mash; if vigorous, the lumps may be subdivided but only with disintegration of much of the cellular structure. This will liberate soluble starch and contribute a pasty consistency t o the mash. On the other hand, if potatoes are overcooked, there appears t o be a weakening of the cell-wall structure, releasing solubilieed starch or, a t least, rendering the individual cells less resistant t o mechanical damage and subsequent release of starch during the process or rehydration operations. A manifestation of undercooking is the presence, in the coarser screen fractions of the product, of particles of dried, vitreous potato tissue appearing much like rice grains. The effects of overcooking are not obvious. Probably the principal effect is one of lower textural quality in the reconstituted product. I n the presence of complicating raw material and processing variables, i t is not simple t o isolate the effect of overcooking of potatoes in potato granule manufacture. However, i t is desirable t o maintain minimum cooking times. Operations are generally controlled by adjusting the cooking time so as t o just avoid the obvious manifestations of undercooking. Because of the time required t o transfer heat through potato tissue, potatoes should be sliced before cooking t o insure uniformity in cooking. Cutting the potatoes t o a maximum heat-transfer depth of about 3.8 in. (potatoes cut t o a 3i-in. slab thickness) appears t o provide the necessary uniformity. As tissue cells are disrupted along the cut potato surfaces, starch is

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released. To avoid undue material loss, slicing and peeling should be held t o a minimum. T o avoid a contribution t o undesirable pastiness in the reconstituted product, the released starch should be thoroughly washed from cut surfaces. Mashing of the cooked potatoes is a critical operation in the production of high-quality granules. The individual tissue cells should be separated by mechanical action but remain whole with cell walls enveloping the swollen, heat-solubilized starch. The type of mixer used, the quality of raw material, and the temperature during mashing are very important. A vigorous mashing action, as with a high-speed, planetary-type mixer, must be of short duration and with a relatively high product temperature t o prevent mechanical damage t o product. I n the add-back method, the immediate addition of previously dried material, as the mashing commences, appears t o reduce product damage. It is possible t o mash potatoes after the cooked product has cooled [to around 140" F. (60" C.)] b y the gentle action of pressing between revolving drums. A 0.050-in. separation between drums appears t o be appropriate. With greater spacing, the mashing is inadequate, with smaller, the flow rate of the product is reduced. The drums should revolve a t the same angular velocity t o prevent shearing action t h a t might damage cell structure. T o self-feed properly in mashing potatoes, drums should be a t least 18 in. in diameter. The tempering operation in the manufacture of potato granules is used following the mashing operakion and converts the mash from a gummy or sticky t o a friable condition. As outlined above, the temperature and product moisture content affect the rate of change in friability during the tempering operation and have a n important effect on the degree of granulation of the product. It appears advisable t o do preliminary mashing of potatoes while they still retain cooking heat and a relatively high temperature [above 140" F. (60" C.)] in order t o achieve adequate cell separation without undue cell damage. On the other hand, i t is advantageous t o provide a mixing operation following product cooling [to below 90" F. (32" C.)] and tempering in order t o adequately granulate or subdivide the product into individual cells. 3. Drying Methods and Equipment

In commercial operations potato granules have been dried in tunnel, rotary-turbo, kiln, and various types of pneumatic dehydrators. The advantages of pneumatic drying systems include efficient utilization of heat and the prevention of agglomeration of separated granules. A dis-

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advantage is the susceptibility of the product t o abrasion and impact damage as i t is conveyed in the duct system and separated from the air stream in the various standard-type collectors. Descriptions of several early developments in pneumatic dehydrators designed specifically for potato granule operations have been published (Fison, 1943; Bar, 1941; Anonymous, 1946). Proctor and Sluder (1943) described a batch-type pneumatic drier for laboratory studies. Studies on certain engineering aspects of duct drying were described by Olson (1947). These studies were not exhaustive but probably provide a useful introduction t o such investigations. A pneumatic drier designed to reduce abrasion and impact damage consisted of a vertical riser and a novel collector that provided for a single change in direction of product flow under decelerating conditions of the air stream (Olson et al., 1953; Neel et al., 1954). Constructed for use in laboratory investigations, the design principles of this drier have had limited commercial application for finish drying, cooling, and conveying of product. One advantage of a vertical duct drier is that it makes possible the use of relatively low air velocities (1500 t o 2000 ft. per min.). Inhorizontal ducts it. is necessary t o maintain higher velocities to prevent the settling of the product on the bottom of the duct. Cooley et al. (1954) indicated that air velocities of 70 ft. per sec. (4200 ft. per min.) were necessary t o keep moist potato powder in suspension in a horizontal pipe. I n this investigation it was found th at product damage was greater (measured by chemical analysis of the free starch) with higher air velocity in the duct. Olson et al. (1953) presented data relating bulk or package density t o the inlet air temperature in a pneumatic drier. In the range of 212 t o 390" F. (100 t o 199" C.) a more dense product was obtained with lowertemperature drying air. Cooley et al. (1954) confirmed this finding and presented data showing that the differences in air temperature (or perhaps drying rate) did not materially affect the particle size distribution found by screen analysis of the product. Although particle size was found t o be correlated with bulk density, the magnitude of this factor was slight compared t o the correlation of drying-air temperature with density. Neel et al. (1954) described a finish drier as a n improved means for reducing moisture content of potato granules from about 12% t o about 4%, a level a t which quality is reasonably stable against browning, even under adverse temperature conditions. This drier utilizes the principle of a bed of granular solid material fluidized b y a uniform, low-velocity flow of air, directed vertically upward from the bottom of a drying chamber. When the drying chamber is filled, the introduction of material

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at one end causes an overflow of product a t the exit end and a continuous drying operation is achieved. Other dehydrators used for finish drying have been the tray, pneumatic, rotary-kiln, and rotary-turbo dehydrators. Questions of economic feasibility and maintenance of acceptable product quality have been raised concerning finish drying to below 6 % moisture content. It remains t o be seen whether or not fluidized-bed drying or some other method will make i t commercially practicable for the interested trade t o reduce moisture-content specification to below 6 %.

4. Product Quality I n the appraisal of potato granules and in the development of a process for their manufacture, a number of factors concerning product quality are considered. Certainly of prime importance is the particle size distribution of the finished product. If in the process tissue cells are completely separated from each other, after drying they will pass without difficulty through a screen of about 60 mesh. By screening t o such a size classification, a uniform appearing material is obtained. Within this material considerable differences exist because of the natural variability of cell size. If the add-back method is used, a major portion of the material that does not pass a 60-mesh screen can be recycled in the process. However, as the particle size gets larger, the effectiveness of the material in mixing and tempering operations is lessened, and a higher percentage of dried product must be added back t o allow successful continuance of the process. Further, the conveying and drying of large particles in pneumatic driers imposes problems connected t o product flow and drying rates. Therefore, all material that will not pass through a 16 t o 20-mesh screen is removed from the system (this includes most of the peel fragments if such are present). This coarse material has salvage value as stock feed but should not exist in excessive amounts because of economic considerations. A specification that all material packed must pass a 70-mesh screen is quite feasible for the add-back process, because larger material can be recycled. It is desirable that about 50% of the material to be recycled should also be of such a fine particle size (minus-70-mesh) as t o insure high product quality in a continuing operation. With methods not involving add-back, the product usually contains appreciable quantities of larger material. In such cases modification in mixing operations to achieve a greater degree of granulation, or use of a partial add-back may be desirable. I n laboratory operations, good yields of fine particlesized granules have been reported for the cold-tempering method (Campbell et al., 1945) and for the solvent-extraction method (Treadway, 1954).

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Considerations of bulk density, stratification of product in packages due t o variation in particle density, and rehydration rate would be determining factors in particle-size specifications. Related t o particle-size distribution and affected by several factors in the process is the bulk density of potato granules. The bulk density of potato granules has been specified as 6 lb., 2 oz. per No. 10 can for military procurement. This requirement establishes potato granules at about 0.9 the density of water. Although this specification can be met without undue difficulty if the add-back process is used, other methods of manufacture, such as the "freeze and squeeze" and solvent-extraction processes, produce a product with a density that is somewhat lower. The quality attributes of potato granules of greatest importance t o the consumer are flavor, color, and texture of the product when prepared for serving. Factors in the process can materially affect all three. Flavor is delicate and subject t o loss or change. Highly volatile flavor components are lost in dehydration. Heat damage may impart a scorched flavor. The raw material itself may be responsible, as in the case of severe sunburning, which makes potatoes bitter and turns them green. The color of potato granules is the result of natural pigment (yellow t o yellow-green) and processing effects. Most prominent is the browning that may result from heat damage in the process. A graying is sometimes observed that is probably related t o holding at high temperature levels while the product is fairly moist. Perhaps consistency is the quality attribute least satisfactorily controlled by present-day practices, although great improvement has been made by all of the commercial dehydrators in recent years. Raw material certainly has significance in this regard. However, nearly every unit operation in any potato granule process has also a n effect on the freeing of solubilized starch from the cooked potato cells and this affects the texture of the reconstituted product. The temperature of the liquid used t o reconstitute potato granules has a marked effect on the texture of product. The temperature must be a t least 160" F. (71" C.) t o satisfactorily reconstitute potato granules.2 An upper temperature limit cannot be defined although it may be generally stated that the higher the temperature of the water of rehydration, the greater the tendency toward an undesirable pastiness. This factor is relative and it is found that an exceptionally high-quality product is better when rehydrated a t nearly boiling temperature than a poor sample rehydrated a t 160" F. (71" C.). An improvement in texture of reconstituted potato granules t o more

* An exception is the selvent-extracted potato granules, which can be rehydrated at lower temperatures.

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closely resemble freshly mashed potatoes is probably a prerequisite t o the establishment of a domestic market of considerable size. I n an earlier volume of this series the storage characteristics of potato products have been reviewed (Ross, 1948). As demonstrated b y Burton (1945, 1949) and confined by Hendel et al. (1951), the two major deteriorative processes that occur in storage are browning, accompanied by a scorched flavor development and an oxidative staling that causes an ‘(off” flavor attributed to deteriorative changes in the fatty constituents of the potato. The browning can be checked by reducing moisture content, adding sulfite, and reducing storage temperature. The reduction of product moisture content, however, enhances the development of oxidative staling. Even a t near freezing temperature the stale, off-flavor may develop. Control of this factor is best achieved by packaging in the absence of oxygen as in an inert atmosphere of nitrogen. The nutritive value of potato granules as served is probably equivalent t o that of dehydrated potato dice. Although processing losses of vitamin C are greater for the granules, the instantaneous reconstitution contributes no further substantial loss whereas considerable loss occurs in the reconstitution of dice (Barker et al., 1943; Green et aZ., 1947, 1948, 1949). Data in Table V suggest that the principal vitamin C loss in granule production occurs in the cooking operation. TABLEV Vitamin C Retention in Potato Granules during Process and Reconstitution“

Type of potatoes

Average of two separate experiments with same raw material (mg./100 g. at moisture content as served)

Vitamin C in raw potatoes, Russet Burbank variety about 3 months after harvest Potatoes, cooked Potato granules (add-back process) Reconstituted potato granules Q

10.6

7.0 8.4 8.5

From Olson and Harrington, unpublished.

X. QUALITYEVALUATION OF POTATO GRANULES 1 . Subjective Appraisal

Quality evaluation of potato granules, not unlike that of other food products, is involved in the development of new and better processing methods and in the control of manufacturing operations. I n development

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work it is important t o determine the effect of new or modified processes on quality attributes of the product. Results are obtained demonstrating significant differences in certain quality attributes and the relationship of such differences t o specific variations in the process. On the other hand, for control of plant operations the objective of appraisal is t o establish and maintain a suitable range of product uniformity and minimum level of acceptability. Techniques for determining differences in foods have been established, using trained test panels and following accepted statistical procedures for determining significance of results. Boggs and Hanson (1949) have provided a general review of the subject. For potato granule appraisal, methods of sample preparation, presentation, and scoring must be developed with consideration not only of product but of the particular quality attribute being judged. For example, flavor judgments should not be influenced by differences in sample color, which can be neutralized by use of colored (amber or red) light in the test area. Color should be judged under controlled conditions of uniform lighting. Off-color and off-flavor can probably be best scored against a standard control sample. Consistency of reconstituted potato granules can also be scored relative t o standard controls. Wood et al. (1955) described a method of ranking mashed potatoes for rubberiness with two coded control samples. The relative ranking placed unknown samples into categories which enabled indirect comparison of samples tested at different times. This has been a valuable tool for research and has eliminated much tedious and costly cross comparison of experimental samples. The evaluation of samples and establishment of levels of acceptability for production control is a problem of greater magnitude than the determination of differences between samples. The final answer t o consumer acceptability is found in the sales data of a commercial concern. Continuing success often will depend upon approximating a level of acceptability, training analysts t o recognize the level, and appraising routine production samples t o provide a control over processing operations. More frequently than not the standard of acceptability is established as a mental concept rather than as a selected material that can be used for comparisons. The reliance on memory and the momentary diligence of the judge during the test may well lead t o an unnecessary lack of precision. Although the replications necessary for statistical analysis in the accepted difference measurements are too cumbersome for routine production control, it is probably advantageous t o establish physical standards for comparison. These standard samples, coded and presented with production samples for judgment of difference, would not give the precision of

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test panel procedures with 30 t o 50 replications and statistical treatment of data t o establish levels of probability of correctness. However, with trained analysts, the method should achieve a higher degree of product uniformity than would be possible with reliance entirely on the analyst’s memory. 2. Microscopy I n recognition of the relationship of broken cell structure, released soluble starch, and pasty or rubbery texture of reconstituted potato granules, microscopic observations have been used as an aid in the appraisal of potato granules. Bunimovitch and Faitelowitz (1936) reported such observations. Greene et al. (1947, 1948, 1949) counted ruptured potato cells in a microscopic field, and established a correlation between subjective evaluation and percentage of cells ruptured. If less than 6% were ruptured, the product was superior; lO-l2%, average; and 20%, a very pasty product. Proctor and Sluder (1944) and Campbell et al. (1945)found counts of broken cells in a microscopic field t o be a suitable method for evaluating texture of reconstituted potato granules. They reported use of this method as a control in experimental investigations. It was possible t o appraise samples taken from the process line and, within limits, predict the general area of acceptability of the product when finished. Hall (1953) has discussed various appraisal methods and postulates t h a t a better value can be obtained by count of broken cells than by measurement of released starch (see below, Objective Mcasurements) . It is noted that, in these references, methods other than the add-back process were involved. [Bunimovitch and Faitelowitz (1936) used the direct two-stage dehydration; Greene et al. (1947,1948, 1949) and Hall (1953)used the “freeze and squeeze” process; and Proctor and Sluder (1943, 1944) and Campbell et al. (1945) used a cold-tempering method.] T o date, no demonstration of successful application of microscopic examination in quality evaluation has been found where the add-back method is used, probably because soluble starch released by cell-wall rupture and then recycled adheres t o intact cells. Microscopic identification of the degree of starch liberation and total cell breakage could not be very precise under such conditions. 3. Objective Measurements

The ultimate appraisal of food products is the response of individuals t o quality attributes. However, cost and lack of precision of subjective appraisals make it desirable t o develop objective measurements that can be correlated with the subjective evaluations. A number of attempts have

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been made t o do so with reconstituted potato granules. I n addition there are objective physical measurements of granules that have descriptive value in themselves. The measurement of viscosity of supernatant liquid obtained by centrifuging a mixture of water and reconstituted potato granules was found a suitable method for control of a potato granule process (Barker et al., 1943; Barker and Burton, 1944). Free solubilized starch extracted from the mashed potatoes was the principal factor related t o the viscosity. Campbell et al. (1945) also used a viscosity measurement but found the method less valuable in their work than microscopic examination. I n preliminary studies a t the Western Utilization Research Branch it was found that viscosity measurement of a starch extract of potato granules is a more difficult analytical procedure than a chemical test for the starch. Viscosity measurements were thus abandoned. Direct measurement of viscosity of potato mash with a Brookfield viscometer3 was not found t o be satisfactorily reproducible. Cooley et al. (1954) reported successful use of viscosity measurement of reconstituted potato granules with a Brookfield viscometer with product supported on a helipath stand. However, i t was found necessary t o rehydrate the granules with appreciably more water than would be used for food preparation. I n no case have viscosity measurements been found t o provide an absolute judgment of potato granule quality. Differences in processing method and probably in raw material influence the relationship of the viscosity of supernatant liquid from potato granule slurry t o the consistency of granules as prepared for serving. However, when raw material is uniform and processing operations do not vary widely, viscosity measurements demonstrate a correlation with subjective appraisal values and appear t o provide a useful purpose in investigations of potato granule processes. Chemical determination of starch extracts from potato granules, as an objective measure of product consistency, is also limited t o similar raw materials and processing methods. A convenient method is extraction of potato granules in an excess of water a t about 150" F. (66" C.). Iodine is added t o a portion of a filtrate from the slurry. The intensity of the starch-iodine color is determined photometrically and designated the Blue Value. A method for determination of the Blue Value has been developed (Olson et al., 1953; Mullins et al., 1955) which has been useful in research and development investigations of potato granule technology. 3 Mention of product name does not imply endorsement by the U. S. Department of Agriculture.

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253

Important quality attributes of potato granules that can be measured objectively include bulk density (weight of sample per unit volume), color, and particle-size classification. Adequately reproducible determinations of bulk density can be obtained by dividing the weight of a sample by its volume, as measured by tamping or jarring t o minimum volume in a graduated cylinder (Olson et al., 1953). Color can he readily measured photometrically by difference comparison with standard color plates in reflectance-type photometers. Such objective data are particularly useful in following changes due t o storage degradation of uniform material stored under different conditions. The establishment of color standards for production cont(ro1 also appears possible, but, of course, depends upon subjective evaluation and judgment t o establish a base point and an allowance for deviations. Particle-size classification is accomplished by screen analysis. I n the add-back process it is perhaps more important t o follow particle size in connection with seed used and total dried product rather than the material that is packaged for sale. For processes where recycling is used, the particle-size classification and the effects of oversized material on rehydration character and bulk density may be a very important indication of operational success.

REFERENCES Allen, A. E. 1921. Process of dehydrating potatoes. U. S. Patent 1,377,172. Anonymous. 1937. Packaged baked potato flour. Food I n d s . 9, 324. Anonymous. 1943a. Potato shreds, Rogers Bros. Pioneered development of dehydrated potato shreds. Western Canner and Packer 36(2), 37, 39, 41. Anonymous. 194313. How potato shreds are made. Food I n d s . 16(3), 47-49. Anonymous. 1946. Simplot shifts production set-up. Food Packer 27(7), 39-40. Anonymous. 1948. Pre-cooked potatoes. Food M a n u f . 23, 295. Bar, P. J. 1941. Process and apparatus for treating solids in gases. Brit. Patent KO. 546,088.

Bar, P. J. 1943. Dehydrated potato product and method of making same. Brit. Patent No. 566,498. Barker, J. 1941. Improved process for producing starchy vegetable products in dry powdered form. Brit. Patent No. 542,125. Barker, J., and Burton, W. G. 1944. Mashed potato powder. I. General characteristics and the “brush-sieve” method of production. J. SOC.Chem. I n d . 63, 169-172. Barker, J., Burton, W. G., and Cane, R. 1943. Mashed potato powder. I. Properties and methods of production, Great Britain Dept. of Scientific and Industrial Research and Ministry of Food. Dehydration; United Kingdom Prog. Rept., Sec. VI, part 6, 10 pp. Boggs, bl. M., and Hanson, H. L. 1949. Analysis of foods by sensory difference tests. Advances in Food Research 2, 219-258. Bosncll, V. R., and Bostelman, E, 1949. Our vegetable travelers. Natl. Geographic Mag. 46(2), 145-153.

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Bowen, W. S. 1931. Spray-drying Idaho’s surplus potatoes. Food Znd. 3, 380-383. Brown, A. H., and Kilpatrick, P. W. 1943. Drying characteristics of vegetables-riced potatoes. Trans. Am. Soc. Mech. Engrs. 66, 837-842. Bunimovitch, M., and Faitelowitz, A. 1936. An improved method of reducing potatoes and other starch containing vegetables to the form of a dry powder. Brit. Pat. No. 457,088. Burton, W. G. 1944a. Mashed potato powder. 11. Spray-drying method. J . Soc. Chem. Znd. 63, 213-215. Burton, W. G. 1944b. Improvements in or relating to the production of dried potatoes. Brit. Patent No. 566,828. Burton, W. G . 1945. Mashed potato powder. 111. The high temperature browning of mashed potato powder. J . SOC.Chern. Ind. 64, 215-218. Burton, W. G. 1949. Mashed potato powder. IV. Deterioration due t o oxidative changes. J. Soc. Chem. Ind. 68, 149-151. Campbell, W. L., Proctor, B. E., and Sluder, J. C. 1945. Dehydration of precooked white potatoes in granular form. Research Reports on Quartermaster Contract Projects, Massachusetts Institute of Technology, July 1, 1944 to June 30, 1945, pp. 248-273. Cooke, E. W. 1912. Dehydrated potatoes and process of preparing same. U. S. Patent No. 1,025,373. Cooley, A. M., Severson, D. E., Peightal, D. E., and Wagner, J. R. 1954. Studies on dehydrated potato granules. Food Technol. 8(5), 263-269. Edwards, C. S. 1845. Improvement in preserving potatoes. U. S. Patent No. 4,337. Erskine, H. L., Jr. 1950. Development of a slurry process for the production of dehydrated mashed potato granules. Master’s Thesis, Kansas State College of Agriculture and Applied Science. Fison, F. J. 1943. Improvements in or relating t o a’method of and a means for drying powdered or other more or less finely divided material. Brit. Patent No. 566,170. Gano, 0. 1940. Treatment of potatoes to produce dried mashed potatoes. U. S. Patent No. 2,190,063. Greene, J. W., Marburger, G. C., and Rohrman, F. A: 1947. Instant mashed potatoes from dehydrated granules. Food Znds. 19, 1622-1625, 1749-1751. Greene, J. W., Rohrman, F. A., Marburger, G. C., Honstead, W. H., Messenheimer, A. E., and Olson, B. E. 1948. Development of a potato granule process. Chem. Eng. Progr. 44(7), 547-552. Greene, J. W., Conrad, R. M., and Rohrman, F. A. 1949. Dehydration process for starchy vegetables, fruits and the like. U. S. Patent No. 2,490,431. Hall, R. C. 1951. Mechanisms of potato cell, rupture resulting from dehydration process, Master’s Thesis. Kansas State College of Agriculture and Applied Science. Hall, R. C. 1952. Cell-rupture closeups aid development of better dehydrated potatoes. Food Eng. 24(8), 91, 221. Hall, R. C. 1953. Better potato dehydrating by slow freezing. Food Eng. 26(3), 90-91, 150, 152. Hall, R. C., and Fryer, H. C. 1953. Consistency evaluation of dehydrated potato granules and directions for microscopic rupture count procedure. Food Technol. 7(9), 373-377. Heimerdinger, H. M. 1926. Food product. U. S. Patent No. 1,571,945. Heisler, E. G., Hunter, A. S., Woodward, C. F., Siciliano, J., and Treadway, R. H. 1953. Laboratory preparation of potato granules by solvent extraction. Food Technol. 7(8), 299-302.

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255

EIendel, C. E., Burr, H. K., and Boggs, Mildred h l . 1951. Factors affecting storage stability of potato granuIes. U. S. Bureau of Agr. and Ind. Chem., Agr. Research Admin., U. S. Dept. Agr. AIC 303, 8 pp. Jones, C. R., and Greer, E. N. 1940. Improvements in or relating to the reduction of potatoes and other starch-containing vegetables t o dry powder. Brit. Patent No. 537,669. Jones, C. R., and Greer, E. X. 1943. Mashed potato powder. 11. Hammer mill process. Great Britain Dept. of Scientific and Industrial Research and Ministry of Food Dehydration. United Kingdom Progr. Rept., Sec. VI, part 6 (11). Kaufman, C. W., Burgess, N. M., and Hollis, F., Jr. 1949. Process of preparing dehydrated mashed potato. U. S. Patent No. 2,481,122. King, J. 1948. Scientific problems in feeding a modern army in the field. Chemislry R: Industry (47), 739-743. Morris, T. N. 1947. “The Dehydration of Food,” Chapman and Hall, London. Mullins, W. R., Harrington, W. O., Olson, R. L., Wood, E. R., Nutting, M.-D. 1955. Estimation of free starch in potato granules and its relation to consistency of reconstituted product. Food Technol. I n press. Neel, G. H., Smith, G. S., Cole, M. W., Olson, R. L., Harrington, W. O., and Mullins, W. R. 1954. Drying problems in the add-back process for production of potato granules. Presented at 1953 Ann. Inst. Food Technol. Convention. Food Technol. in press. Nixon, E. L. 1944. Preservation of potatoes, fruit and vegetables. U. S. Patcnt No. 2,339,028. Olson, B. E. 1947. A study of the fundamentals of parallel flow drying in ducts. Master’s Thesis, Kansas State College of Agriculture and Applied Science. Olson, R. L., and Harrington, W. 0. 1951. Dehydrated mashed potatoes-A review. Bureau of Agr. and Ind. Chem., Agr. Research Admin. U. S. Dept. Agr. AIC 297, 23 pp. Olson, R. L., Harrington, W. O., Neel, G. H., Cole, M. mi.,and Mullins, W. R. 1953. Recent advances in potato granule technology. Presented a t 1952 Ann. Inst. Food Technol. Convention. Food Technol. 7(4), 177-181. Potato Corporation of Idaho. 1939. Treatment of potatoes to produce dried mashed potatoes. Brit. Patent 526,474. Potter, A. L., Jr. 1954. Changes in physical properties of starch in potatogranules during processing. Agr. and Food Chem. 2(10), 516-519. Poulton and Noel, Ltd., and Bostock, B. R. 1946. Improvements in or relating t o the dehydration of potatoes. Brit. Patent No. 589,830. Proctor, B. E., and Sluder, J. C. 1943. Dehydration of pre-cooked white potatoes in granular form. Research Reports on Quartermaster Contract Projects, Massachusetts Institute of Technology, July 1, 1942 t o June 30, 1943. pp. 140-151. Proctor, B. E., and Sluder, J. C. 1944. Dehydration of pre-cooked white potatoes in granular form. Research Reports on Quartermaster Contract Projects, Massachusetts Institute of Technology, July 1, 1943 to June 30, 1944. pp. 227-243. Remmers, B. 1918. Method of preparing pre-cooked food products. U. S. Patent hTo. 1,258,047. Rendle, T. 1943. Improvements in and relating to the preparation of cooked starchy vegetables in powder form. Brit. Patent No. 566,167. Rendle, T. 1945a. Preparation of cooked starchy vegetables in powder form. U. S. Patent No. 2,381,838. Rendle, T. 194513. The preservation of potatoes for human consumption. Chemistry & Industry (45), 354-359.

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Rivoche, E. J. 1948. Improvements in and relating to the drying of vegetables. Brit. Patent No. 601,151. Rivoche, E. J. 1950. Drying of starchy foodstuffs. U. S. Patent No. 2,520,891. Rivoche, E. J. 1951a. Method and technique of food drying. U. S. Patent No. 2,572,761. Rivoche, E. J. 1951b. Process of preserving moisture-containing cellular foodstuffs. U. S. Patent No. 2,572,762. Ross, A. F. 1948. Deterioration of processed potatoes. Advances in Food Research 1, 257-290. Salaman, R. N. 1949. “The History and Social Influence of the Potato,” Cambridge Univ. Press, England. Salaman, R. N. 1953. Potatoes as a crop and a n industrial raw material. The potato’s influence in shaping society. Chemistry & Industry (35), 907-912. Stamberg, 0. E., and Beresford, H. 1943. Potatoes baked, then dehydrated to avoid loss of product. Food. Inds. 16(9), 78-79. Stoddard, E. S. 1922. Precooked food. U. S. Patent No. 1,402,108. Tjomsland, Anne. 1950. The white potato. Ciba Symposia 2(6), 1254-1284. Treadway, R. H. 1954. Personal communication. Van Arsdel, W. B., Brown, A. H., and Lazar, M. E. 1947. Drying-rate nomographs. I. Riced white potatoes. Bureau of Agr. and Ind. Chem., Agr. Research Admin., U. S. Dept. of Agr., AIC 31-1 (revised). Volpertas, Z. 1937. Improvement in process and device for reducing vegetables containing starch t o dry powder. Brit. Patent No. 496,423. Volpertas, Z. 1939. Process for reducing vegetables containing starch to a dry powder. Brit. Patent No. 525,043. Volpertas, Z. 1944. Art of dried starch bearing food. U. S. Patent No. 2,352,670. Werts, M. 1947. They’ve lost their eyes. Kansas Agr. Student 23(3), 11. Willets, A. K., and Rendle, T. 1946. Improvements in and relating to the production of mashed potato powder. Brit. Patent No. 589,876. Willets, A. K., and Rendle, T. 1948. Production of mashed potato powder. U. S. Patent No. 2,439,119. Wood, Elizabeth R., Olson, R. L., and Nutting, M.-D. 1955. A method for the comparison of consistency in potato granule samples appraised at different times. Food Technol. 9(4), 164-168.

The Thermal Destruction of Vitamin B1 in Foods BY K . T. H . FARRER Research Laboratories. Kraft Foods Ltd., Melbourne. Australia Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 I1. Factors Influencing the Thermal Dcstruction of Vitamin B1. . . . . . . . . . . . . 253 1 . Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 3. 4 5. 6. 7.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolyte System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heavy Metals . . . .................................... Concentration of Electrolytes-the Salt Effect . . . . . . ...... Nonelectrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Form of the Vitamin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 . Concentration of Thiamine and Cocarboxylase . . . . . . . . . 10. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Losses in Cereals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Loss during Baking (or Toasting) of Bread . . . . . . . . . . . . . . . . . . . 2 . Stability of Vitamin BI from Different Sources during Baking . . . . . . 3 . Loss during the Cooking of Breakfast Cereals . . . . . . . . . . . . . . . . . . . . 4 . Effect of Various Baking Powders on Stability in Baked Products . . . 5 . Losses in Other Cereals.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Losses in Meats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Losses in Processing Vegetables . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Losses in Other Foodstuffs . . . . . . . . . . . . . . . . . . . . . 1. Dairy Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Peanuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Losses on Storage . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Unprocessed Foods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Canned Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Dehydrated Products . . . . . 4. Pharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

111.

I V. V. VI .

VII .

VIII .

259 259 259 260 260 261 261 262 262 263 261 264 268 269 271 274 275 285 293 293 293 294 294 294 296 299 301 303 306

I . INTRODUCTION I n the course of the last ten to fifteen years. a considerable amount of information concerning the thermal destruction of vitamin B1 during the 257

258

K. T. H. FARRER

cooking, processing, and storage of foodstuffs has been published. However, by far the greater part of these data has been concerned with the retention of the vitamin during the treatment of a particular foodstuff under certain specified conditions and, generally speaking, no attempt has been made to relate the results obtained t o any other foodstuff or any other set of conditions. During the same period, some work has been done to determine the fundamental principles underlying the thermal destruction of vitamin Bl* and several factors are now known to be involved. This paper aims: (1) t o survey the data available on factors influencing the thermal destruction of vitamin B1, and the thermal losses of vitamin B1 from foodstuffs; (2) t o show that the most reliable and satisfactory method of approaching vitamin B1 destruction is through simple reaction kinetics; (3) by means of the kinetic approach, to show: (a) that of the data available, much is worthless and the rest can be correlated and simplified, (b) that information available can be used through the first order reaction and the Arrhenius equation t o predict the behavior of vitamin Bl under specified conditions. The term "thermal destruction" is used t o differentiate this hydrolysis from the results of enzymic or microbiological activity, and covers losses of vitamin B1on storage as well as a t the higher temperatures associated with cooking or processing.

11. FACTORS INFLUENCING THE THERMAL DESTRUCTION OF VITAMINB1

I. Temperature Most workers have understood the importance of varying temperatures in considering the retention of vitamin B,, but very little has been done t o study quantitatively the effect of temperature from the point of view of reaction kinetics. Rice and Beuk (1945) systematically studied thiamine decomposition in pork a t different temperatures. Farrer and Morrison (1949) have shown that there is no deviation from the Arrhenius equation for thiamine destruction in buffer solutions a t temperatures between 50 and110" C. (122 and 230" F.). Farrer (1950, 1953a) has further used the Arrhenius equation for studies on processed cheese and yeast extract at storage temperatures, while Bendix et al. (1951) have also used kinetic methods for a study of the mechanism of thiamine destruction in peas, corn, Lima beans, and tomato juice from 104.5 to 132" C. (220 t o 270" F.). *For the purposes of this discussion "vitamin B," is considered to consist of thiamine, cocarboxylase, and "bound " thiamine.

THE THERMAL DESTRUCTION O F VITAMIN B1 I N FOODS

25'3

2. Time Early in the history of this work, Sherman and Burton (1926) showed t h a t destruction of vitamin B1 was greatly increased if the time of heating was increased from 1 t o 4 hr. The effect of time was clearly demonstrated by Farrer (1941), and much work since then has underlined the importance of this factor. Feaster et al. (1948) have shown clearly the interrelationship of time and temperature in thiamine retention in canned foods. 3. p H Here again early work pointed t o the importance of the p H (Sherman and Burton, 1926; Guha and Drummond, 1929), but i t was not until the thiochrome method (Jansen, 1936) for the determination of vitamin B1 became available that the full importance of p H was demonstrated (Farrer, 1941; Booth, 1943; Farrer, 1945,). Roy (1953) has emphasized the destructive effect of high p H in a study of losses from various samples of water used for cooking in Western Bengal.

4. Electrolyte System Beadle et al. (1943) first made the suggestion that the buffer system of the solutions under examination is important. This idea was greatly extended by Farrer (1945a) t o reconcile a variance between the results obtained earlier b y him (Farrer, 1941) and by Booth (Zoc. cit.). As a result of the study of thiamine destruction a t 100' C. (212" F.) in 4 buffer systems (citric acid-phosphate; phosphate; succinic acid-borate ; phosphate-borate) Farrer (1945a) believed that the rate of destruction of thiamine changes as the ionic constitution of the buffer changes with rising pH, and t h a t where the latter change is accompanied by a marked change in pH, there is a marked change in rate of destruction of thiamine. Similarly, where the pH is only slightly affected by the changing ionic constitution of the solution, there is a correspondingly slight change in the rate of thiamine decomposition. This close association of anionic constitution with p H in catalyzing the pseudo-unimolecular decomposition of thiamine has been underlined by subsequent work (quod vide) on various forms of thiamine under various conditions, and it is the author's belief that this same effect operates in the more complicated systems met with in footstuffs. I n the meantime, McIntire and Frost (1944) had claimed that a- and @-aminoacids decrease the rate of thiamine destruction. This fact, too, would be in accord with the recognition of the special part played by the electrolyte system as amino acids are well-known as buffer agents.

260

K. T. H. FARRER

I n 1944 some Japanese workers (Watanabe et al., 1944) published a study on the effect of acids on the stability of vitamin B1 solutions. This was not brought t o notice until 1951 (Chem. Abstr. 46,3566).' They used 0.001 M solutions of thiamine and 0.001 M-O.1 M concentrations of acids (hydrochloric, sulphuric, oxalic, maleic, acetic, and boric), and determined reaction velocities for thiamine destruction at 100" C. (212" F.). They found that all acids increased thiamine stability, although acetic and boric acids were less effective than the others. They appear t o have noted the general pH effect while overlooking the significance of the effect of the individual buffer anions. The same group (Watanabe et al., 1949) reported t ha t lithium, sodium, and potassium chlorides and potassium nitrate had no effect, and th at sodium sulphate had a slight accelerating action on thiamine destruction. Watanabe and Marui (1949) stated th a t although the decomposition was strongly accelerated b y sodium sulphite, borate, thiosulphate, acetate, carbonate, and monohydrogen phosphate as well as potassium dihydrogen phosphate, the effect was the same as that of alkali. Again, it would appear from the abstract th a t the significance of the individual buffer anions was overlooked. Ache and Ribeiro (1945) have claimed that sodium sulphite and chloride accelerate decomposition and Murphy and Goodyear (1949) report th a t potassium iodide is without effect. Much earlier, Escudero and de Alvarez Herrero (1942) had shown that potassium bromate (a bread improver) rapidly destroys thiamine in solution and in doughs. This is almost certainly a n oxidation effect. Similarly, Tavares and Rodrigues (1947), while showing that the stability of thiamine in pharmaceutical solutions is unaffected by sulphate, chloride, iodide, sodium, or magnesium ions, were emphatic that thiosulphate ions caused considerable destruction. This, too, is not a true ionic effect but, in this case, a reduction. 5. Heavy Metals

Farrer (1947a) has shown that heavy metals which can form complex anions with constituents of the medium can influence thiamine destruction very significantly. This is probably a special case of section 11, 4. 6. Concentration of Electrolytes-the

Salt Eflect I n certain circumstances, a t least, the concentration of the buffer salts can influence the rate of destruction of thiamine (Farrer, 1947b; 1949b) and cocarboxylase (Farrer, 1949b). Whereas almost all the papers cited in this review have been read in their original

form, most Japanese and some obscure foreign journals are unobtainable in Australia.

THE THERMAL DESTRUCTION

OF VITAMIN

~1

IN FOODS

261

7. Nonelectrolytes Melnick et al. (1941) suggested t h a t a t low temperatures thiamine is more stable in digestive juices than in synthetic solutions. Greenwood et al. (1943) have reported that both gelatin and egg albumin considerably retard the rate of aneurin destruction at 98" C. (208" F.). Atkin et al. (1943) also refer t o the protective effect of gelatin and albumin (at atmospheric temperature) and also of gums, dextrin, and soluble starch. Rice et al. (1943, 1944), too, have described the stabilizing effect of certain cereals on thiamine during the storage of dehydrated pork and, in addition, McIntire and Frost (1944) have described the effects of a number of nonelectrolytes (e.g. certain amines) on thiamine destruction. Watanabe and Sakaki (1944) studied the effect of sucrose, lactose, and glucose on thiamine destruction a t 110" C. (230" F.) and claimed t o show a slight accelerating effect. Watanabe (1951) used 0.1% of thiourea t o stabilize a 1% solution of thiamine, which is then claimed t o be stable for 1 hr. a t 100" C. (212" F.) or 30 min. a t 115" C. (221" F.). Terao and Kawaziri (1951) use methionine for a similar purpose and Watanabe et al. ( I 952) additionally reported that thioglycollic and thiolactic acids also stabilized thiamine solutions. Ache and Ribeiro (1945) have stated that glycine, xanthine, chloretone, and riboflavin are without effect on thiamine destruction, but that fructose, invertase, and inositol in 5 % solutions retard decomposition. Murphy and Goodyear (1949) claim that glucose and ethyl alcohol are without effect. An interesting report is that of Inagaki and Takeda (1950, 1951) whose figures suggest t h a t thiourea has a considerable protective effect on thiamine over a wide range of pH. No clear picture emerges from this brief survey, but it is apparent that nonelectrolytes cannot be ignored. 8. F o r m of the V i t a m i n

It is well-recognized that vitamin B1 can occur in three forms. Free thiamine and the pyrophosphate ester (cocarbouylase) are well-known ; the occurrence of protein-bound thiamine in milk (Houston et al., 1940) and cheese (Dearden et al., 1945; Evans et al., 1946 has also been described. Greenwood et al. (1943) stated that thiamine is less stable than cocarboxylase. However, Booth (1943), Lincoln et al. (1944), Farrer (1945b), and others later, agree t h a t cocarboxylase is a good deal less stable than thiamine. Kandutsch and Baumann (1953) have referred t o the apparently greater thermal stability of protein-bound thiamine in brewers' yeast and pig kidney a s added t o laboratory diets. Thiamine mononitrate is being used in the reinforcement of certain

262

I$ in. x 3/4 in.) were able t o obtain very rapid heating and cooling of

282

K . T. H. FARRER

the meat, and so studied the rate of destruction of vitamin B1 in pork a t temperatures of 99, 110, 118.5, and 126.5" C. (i.e. 210, 230, 245, and 260" F.). The rate constants, calculated from the data presented by the methods previously used by the present author (Farrer, 1945a), are tabulated (Table V). TABLEV Rate of Destruction of Vitamin BI ( k ) in Pork"

,, From

Temperature, ("C.)

k

99.0 (210" F.) 110.0 (230" F.) 118.5 (245' F.) 126.5 (260" F.)

,0025 ,0056 ,011 .022

Greenwood et al. (1944).

The value of k a t 99" C. (210" F.) compares very favorably with that calculated from Greenwood et al. (1943). The lower value found in this case is more likely t o be correct as it is based on observations at several different time intervals and, in addition, the earlier result is likely to be higher because of abnormally rapid destruction frequently met with in early stages of heating in thiamine solutions and of which there is evidence in the data of Greenwood et al. (1944). No account could be taken of this short-lived abnormality in calculating k from only one time interval, but in a series of readings, the correct destruction rate is readily assessed. Rice and Beuk (1945) determined the rate constant for thiamine destruction in pork a t 9 different temperatures from 49 t o 121" C. (120 to 250" F.). At 99" C. (210" F.), k , calculated from their data, is 0.0048. (The actual value tabulated by these authors is 80 X 10W from which it seems that they have taken t in seconds instead of minutes, as is usually the case in the application of the first order reaction equation.) This value is roughly double that of Greenwood et al. (1943). Rice and Beuk (loc. cit.) applied the Arrhenius equation t o their data which may be expressed as log, k = I - E / R T (where k = rate constant, T = temperature in degrees Absolute, and I , E and R are constants) and found that the curve log k 1/T was a straight line down t o 77" C . (171" F.). There are sufficient data in the paper of Greenwood et al. (1944) t o permit the application of the Arrhenius equation to this work also, and in Fig. 2, log k is plotted against l / T for the work of Rice and Beuk (the values of k are calculated from time in minutes, not seconds) and Greenwood et al. Two curves result,

T H E THERMAL DESTRUCTION O F VITAMIN B1 IN FOODS

283

from which it is apparent that vitamin B1 is destroyed more rapidly at each temperature in the work of Rice and Beuk than in that of Greenwood et al. The reason for this is not apparent. It is most unlikely that pH differences were such as t o cause the different values of k , but it could be t h a t the pork used by Rice and Beuk contained a higher proportion of cocarboxylase than that used by Greenwood et al. However, the two curves are approximately parallel, i.e. the temperature coefficient is approximately the same in each case, as would be expected.

Greenwood el 01. (1944) hlyrback & Vallin ( 1 9 4 4 ) x Rice & Beuk ( 1 9 4 5 )

0

-3.OL 0.00250

, 0.00260

I

0,00270

%\

0.00280

l/T' abs.

FIG.2. Effect of temperature on log k for thiamine in pork.

The only direct information permitting the application of this reasoning t o other meats is that supplied by Myrback and Vallin (1944) and Cover and Dilsaver (1917). Myrback and Vallin examined beef before and after processing for 2 hr. a t 115" C. (239" F.) and found a loss of vitamin B1 similar t o that found in pork. Cover and Dilsaver compared average values obtained for losses in stewing beef and lamb (12 samples of each) with losses obtained by Rice and Beuk (1945) in their examination of pork. Their table, however, records varying cooking times for cach meat, and the present author has plotted the logarithm of the percentage retention [in fact, log (a - .)] against time (Fig. 3) for the 3 temperatures mentioned, indicating which points are for pork, beef, and Iamb. As the curves must be linear, i t is very easy t o see that the values for pork and beef are in line a t each temperature (thus confirming Myrback and Vallin's finding), but that those for lamb are somewhat lower than would be anticipated. Nor is the explanation for this t o be found in the cooking procedures which are described by Cover et al. (1947) and

284

K. T. H. FARRER

which would, if anything, tend to favor a more rapid destruction in the lamb. This discrepancy also can most logically be attributed to the state of combination of the vitamin, as the variations in chemical composition, etc., from one type of meat to another are such as to make it very likely that observations on one type of meat are applicable t o another. If this is accepted, it simplifies very greatly the fundamental considerations underlying the thermal destruction of vitamin B1 in meats reducing them to t'ime, temperature, and state of the vitamin as, indeed, the other factors remain constant.

121°C. 1.6

I

0

60

I

I

120 180 Time, min.

240

FIG.3. log ( a - z ) / ' t curve for beef, pork, and lamb (from results of Rice and Beuk (1945);Cover and Dilsaver, 1947).

At least one attempt has been made to make use of the reported protective effect of starch on thiamine. Cover and Smith (1948) found a loss of 46% of the thiamine in a beef stew but only 8% from potatoes. When, however, they added potatoes to a beef stew and boiled for 15 min., they could find only the thiamine which could be calculated from the known behavior of the separate ingredients. This is scarcely surprising, as the meat had been pressure cooked for 17 min. a t 15 lbs. pressure before the addition of the potatoes. One may say, then, that most of the data available on losses on cooking meats are indefinite, lacking precise information regarding time and temperature, and taking no account of temperature gradients. The data which do permit a systematic study of vitamin B1 losses in meats point to a similar set of conditions in each case, with the main variable likely to be the proportion of cocarboxylase present. This generalization applies particularly to pork and beef, but less definitely to lamb.

THE THERMAL DESTRUCTION

OF VITAMIN

B~ IN FOODS

285

V. LOSSESIN PROCESSING VEGETABLES

A large amount of information on losses of vitamin B1 in vegetables during processing has been published. It relates both t o thermal destruction and t o leaching of the vitamin into the cooking water. Only the former is considered in this report, and it must be recorded t h a t not only is by far the greater part of the information useless so far as study of the fundamentals of thiamine destruction are concerned, but that many of the data are mutually contradictory. Available figures are summarized in Table VI. Only the most general conclusions can be shown regarding the effect of the different forms of processing. Boiling, steaming, simmering, i.e. any treatment a t or near 100" C. (212" F.) rarely causes a greater loss of thiamine than 20%. Much smaller losses are frequently found. Losses on canning are of the order of 40% where the conventional procedures are used. High-temperature, short-time methods, as are described by Feaster et al. (1948), will reduce such losses t o less than 20% as will agitation, leading t o shorter processing times a t normal temperatures. Pressure cooking gives results similar t o those obtained in canning, although McIntosh and Jones (1947) found no destruction (within experimental error) in 6 vegetables pressure cooked a t pressures of from 0 t o 20 p.s.i. The losses on baking are analogous t o those found on boiling, probably because the comparatively low internal temperatures rarely exceed 95" C. (203" F.). Stevens and Fenton (1951) examined the comparative effects of dielectric and stew-pan cooking of peas on thiamine retention and found no differences. The numerous exceptions t o these general conclusions may be att,ributahle to the following factors which do not seem t o have been taken into account: (1) Variations (of pH etc.) between varieties, and within the same variety. This is not a likely source of error because of the relative constancy of composition of vegetables, etc. ( 2 ) Variations in the size of the pieces cooked, leading to variations in (a) rate of heat penetration, (b) penetration of cooking water, (c) leaching away of vitamin from "native tissues" thus possibly making it more vulnerable, (d) leaching away of electrolytes, etc. (3) Variations in strength and permeability of cellulose envelope of peas, beans, etc., which must cause the same sort of variations as are listed above. Some support for the belief in the importance of these factors is found in the work of Ashikaga (1916) on soybeans and green peas, the report of which has only just become available.

286

K. T. H. FARRER

TABLEVI Loss of Vitamin BI on Processing Vegetables

Vegetable

Method of processing

TemTime peraof ture Loss proco€ of essing proc- thi(min- essing amine utes) ("C.) (%)

Reference

45 70 0.63 90 30 55 0.63 30 20

18 Fellers et al., 1940 36 Clifcorn and Heberlein, 194413 Nil Gleim et al., 1944 100 100 13, nil Hinman et al., 1944 100 2 1 , s Connolly et al., 1947 18 Aughey and Daniell, 100 1940 100 Nil Aughey and Daniell, 1940 100 59 Aughey and Daniell, 1940 Nil Aughey and Daniell, 100 1940 115.5 40 Fellers et al., 1940 115.5 38 Feaster et al., 1948 140 14 Feaster et al., 1948 . 204 9, 12 Hinman et al., 1944 100 8 Hinman et al., 1944 115.5 41 Feaster et al., 1948 140 22 Feaster et al., 1948 100 Nil, 12 Hinman et al., 1944 115.5 46 Guerrant et al., 1946

35

115.5

33 Guerrant et al., 1946

20

115.5

24

30

115.5 2 9 4 2

Beans, Lima

Canned, (No. 2) Boiled

25

100

22

Beans, Lima

Canned

45

116

38

Beans, Lima

Bottled

45

116

44

Beans, Lima Beans, Green

Canned Canned

-

-

-

-

78 38

Asparagus Asparagus

Canned Canned

23 14

Asparagus Asparagus Asparagus Beans, snap

Boiled Boiled Boiled Boiled, (PH 5.8) Boiled, (PH 5.7) Boiled, (PH 6.6) Boiled, (PH 6.0) Canned Canned Canned Baked Boiled Canned Canned Boiled Canned, (No. 2) Canned, (No. 10) Canned

8 30 15 40

Beans, snap Beans, snap Beans, snap Beans, Lima Beans, Lima puree Beans, Lima puree Beans, Lima puree Beans, green cut Beans, green cut Beans, green cut Beans, Lima Beans, Green, Wax, Lima Beans, Green, Wax, Lima Beans, Green Beans, Lima

85 40 53

115.5 120

Clifcorn and Heberlein, 1944b Clifcorn and Heberlein, 1944b Guerrant and O'Hara, 1953 Guerrant and O'Hara, 1953 Guerrant and O'Hara, 1953 Ingalls et al., 1950 Ingalls et al. 1950.

THE THERMAL DESTRUCTION OF VITAMIN B~ IN FOODS

287

TABLEVI ( C o n t i n u e d )

Method

of Vegetable Beans* Beans, snap Beans, green Beet (dehydrated) Beet (dehydrated) Beet Broccoli Broccoli Brussel Sprouts Cabbage Cabbage, dehydrated Cabbage, dehydrated Cabbage, fresh Cabbage, stored Carrots Carrots Carrots Carrots Carrots Carrots, dehydrated Carrots Carrots Carrots Carrots Carrots, puree Carrots, puree Cauliflower cauliflower Cauliflower Cauliflower Corn Corn Corn

processing Boiled Boiled Canned Boiled Steamed Boiled Boiled Pressure cooked Pressure cooked Pressure cooked Pressure cooked Steamed Boiled Boiled Boiled Pressure cooked Boiled Boiled Steamed Simmered Simmered Simmered Canned Boiled Canned Canned Boiled Boiled Steamed Canned Boiled Boiled Canned

TemTime peraof ture Loss of of processing proc- thi(min- essing amine Utes) ("C.) (%)

Reference

100 2 Connolly et al., 1947 100 41, 71 Lane et al., 1942 38 Ingalls et al., 1950 100 Nil Fenton et al., 1943 100 Nil Fenton et al., 1943 80 Lane et al., 1942 100 5 Oser et al., 1943 15 100 5-13 100-121 Nil Trefethen et al., 1951

20 25 25

20

100-121

-

-

10

-

25

Connolly et al., 1947

17 Lane et al., 1942 Nil

Fenton et al., 1943

Nil Fenton et at., 1943 27 Connolly el al., 1947 5 Connolly et al., 1947 Nil Aughey and Daniell, 1940 Nil Aughey and Daniell, 121 13 1940 33 Lane ec al., 1942 100 30 5 Hinman et al., 1944 100 18 Gleim et al., 1944 15 100 10 Gleim et al., 1946 30 25 37 Gleim et al., 1946 -~ 20 Nil Gleim et al., 1946 115.5 Nil Guerrant et al., 1946 25 15-20 100 Nil Connolly et al., 1947 23 Feaster et al., 1948 115.5 60 0.63 140 13 Feaster et al., 1948 100 15 48 Connolly et al., 1947 12>< 100 13 Lunde el al., 1940 1235 100 17 Lunde et al., 1940 10.5 Lunde et at., 1940 110 10 Nil Barnes et al., 1943 100 10 100 22 Hinman et al., 1944 30 40-50 115-120 75-66 Guerrant et al., 1946

10 20 25 23

-

~~

288

K. T. H. FARRER

TABLEVI (Continued)

Vegetable Corn Corn Corn, cream style Corn, cream style Corn, cream style Kale Onion Onion Parsnip Parsnip Peas Peas Peas, sugar Peas, sugar Peas, sugar Peas, split Peas, split Peas, split Peas, green Peas, green Peas, green Peas, green Peas, green Peas, dried Peas, fresh Peas. frozen Peas. frozen Peas, frozen Peas, frozen Peas, frozen

Method of processing

TemTime peraof ture Loss of of processing proc- thi(min- essing amine utes) ("(3.1 (%)

10 Boiled Canned Canned 70 Canned 85 Canned 0.63 Boiled 30 Boiled 15 Boiled Boiled 10 Boiled 15 Roiled 20 Canned 35 Boiled 1235 Steamed 1234 Canned 10 Boiled 30 Boiled 15 120 min. then baked 1 Pressure cooked Canned 15 Boiled, 12 (PH 6.4) Boiled, 12 (PH 7.4) Boiled 20 Boiled 5 Boiled Boiled 8 Simmered, 6 (PH 7.7) Simmered, 4 (PH 8.8) Simmered, 6 (PH 7.7) Simmered, 4 (PH 8.7) Pressure 9 cooked

100

-

115.5 115. 5 140 100 100 100 100 100 100 115.5 100 100 110 100 375

Nil 62 68 64 2-3 Nil 11 27 3 3-11 30 40 4 16 Nil 50 27

121

32

119 100

9 9

100

22

100 100 100 100 100

13.2 2.2 48 Nil Nil

Reference Connolly et al., 1947 Ingalls et al., 1950 Feaster el al., 1948 Feaster el al., 1948 Feaster e l al., 1948 Connolly et al., 1947 Connolly et al., 1947 Lane et al., 1942 Connolly et al., 1947 Connolly et al., 1947 Connolly et aZ., 1947 Fellers et al., 1940 Lunde et al., 1940 Lunde et nl., 1940 Lunde et al., 1940 Murray, 1948 Murray, 1948 Murray, 1948 Lunde et al., 1940 Aughey and Daniell, 1940 Aughey and Daniell, 1940 Ashikaga, 1946 Ashikaga, 1946 Lane et al., 1942 Barnes et al., 1943 Johnston et aZ., 1943

100

8 . 5 Johnston et al., 1943

100

3 Johnston et al., 1943

100

38 Johnston et al., 1943

100-121

2-7

Trefcthen and Fenton, 1951

T H E THERMAL DESTRUCTION OF VITAMIN B1 I N FOODS

289

TABLEVI (Continued) Temperaof ture Loss of of processing proc- thi(min- essing amine Utes) ("C.) (%) Time

Vegetable l'cas, fresh

hlethod of processing

Peas " Peas, Alaska

Simmered, (PH 7.3) Simmered, (PH 8.8) Boiled, (pH 7.5) Boiled, (PH 9.4) Boiled Canned

Peas, sweet Peas, sweet Peas, sweet

Peas, fresh Peas, fresh

Peas, fresh

17

100

Nil

Johnston et al.. 1943

8

100

Nil

Johnston et al., 1943

-

45 Johnston et al., 1943 -

60 Johnston et al., 1943

15 35

100 115.5

6 37

Canned Canned

35-40 35

115.5 115.5

39 33

Canned

45

120.5

40

35 35 70 0.63 25

120 42 120 35 115.5 21-37 13-14 140 46-50 7 100

Peas, sweet Canned Peas, sweet (immature) Canned Peas, puree Canned Peas, puree Canned Peas, puree Canned Peas, green Boiled

Reference

Peas, green

Canned

35

116

45

Pens, grern

Bottled

35

116

34

Potato Potato Potato, whole

Boiled Boiled Boiled

15-20 15 36

100

21 8 20

Potato, whole

Baked

63

190

16

Potato" Potato, dehydrated Potato, dehydrated Potato * Potato, Green Mountain Potato, Green Mountain

Boiled Boiled Steamed Steamed Steamed Steamed

15 15 20 60 90

-

-

107 70

19 Nil Nil 4 15.5 7.2

Oser et al., 1943 Clifcorn and Heberlein, 1914b Guerrant et ul., 1946 Clifcorn and Heberlein, 194413 Clifcorn and Heberlein, 1944b Heberlein et al., 1950 Heberlein et al., 1950 Feaster et al., 1948 Feaster et al., 1948 Ingalls et al., 1950 Guerrant and O'Hara, 1953 Gurrrarit and O'Hara, 1953 Gucrrarit and O'Hara, 19.53 Connolly et al., 1947 Cover and Smith, 1948 Aughey and Daniell, 1910 Aughey and Daniell, 1940 Lane et al., 1942 Fenton ez al., 1943 Fenton et al., 1913 Oser et al., 1943 Wertz and Weir, 1914 Wertz and Weir, 1944

290

K. T. H. FARRER

TABLE VI (Continued)

Vegetable

Method of processing

TemTime peraof ture Loss of of processing proc- thi(min- essing amine utes) ("C.) (%)

Pumpkin, fresh Pumpkin, stored Rutabagas, dehydrated Rutabagas, dehydrated Spinach

Boiled Boiled Boiled Steamed Boiled

15 20 25 45 7

100 100 100 100

Nil 49 Nil Nil 22

Spinach Spinach Spinach Spinach Spinach Spinach Soybeans

5 5 8 30 45-60 60

100 100 100 100 100 122 121

40 Nil Nil Nil Nil 75 74

Squash Sweet potatoes Sweet potatoes

Boiled Boiled Steamed Boiled Boiled Canned Pressure cooked Boiled Baked Baked

10

100

Sweet potatoes

Boiled

Taro

Commercial > 4 hr. steaming 121 45 Pressure cooked 35 Boiled 60 Steamed 30 Boiled 30 Boiled 40-60 Canned 110-130 Canned Boiled 15-20 Boiled

Taro Taro Taro, leaves Tomatoes Tomatoes Tomatoes Tomatoes Turnips Turnips

* Variety not given.

-

-

Reference Connolly et al., 1947 Connolly et aZ., 1947 Fenton et al., 1943 Fenton et al., 1943 Aughey and Daniell, 1940 Lane et al., 1942 Cutlar et al., 1944 Cutlar et al., 1944 Gleim et aZ., 1944 Hinman et al., 1944 Guerrant et al., 1946 Miller, 1945

11 Connolly et al., 1947 75 Lane et al., 1942 25 Pearson and Luecke, 1945 8 Pearson and Luecke, 1945 40-60 Miller et al., 1952

20-40 11 20 29 6 Nil Nil 5.5 40

Miller ef al., 1952 Miller et al., 1952 Miller et aZ., 1952 Hinman et aZ., 1944 Guerrant et al., 1946 Guerrant et al., 1946 Lane et aZ., 1942 Connolly et al., 1947

THE T H E R M A L DE S T RUCT ION O F VITA MI N B1 I N FOODS

291

Only a few of the many papers appearing have made any attempt t o consider fundamental conditions. Jackson et al. (1945), in a discussion of the effect of canning procedures on the retention of vitamins in canned products, included in their survey a consideration of the effect of the thermal processing on vitamin B1. They were able t o shorn that, in commercial practice where the first essential is t o obtain the highest degree of bacterial mortality, retention of vitamin B1,other things being equal, is favored by higher temperatures for shorter times rather than by lower temperatures for longer times. They point out that this does not hold for slow-heating products such as minced meats, but applies under conditions which permit more rapid heating of the mass of the product, viz. fruit and vegetable packs. Feaster et al. (1948) have added results obtained by the conventional canning [55-70 min. a t 115.5" C. (240" F.)] and flash sterilization [0.63 min. a t 139" C. (282" F.)] of pea, carrot, green Lima bean, and green bean purees. The average loss of thiamine in the conventional procedure was 32%. I n the high-temperature, short-time process i t was 15%. Similarly, in comparing the conventional method for corn [60 min. a t 121" C. (250" F.)] with the agitated-can technique [17 min. a t 121" C. (250" F.)] losses were much lower, as would be expected over the shorter time. Bendix et al. (1951) have discussed the factors influencing the stability of thiamine during heat sterilization. They used kinetic procedures in a study of thiamine destruction in peas, corn, tomato juice, and Lima bean puree over the temperature range 104.5 t o 132" C. (220 t o 270" F.). I n peas, the first order reaction curves are linear and the rate constant k , was calculated for 104.5, 118, 126.5, and 132" C. (220, 245, 260, and 270" F.). A straight line for the graph log k 1

ToAbs. showed that the Arrhenius equation was valid for thiamine destruction in peas. With the other vegetables these authors found a sharp initial drop in the log (a - z ) / t curve over the first 10-20 min., followed b y a much more gradual linear relation. Booth (1943) and the present author have sometimes encountered this in thiamine solutions buffered with inorganic buffers, and there appears t o be no obvious explanation for it. Experience has shown that the second part of the curve yields values of k which are comparable with those obtained when no such inflected curves are met with. However, Bendix et a1 (1951), even using log k calculated in this way, could not satisfy the Arrhenius equation for thiamine in the other

292

K . T. H . FARRER

three products, nor could they obtain a satisfactory lead from the study of mixtures buffered with either phosphate or citrate. They rightly conclude "that the rate and nature of thiamine destruction in foodstuffs is governed by the combined influence of numerous factors," and go on to say t ha t high-temperature, short-time processing will favor thiamine retention. This, of course, follows from the results recorded by Feaster et al. (1948). Farrer (1953b) has, in some preliminary experiments, obtained the following rate constants for thiamine destruction in purees a t 100" C. (212" F.): peas, 0.0021; carrots, 0.0022; cabbage, 0.0027; potatoes, 0.0020 and 0.0032. No difficulties such as Bendix et al. (1951) report were encountered, but the gel formed by the potato starch gave rise t o temperature gradients and, therefore, doubtful results. Basu and Malakar (1946) attributed variations in the destruction of thiamine in different Indian vegetables t o variations in pH, electrolyte systems, and possibly protein systems in the tissues. At the same time, they presented figures t o show that there was concurrently virtually no loss of cocarboxylase in the same vegetables, whereas thiamine losses ranged from 50 to 100%. This is completely a t variance with studies in buffer solutions (Farrer, 1915b), cereals (Lincoln et al., 1914), and meat (Myrback and Vallin, 1944) which show cocarboxylase t o be more vulnerable than thiamine. Kohman and Rugala (1949) have considered the enzymic production of thiamine in sweet potatoes during cooking, and conclude th a t the temperature optimum for this enzymic process is probably as low as 40" C. (101" F.). They refer t o a loss of thiamine with continued heating a t relatively low temperatures, but their use of the fermentometric assay limits the value of the figures presented. Ashikaga (1951) has examined the behavior of thiamine during the cooking of 10 vegetables. He noted leaching of the vitamins into the cooking water and the relative losses during steaming (smallest loss), boiling, parching, frying, and baking (largest loss, 25 %). Loss was much influenced by time and temperature of cooking. These observations are in accord with those made by a number of authors. The present author believes that the conditions which govern thiamine destruction will be found t o be very similar in all types of vegetables. If this is so, i t will greatly facilitate the prediction of thiamine losses during processing and cooking. As has already been pointed o u t by Elvehjem and Pavcek (1943), it cannot be too greatly emphasized th a t full details of conditions used should always be included with the results of thiamine retention studies. Even so, more specific information can only be expected when "homogeneous" purees are used for proper kinetic studies, as the

THE THERMAL DESTRUCTION

OF VITAMIN

B~ IN FOODS

293

misleading effects of temperature gradients, size of vegetable portions, etc., are only too evident in the literature.

VI. THERMAL LOSSESIN OTHER FOODSTUFFS I . Dairy Products The loss of thiamine on the heat treatment of milk has been studied to some extent. Weckel (1938) reported loss of one-third of the thiamine in milk during commercial sterilization. Houston et al. (1940) state that 10% is lost during pasteurization, and up t o 50% on sterilization. Kon (1941) gives the following percentage losses during the processing of milk : Pasteurizing 10% Sterilizing 30 % Spray drying 10% Roller drying 15 % Condensing 40 % The figure quoted by Elvehjem (1941) for loss on pasteurization is 20%, but Holmes et al. (1943) agree with the earlier workers in quoting a loss of 9 % on pasteurization. Hitherto, attention was directed towards holding pasteurization, but Holmes et al. (1945) found a loss of not more than 3 yo during H.T.S.T. (high-temperature, short-time) pasteurization. Randoin and Perroteau (1950) have reported a loss of 23 % on pasteurization of human milk for 20 min. a t 65" C. (149' F.) and van der Mijll Dekker and Engel (1952) record a 14-30% loss of vitamin B1 on sterilizing milk, which is in general agreement with Kon (1941). Dearden and co-workers (1945) found t h a t 6-20% of the thiamine in the milk could not be accounted for in the curd and whey during the manufacture of Cheddar, Cheshire, and Stilton cheese in the winter months. I n the summer this loss was 6-7% for Stilton and vanishingly small for Cheddar and Cheshire. No further losses occurred during ripening up t o 42 weeks. The present author (Farrer, unpublished data) has shown that there is no loss of thiamine during the processing of cheese. 2. Peanuts

Pickett (1944) examined 60 samples of peanut butter and related lower thiamine contents t o the dark color which indicates more severe roasting conditions. When the temperature of the nuts reaches 147" C. (300" F.), large percentages of thiamine are lost. Fournier et al. (1949) showed that losses on roasting for 20 min. a t

294

K. T. H. FAHRER

155-160' C. (311-320" F.) were over 90%. A log (a - z)/time curve from tabulated results shows clearly the lag in thiamine destruction as the peanuts heated up over the first 10 min., but thereafter the destruction was equivalent to a rate of k = 0.152, i.e. a loss of well over 50% in 70 min. A cooling gradient is also apparent from the loss of more thiamine (about one-third of th at remaining) after roasting has ceased and while the nuts cool. Storage losses were also shown. French et al. (1951) and Willich et al. (1952) have also reported large losses of thiamine during the roasting of peanuts, the latter confirming Pickett's correlation of darkest color with greatest loss. 3. Miscellaneous

Stamberg and Peterson (1946) found an average thiamine loss of 15% on cooking eggs by any one of a number of ways. Farrer (1946) has shown th at no thiamine is lost during the autolysis of yeast for up to 24 hr. a t 50" C. (122" F.). Further work by the same author (Farrer, unpublished data) has shown that it is possible t o make yeast extract commercially without thermal loss. Farrer (19534 has further recorded the effect of pH on the rate of destruction of thiamine in yeast extract solutions a t 100" C. (212" F.). VII. THERMAL LOSSESO N STORAGE The storage of food for varying periods is commonplace. Fresh, unprocessed foodstuffs, e.g. meat, fruit, vegetables, and cheese, are sometimes held for months a t a time a t freezing temperatures, and canned products can be stored for years. I n the former, enzyme and microbiological activity still proceeds and can cause changes in vitamin potencies. Furthermore, vitamin losses can occur by release of fluids during the thawing process prior to cooking or processing. It is not with these losses th a t this discussion is primarily concerned, but, because some of them are undoubtedly thermal, some account must be given of them. I . Unprocessed Foods

Those who have studied the behavior of thiamine during storage a t low temperatures generally agree th at losses either do not occur, or are very small. Cook et al. (1949) found no significant loss from turkey tissues in 3 t o 9 months a t -23" C. (-9" F.). Morgan et al. (1949) reported similarly for chicken held frozen for 8 months. Hartzler et al. (1919) stored 16 samples of various pork cuts for 5-9 months a t -21 t o -23" C. (- 6 t o - 9" F.) and found a very slight loss of thiamine from shoulder and loin, but no loss from liver. Westerman et al. (1952a) studied the effect of storage time u p t o 72 weeks and temperatures from -29 t o

295 ' - 12" C. (-20 t o 10" F.) on thiamine in pork and concluded from their statistical analysis of the results that neither factor had any significant effect on the thiamine content. The same authors (Westerman et al., 195213) showed that there was no correlation between storage time and temperature, and the subsequent loss of thiamine on roasting. Results rather less satisfactory were obtained by Lehrer et al. (1951) who claim losses of 21% and 40% after the frozen storage of pork chops for 3 and 4 months, respectively. Chops quick frozen at -26" C. ( - 15" F.) or held a t - 18" C. (0" F.) gave substantially the same results. A similar study on lamb chops (Lehrer et al., 1952) showed larger amounts of thiamine in those held a t -26' C. (-15" F.) for 48 hr. than in those held a t -28" C. (-18" F.). There was no difference in 3 t o 6 months between those held a t the 2 different temperatures, although it was claimed that there was a loss of 1.2 pg per gram of tissue u p t o 3 months. Rice et al. (1946) showed very good retention of thiamine in pork a t 4" C. (39" F.) but much higher losses a t room temperature, accompanied by putrefaction. It is apparent t h a t in frozen storage there is a balance between three processes : (1) Slow "thermal" destruction over a long period of time. This is scarcely perceptible. (2) Thiamine destruction by enzyme and microbiological activity. (3) Thiamine synthesis by enzyme and microbiological activity. These same factors are possibly important during the storage of unprocessed foods a t atmospheric temperatures, although, under these conditions, the thermal destruction will assume greater importance. Connolly et al. (1947) studied the cool storage of certain raw vegetables for various periods from 100 t o 300 days, and obtained confusing results which nevertheless led them t o conclude that storage did not decrease the thiamine contents appreciably. Even cucurbits stored a t 14.5-19" C. (58-66" F.) for 21 weeks gave contradictory results. Ingalls et al. (1950) were concerned with the short storage of vegetables (corn, spinach, and legumes) in the shade (up t o 48 hr.), cold (up t o 120 hr.), and iced (up t o 120 hr.) prior t o canning, and their results showed that, under the conditions studied, either the precanning loss was very small, or a n apparent gain occurred. Carrots were subjected t o longer storage periods of up t o 90 days and losses of 63% (held out of doors) and 44% (cold storage) were recorded. It is obvious that under these conditions enzyme activity must also play a part. Ashikaga and Chachin (1951) report a fairly rapid loss of thiamine from orange and radish juices (17 and 22%, respectively) after 24 hr. a t 9" C . (48" F.). At 10" C. (50" F.) grated radish lost 27%, and a t 14" C. (5'7" F.) grated sweet potato lost 14.5% in the same time. These losses are T H E T H E R M S L DESTRUCTION O F VITAMIN B1 I N FOODS

296

K. T. H. FARRER 8

greater than would be normally expected, and the influence of oxygen or enzymes either separately or together seems t o be the likely explanation for the high figures, especially as the authors report that the losses from radish and sweet potato were greater when the samples were chopped finely than when chopped coarsely. According t o Caileau et al. (1945) brown rice, bran, and polishings held for 6 months a t a n average room temperature of 20" C. (68" F.), lost from 0 t o 30% of thiamine. After 24 months, the loss was 50-67%. Canned and parboiled brown and parboiled undermilled rice showed little or no loss over 6 and 3 months, respectively. Kik (1945) after storing rice a t high room temperatures for u p t o 245 years, reported a n average loss in 3 months of 5-7oJo; after 9 months, the loss was 10-15%. The loss after 235 years storage in the dark was about 30%. I n the same period, rice stored a t -10" C. (14" F.) lost only about 1% of its thiamine. Pelshenke and Schulz (1951) found only small losses from bread after storage for six months. Pearce (1943) found no loss from wheat germ after 6 months storage a t 15.5" C. (60" F.). On the other hand, Bayfield and O'Donnell (1945) found losses of 32% in 5 months and 40% in 7 months from wheat stored a t atmospheric temperatures. 2. Canned Products

Guerrant et al. (1945, 1948) have studied losses of thiamine from fruit and vegetable products a t temperatures of from -1" t o 43.5" C . (30 t o 110" F.) for periods of up t o 2 years. They found very limited losses a t the two lowest temperatures, - 1 and 5.5" C. (30 and 42" F.), but very definite losses a t the higher temperatures. Clifcorn and Heberlein (1944b) obtained results a t room temperature on asparagus, Lima beans, corn, and peas which, in some cases, were contradictory. Moschette et al. (1947) found loss of thiamine from canned fruits and fruit juices a t 26.5" C. (SO" F.), but not a t 18.5" C. (65" F.) or less. They were concerned with warehouse storage and considered temperatures likely t o be encountered in practice. They found losses, after 1 year a t 26.5" C. (80" F.), of from 9 % with citrus fruits and fruit juices, t o 19% for peaches. Pineapples and tomatoes were intermediate. Sheft and her co-workers (1949) carried out a similar study over 18 t o 24 months with a number of canned fruits and fruit juices (citrus, pineapple, tomatoes, peaches). Losses in 2 years a t 10 t o 18.5" C. (50 t o 65" F.) did not exceed 13%; a t 26.5" C. (80" F.) they were higher. The authors state that losses of thiamine seemed to depend more on temperature than time of storage. This is hardly in accord with the Arrhenius equation which relates temperature with rate of loss. Feaster et al. (1949) have recorded results for

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tomato juice in warehouse stacks. Losses were very small a t 4 and 21" C. (39 and 70" F.)) and it was concluded that thiamine is not seriously affected during stack cooling. Cover et al. (1949) showed high losses from canned beef and veal in 3 months a t summer temperatures [35 t o 36.5" C. (95 t o 98' F.)]. None of these investigations were kinetic studies, nor do the results permit any such calculations t o be made. They are therefore of limited value. Although they cannot be used for purposes of strict comparison or prediction of thiamine behavior, they do have a cumulative effect in showing the trend of thiamine stability in stored canned products, especially fruit and vegetables. Freed et al. (1949) have presented nomographs for thiamine retention in canned apricots, green beans, Lima beans, tomato juice, peas, and orange juice. They are based on data obtained on these products over 18 months storage a t 21, 32, and 38" C. (70, 90, and 100" F.) and were used t o compare the results obtained by Guerrant et al. (1945) and Moschette et al. (1947) for the same products, with predictions obtained from the nomographs. These comparisons were very promising, and there is no doubt that these nomographs are valuable in predicting the storage life of thiamine in the products studied and in other similar ones. Storage temperatures of 29.5" C. (85" F.) for tomato juice and peas, and 35' C. (95" F.) for orange juice led t o losses not greater than 20%, whereas the temperatures required for the same retention in Lima beans, apricots, and green beans were 18, -9, and -2" C. (64, 16, and 28" F.), respectively. The following table is taken from Freed et al. (1949) : Retention for 12 months storage at

38" c. (1000 F.) 1.5" C. (35' F.) %7

Apricots Grcen beans Lima beans Tomato juice Peas Orange juice

35

8 48 60

68 78

70 72 76 92 100 100 100

Although the paper of Freed et al. is very valuable and is soundly based on kinetic considerations, i t still leaves unexplored the factors affecting the thermal stability of thiamine. It does, however, permit a comparison with the results obtained by Guerrant et al. (1945) and Moschette et al. (1947). Farrer (1950) approached the problem in a different way by suggesting that the results of storage studies should be used t o calculate the rate

298

K. T. H. FARRER

constant, k,, where time is measured in weeks, not minutes. ( k , very nearly equals k X lo4.) The advantages ascribed t o this are: (1) the ability t o compare quickly and conveniently results obtained in different laboratories, and (2) the means of predicting, through the Arrhenius equation, the life of the thiamine under given conditions of temperature and time. I n assessing the latter conditions, the paper of Monroe et al. (1949) is of considerable assistance. Brenner et al. (1948b) have published thiamine retention curves for the storage of certain canned fruit and vegetable products. They make the general claim that 60% of the thiamine, on the average, is retained in these products after storage for 6 months at 38" C. (100" F.) or for 12 months a t 32" C. (90" F.). Farrer (Zoc. cit.) showed th a t their results could be treated kinetically, and presented rate constants for thiamine destruction in the various products and Arrhenius curves for apricots and orange juice. I n the same way, Farrer demonstrated the identity of the results of Feaster et al. (1946) and Rice and Robinson (1944) for thiamine losses on storage of canned pork. Farrer's own results give rate constants and Arrhenius equations for thiamine destruction in processed cheese (Farrer, 1950) and, more recently (Farrer, 1953a), in yeast extract as well. I n the latter paper the behavior of the mononitrate is compared with th a t of the naturally occurring thiamine. A similar approach to storage losses from dehydrated products was also found t o be possible. A further example of the usefulness of the kinetic approach may be found in the results of Guerrant and O'Hara (1953). They summarized many data obtained on the storage a t 20-29.5" C. (68-85" F.) of canned and bottled peas and Lima beans. If the results are examined by kinetic methods, i t is seen, firstly, that some sets show the initial unexplained rapid rate of destruction which has already been referred to, secondly, that the rate of destruction was the same as far as can be determined, in all samples of peas and in all samples of beans, and thirdly, that destruction was faster in beans than in peas. Approximate values of k, are 0.0049 for peas and 0.015 for Lima beans. It is possible now t o compare these results with those of Brenner et al. (1948b), from which Farrer (1950) calculated k , values of 0.013 [at 38" C. (100" F.)] and 0.0097 [at 32" C. (90" F.)] for beans. It is apparent that Guerrant and O'Hara's results for peas are of approximately the same order, having regard t o the lower temperatures, but are quite different for Lima beans. There is no doubt that the suggestions of both Freed et al. and Farrer are far better than the unrelated results which have appeared in the literature hitherto. The nomographs of Freed et al. are possibly simpler, whereas Farrer's approach is possibly more precise. Both, however, are based on fundamental kinetic considerations.

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Fournier et aE. (1949) have given results for losses of thiamine on storage of peanut butter a t temperatures ranging from -29 to 54" C. (-20 to 129" F.). At -29 and ' 4 C. (-20 and 39" F.). There was no loss in 5 months, but losses at 18, 27, 37, and 54" C. (64, 81, 99, and 129" F.) were significant and can be treated kinetically. The present author has calculated Ic, for thiamine destruction a t each temperature, and from the values obtained the Arrhenius curve (Fig. 4) has been drawn.

-2.5

I

I

0.00300

I

I

I

0.00320 1IT" abs.

I

0.00340

FIG.4. Arrhenius equation curve for loss of thiamine on storage of peanut butter at different temperature (from data of Fournier et al., 1949).

It is clear that the pattern of thiamine destruction is the same in different samples of the same product, and there is good reason for believing that it is very similar within groups of products. 3 . Dehydrated Products

Schultz and Knott (1939) and Knott (1942) have studied the storage loss of thiamine from evaporated milk. They show considerable losses over a period of some months. Knott (1942) claims that destruction on storage eventually reaches equilibrium, and that the rate of destruction varies with each lot of milk. By applying simple kinetics to the values given, it can be shown that neither statement is true. Photography and projection onto graph paper were used by the present author to measure the position of the points in Knott's curves for thiamine destruction. The rate of destruction k,, (Farrer, 1950) (where time is measured in weeks) was determined, and for 2 curves of 4 points each the values obtained were 0.0172 and 0.0205. Applying the same method t o Schultz and

300

K. T. H. FARRER

Knott's (1939) values gave 0.0182 and 0.0209 for 2 other milks, whereas another sample recorded by Knott (1942) gave the value, at one time interval only, of 0.0204. One other sample gave anomalous results, the same amount of thiamine remaining after two different time intervals and corresponding with k , values of either 0.044 or 0.097. However, it is clear that equilibrium is never reached as a simple time retention curve must be logarithmic, and that the rate of destruction of thiamine is, generally speaking, the same in each sample, as would of course be expected in products of similar composition. I n spray-dried whole egg, Klose ef al. (1913) found virtually no loss of thiamine a t -9.5" C. (15" F.) in 9 months. At 21" C. (70" F.) only 14% was lost in 3 months, but 48% in 9 months. At 37" C. (99" F.) the losses were 25, 32, and SO%, respectively, a t 3, 6, and 9 months. Olsen et al. (1948) showed that in spray-dried whole egg the thiamine retained after 20 and 57 weeks' storage a t 37" C. (98.5" F.), varied inversely with the moisture content in the range 1% t o 6%. A kinetic interpretation of their results suggests that at all moisture levels studied the rate of destruction is the same from 20 to 57 weeks and that the rates vary in the initial stages. This change of rate constant has been observed in buffer solutions and other food products. Tressler et al. (1943) report no loss of thiamine from dehydrated vegetables (rutabagas, beets, cabbage, and potatoes) on storage under air or carbon dioxide, or in cellophane or pliofilm bags for 3 to 4 months a t -40, 0.5, 14.5, and 24" C. (-40, 33, 58 and 75" F.) The Research Staff of the Continental Can Co. (1944, 1945) have published figures for similar samples incubated for longer times a t higher temperatures [up to 54.5" C. (130" F.)]. Losses up to 76% in 3 months a t 54.5" C. (130" F.) were found, and the effects of time and temperature were clearly shown. Rice et al. (1944) made a thorough study of the destruction of thiamine in dehydrated pork a t -29,3,27,37,49, and 63" C. (-20,37,81,98,120, and 145" F.). There was no detectable loss in 3 weeks a t the 2 lowest temperatures but Farrer (1950) showed that the results for the 4 highest temperatures permitted a kinetic study which, conforming with the Arrhenius equation, gave a clear-cut relationship between rate of destruction and temperature. Nymon and Gortner (1948) in a similar study, investigated the effect of temperature, incorporation of soya flakes, container, and moisture content on the retention of thiamine in dehydrated pork loaves. Farrer has also treated their results kinetically giving quantitative expression t o Nymon and Gortner's conclusions; via. that soya flakes retard thiamine destruction, and th at the loss of thiamine from dehydrated pork, with and without soya flakes, is faster in moistureproof packages than in cellophane. Nymon and Gortner attribute this t o

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further loss of moisture through the cellophane, and this agrees with the emphasis laid by Rice et al. on the importance of the moisture content of dehydrated pork on the stability of thiamine. The importance of moisture content has also been emphasized by Hollenbeck and Obermeyer (1952) who worked with enriched flour. Storage was carried out for 4 months a t 38" C. (100" F.) and a t room temperature. Losses of thiamine hydrochloride were 40 and 27 %, respectively, but not more than 5% of thiamine mononitrate in flour of 14.5% moisture. Increasing moisture content favored destruction of thiamine hydrochloride which the authors attributed t o its greater hygroscopicity. Kandutsch and Baumann (1953) have studied the storage loss of thiamine from laboratory rations and confirmed that it follows a first order reaction. Losses in basal diets amounted t o about 70% in 1 week a t room temperature and attempts were made to modify the rations t o avoid these losses. They were reduced by removal of fat or salts, by the addition of antioxidants, such as ascorbic acid, or by storage under nitrogen. However, desiccation in air gave greater protection than storage under nitrogen, and desiccation in vacuo gave complete stability for 2 weeks a t room temperature. Thiamine was completely stable when mixed with sucrose, or with sucrose and other vitamins. Kandutsch and Baumann argued that destruction occurred in the aqueous film on the ration particles and that it was primarily a n oxidation. The formation of a film of glycerol in the particles successfully prevented losses. Waibel et al. (1954) in a similar study found dipotassium hydrogen phosphate t o be the ingredient most responsible for destruction. Calcium carbonate and manganese sulphate also contributed. Alodification of t h e formula and the addition of 1% glycerol greatly reduced losses. The importance of humidity was emphasized. That oxygen can accelerate thiamine destruction under certain conditions was shown by Farrer and Morrison (1919). Although these authors worked with buffer solutions from 50 t o 110" C. (122 t o 230" F.) there is, nevertheless, support for Kandutsch and Baumann's contention. The catalytic effect of salts is only t o be expected from the work already outlined earlier in this paper, and the protective effect of drying follows from what is known of the destruction of thiamine in aqueous solutions.

4. Pharmaceuticals It is generally recognized that thiamine is very stable in acidified solution, especially if stored in the cold. It is not possible, however, t o acidify pharmaceuticals t o the same extent as, for example, standard solutions, and losses of thiamine potency on storage of these materials is also recognized. Nevertheless, Watanabe et al. (1942) reported

302

K. T. H. FARRER

the high stability on storage of sterilized, aqueous solutions of thiamine hydrochloride, and Myrback et al. (1942) found no loss of thiamine hydrochloride in 9 months from solutions a t pH 2-6.5 and containing sodium pyrophosphate. Taub et al. (1949) found maximum stability of thiamine in parenteral solutions with riboflavin and niacinamide under nitrogen a t pH 4. They carried out storage tests at 22 and 45" C. (72 and 113" I?.) and found the mononitrate to be rather more stable than the hydrochloride. In iron compounds, ferrous gluconate favored thiamine stability more than iron peptonate or ferric ammonium citrate. The effects of time, temperature, pH, anions, thiamine salt, and, t o a lesser extent, air are evident in this work without there being sufficient data for the calculation of reaction rates. Macek et aZ. (1950) have compared the relative stability of the mononitrate and hydrochloride in a number of pharmaceutical products a t room temperature and 40" C. (104" F.). Quite clearly, their results are capable of kinetic treatment. They show the hydrochloride to be less stable than the mononitrate, losses of up to SO% of the former in 6 months at room temperature being recorded. Use of the mononitrate, and attention to methods of preparation and changes of formula, reduced storage losses to 10% or less. Bird and Shelton (1950) report the results of collaborative studies on the stability of thiamine mononitrate in tablets, capsules, and solutions also at room temperature and a t 40" C. (104" F.). Losses in dry preparations were very small (6-80/, after 1 year) except for those from capsules containing tartaric acid and held at 40" C. (104" F.). I n this case the loss was about 20%. Losses in solution were 7 and 12%, respectively, at room temperature and 40" C. (104" F.) in a solution adjusted to pH 3.8 with nitric acid, but in an elixir at pH 4.0, 15% was lost over a year a t room temperature and 42% a t 40" C. (104" F.). These figures illustrate wellknown variations in thiamine stability, but no comparisons with the hydrochloride were recorded. Partington and Waterhouse (1953) also studied losses on storage of dry pharmaceutical preparations at 15.5-21' C. (60-70" F.). I n formulas which give pH values of 4-5 in solution they showed losses of about 10% after 2-3 years. This confirms Bird and Shelton's work. I n addition, Partington and Waterhouse showed that formulae which gave solutions of higher pH suffered much greater loss on long storage, practically the whole of the vitamin disappearing. Copper and cobalt had little effect on thiamine loss under the conditions studied. Dutta et al. (1952) have studied the stability of thiamine in oral preparations at 25-32" C. (77-90' F.) and concluded that thiamine mono-

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nitrate is more stable than the hydrochloride, but that in water or aqueous alcohol both forms are subject t o considerable losses over 40 weeks in ferrous sulphate solution (93 %). Their results allow for kinetic treatment. For the mononitrate in water and alcohol, k, has been calculated t o be 0.00345; for the hydrochloride, k , is approximately 0.0057. I n copper sulphate solutions, k , for both forms was approximately 0.01. I n all solut,ions the log (a - $)/time curves showed a very fast initial rate of destruction which quickly gave place t o a slower rate over almost all the time period. As already stated, this was noted by Booth (1943) and Rendix et al. (1951) and has frequently, but certainly not always, been encountered by the present author. The cause is still obscure. I n 2 cases in the paper of Dutta et al., the log ( a - $)/time curves were clearly 2 intersecting straight lines representing 2 distinct rates of reaction. Both applied t o the hydrochloride, and in each case (ferric sulphate and ferric ammonium citrate) the more rapid rate of reaction corresponds with th a t for the mononitrate, whereas the slower rate, attained after 12 weeks, was very much slower than th at for the mononitrate. These points are clearly evident from the kinetic treatment of the results, but are not apparent from the more usual time/retention curves or tables. VIII. CONCLUSION It is clear from the evidence available that the thermal destruction of vitamin B1 in foods can be followed by simple kinetic methods. First order reactions are relatively easy to follow, and the result may be expressed as a rate constant, k , in set.-', min.-' or weeks-', or as a halflife time, 2 t0.5 = log, k Whichever method is preferred, the result may then be compared with other results obtained under the same conditions. I n addition, it is a neat and quantitative expression of the percentage retention (or loss) after a given time, but, most of all, it is the first step in correlating losses with temperature as well as with time. The temperature coefficient of first order reactions is of the order of 2 or 3, i.e. the rate constant, k , approximately doubles or trebles for each 10" C. rise in temperature. The Arrhenius equation, log, k

=

I -

E RT

-

which is so useful in the study of first order reactions, has been shown, theoretically and experimentally, t o be of great value in the prediction of

304

K. T. H. FARRER

thiamine retention in foods, both in the canning process and in the storage of canned or dried foods. I n the graphical representation of the Arrhenius equation, where log k is plotted against 1 / T , the slope of the line is proportional to E , the energy of activation ( R = gas constant), and E can thus be calculated easily. Values of E from all the data which can be treated kinetically are given in Tables VII and VIII, and wide variations are evident. There is a TABLEV I I The Energy of Activation for Thiamine Destruction At Storage Temperatures Product

E in calories

Reference

Yeast extract (natural B,) Yeast extract (mononitrate) Cheese (natural BI) Cheese (mononitrate) Pork, canned Pork, canned Pork, dehydrated Peanut butter Apricots Orange juice Yeast extract, I % , solution a t 100-110" C.

10,200 9900 8450 8040 7440 7440 11,180 6520 6920 6210 6760

Farrer, 1953a Farrer, 1953a Farrer, 1950 Farrer, 1953a Rice and Robinson, 1944 Feaster et al., 1946 Rice et al., 1944 Fournier et al., 1949 Brenner et al., 194810 Brenner et al., 194813 Farrer, unpublished data

tendency for E to be similar in the high-protein products to the values obtained in buffer solutions, but whether or not this is a real trend cannot be determined from the limited information available. The big difference between the value obtained for canned pork and that for dehydrated pork is noteworthy. This may be related t o the claim of several authors that moisture content in dehydrated products is important in this connection and t o the belief of Kandutsch and Baumann (1953) that oxidation plays a part in such products. Some support for this is found in Table VIII which shows that E for thiamine destruction in buffer solution with oxygen is close t o the value obtained for dehydrated pork. I n addition, the great increase in E found in phosphate buffer solutions sealed in a n atmosphere of oxygen and heated in the range 90-110" C. (194230' F.) suggests that here there is some other reaction besides hydrolysis. The very low values for E obtained with several foods calls for comment, since i t is evident th at the reaction involved cannot be the same asmgivenin buffer solutions, yeast extract, etc., with values of the order of 10,000 cal. There appear t o be two possible reactions. One is the breaking

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305

TABLEVIII E, the Energy of Activation for Thiamine Destruction in Buffer Solutionse Temperature range Buffer

(“C.1

Other conditions

E in calories

Citric acid-phosphate Citric acid-phosphate Citric acid-phosphate Citric acid-phosphate Phosphate Phosphate Phosphate Phosphate Phosphate Phosphate Phosphate Phosphate-phthalate Phosphate-phthalate Phosphate-phthalate Phosphate-phthalate

70-110 70-110 90-110 80-100 50-100 50-70 90-110 70-110 80-90 90-110 80-100 70-100 70-100 70-100 80-100

Sealed with air Sealed with O2 Sealed with n - 2 Reflux Reflux Sealed with air Sealed with Nz Sealed with air Sealed with 0 2 Sealed with 0 2 Reflux, with 0 2 stream Reflux, with 02 stream Reflux Sealed with air or Nz Sealed with O2

11,460 11,460 11,460 11,460 8630 8630 8630 11,380 11,380 20,060 11,380 10,100 8190 10,750 11,800

From Farrer and hloriison (1‘349)

of the CH, “bridge” leaving the pyrimidine and thiazole moieties, and the other involves the breakdown of the thiazole ring with production of HZS. Attempts are now being made t o obtain information on the thermal destruction products of thiamine, and from these and from further studies on foods, specifically designed t o yield kinetic data, the explanation will no doubt be obtained. Another point of interest is the discrepancy between the values obtained for yeast extract and yeast extract solutions in Table V I I . _4t storage temperatures, a commercial intermediate (about 40 % moisture) was used, but a t the higher temperatures a 1% solution mas studied. I n addition, the pH was lower in the latter case. The big difference in E; confirms the observation made by the present author (unpublished datda) that thiamine becomes more vulnerable as the concentration of yeast extract solutions increases. The effect of buffer salts on thiamine destruction is now well recognized, and i t is t o be expected that the observed rate constant is, as a result of the primary salt effect, related t o the ionic strength. Glasstone (1943, for example, gives the following relationships : (1) in noncatalytic reactions log k

=

log ko

+ 1.02ZAZ8-&

306

K. T. H. FARRER

where Z A and Z B are the charges carried by the reactants and p is the ionic strength. (2) in catalytic reactions, as between a neutral molecule and an ion.

IC =

kO(1

+

PP*)

where P8 is a constant. Since thiamine forms salts (hydrochloride and mononitrate), one would expect the first relationship to hold, but there is clear evidence for the catalytic effect of various ions on thiamine destruction so that the second relationship, Ic being a linear function of ionic strength, may apply. So far as the author is aware, no account has been taken of ionic strengths in studies on thiamine. However, all basic work on the pure substance has been done in buffer solutions and calculations will show that the changes in ionic strength from solution to solution are not great enough t o alter the general conclusions. This follows, too, from the work of the present author with buffer solutions of different concentrations (Farrer, 1947b; 194913). I n foods, the actual concentrations of ions in the aqueous phase are likely to be high, and ionic strengths difficult to calculate and of little practical importance. From studies on cereals, particularly, the catalytic effect of various ions appears to be more important, and, in any case, conditions within a food group will tend to be similar from sample to sample. From the many data available, a fairly clear picture is emerging, and the value of the kinetic approach is evident. If further work is done along these lines, one may expect the rapid solution of problems still outstanding. REFERENCES Ache, L., and Ribeiro, 0. F. 1945. Rev. fac. med. vet., univ. Sao Paulo 3, 27 (Chem. Abstr. 40, 7525). Ahmad, B., Mehra, S. L., and Bharihoke, G. 1948. Ann. Biochem. Exptl. Med. (India) 8, 89. Arnold, A., and Elvehjem, C. A. 1939. Food Research 4, 547. Amy, E., and Hanning, F. 1947. J . Am. Dietet. Assoc. 23, 690. Ashikaga, C. 1946. J . Fermentation Technol. (Japan) 24, 85 (Chem. Abstr. 47, 5039, 1953). Ashikaga, C. 1951. Vitamins (Japan) 4, 23. (Chem. Abstr. 46, 10428). Ashikaga, C., and Chachin, T. 1951. Vitamins (Japan)3,285 (Chem. Abstr. 46,10428). Ashikaga, C., and Koshimizu, M. 1951. Vitamins (Japan) 3, 110 (Chem. Abstr. 46, 10428). Association of Vitamin Chemists. 1951. “Methods of Vitamin Assay,” p. 110. Interscience, New York. Atkin, L., Schultz, A. S., and Frey, C. N. 1943. U.S. Patent 2,322,270. Aughey, E., and Daniell, E. P. 1940. J . Nutrition 19,285.

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Tunnel Dehydrators for Fruits and Vegetables BY P . W . KILPATRICK. E . LOWE.

AND

W . B . VAN ARSDEL

.

Western Utilization Research Branch. Agricultural Research Service. U S. Department of Agriculture. Albany. California

Page I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 1. The Development of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . 314 2. Production of Dehydrated Fruits and Vegetables . . . . . . . . . . . . . . . . . . . 315 I1. Classification of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . 316 d Arrangements . . . . . . . . . . . 316 1. General Discussion, Characteris 318 2. Longitudinal Air Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a . Counterflow Circulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 b . Parallel-Flow Circulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 c. Two-Stage Tunnels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 3 Transverse Air Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 a Combination Compartment and Tunnel . . . . . . . . . . . . . . . . . . . . 324 ........................ 325 4 Other Tunnel Arrangements . . . . . . . . I11. Mechanical Elements of Tunnel Construction . . . . . . . . . . . . . . . . . . . . . . . . . 326 326 1. Fans and Blowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Heating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 3 . Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 4 . Materials of Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 337 5. T r a y s a n d T r u c k s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Typical Commercial Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 1 . Twin-Tunnel Counterflo-w Dehydrator . . . . . . . . . . . . . . . . . . . . . . . . . . 339 342 2. The Miller Tunnel Dehydrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . The Carrier Compartment Drier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 V. Criteria for Selection of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . 345 VI . Basic Theory of Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 350 1. Theoretical Tunnel Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Optimum Tray-Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 ............................. 357 3 Optimum Recirculation of Air . 4 . Product Temperature in the D rator . . . . . . . . . . . . . . . . . . . . . . . . . 359 362 5. Departures from Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII . Operating Procedures for Tunnel Dehydrators . . . . . . . . . . . . . . . . . . . . . . . . . 363 VIII . Recent Trends in Tunnel Dehydration of Fruits and Vegetables . . . . . . . . . 367 ........................................ 369 IX List of Symbols Used . References . . . . . . . . . . . .................................. 369 313

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P. W. KILPATRICK, E. LOWE, A N D W. B. VAN ARSDEL

I. INTRODUCTION This discussion of tunnel dehydrators, as used to dehydrate certain fruits and vegetables, is intended to provide a general introduction t o the subject, primarily for the use of students, food technologists, and engineers. Although it goes into a number of matters concerned with the design and operation of this kind of equipment, it is in no sense a manual either of design or of operation. Emphasis is laid on discussion of underlying principles and the more recent advances in application of these principles. An effort has been made to bring together published information from many different sources, some of which are not widely available. Unfortunately, it has not been possible for the authors t o survey publications in other than English-language journals and books. 1. The Development of Tunnel Dehydrators

The germ of the idea of the tunnel dehydrator is a t least a century old. Various features of present-day, typical tunnels were undoubtedly added one a t a time t o as simple a basic idea as th at described b y Yule (1845) in an English patent for “improvements in preserving animal and vegetable matters.” Yule, a “preserved provision manufacturer,” placed the cooked or uncooked animal or vegetable product on shelves in “ a chamber of oblong form,” and passed through the chamber a current of air which had been dried by passage through a receptacle of lump calcium chloride or other chemical absorbent of moisture. Yule says nothing in his patent about heating the air stream, but Prescott and Proctor (1937) say that Eisen, in 1795, dried vegetables on racks arranged around a stove in a dry-room, so Yule undoubtedly was acquainted with warm-air drying. No records have come t o light about the kind of equipment used to dry the dehydrated vegetables used by the Union Army in the Civil War. Something strongly resembling a counterflow tunnel dehydrator was being used later in the 19th century t o dry glue, according to Thorp (1905). Cruess (1938) says the Oregon tunnel drier was invented by Allen about 1890; this drier, which consists of a long sloping box with spaced ledges on the interior vertical sides to support trays of fruit, mas originally ventilated only by convection of warm air from a furnace room a t the lower end, but this was later supplanted by fan ventilation. The trays of fruit were pushed downhill through this tunnel, counterflow t o the air movement. Still later, as described b y Wiegand (1923)) the design was modified t o permit recirculation of part of the air, and finally the tunnels were constructed level and wheeled trucks replaced the sliding trays. During the First World War a considerable flurry of interest in vegetable dehydration had led t o the construction and operation of a number

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of dehydration plants. Some of the driers were extremely elaborate and complex, and were abandoned at the end of the war. Soon thereafter, however, untimely rains in the prune-drying area of California led t o a great demand for practical farm fruit dehydrators. Within the next few years the work of investigators like Cruess (1919) and Cruess and Christie (1921a,b), engineers like Ridley (1921), and designers and builders like Chapman (1922a,b), Rees (1922), Puccinelli (1923), and Pearson (1923) resulted in the rapid development of simple tunnel dehydrators basically similar t o those used today. The design of the Oregon tunnel had converged toward a similar pattern. Many hundreds of these dehydrators went into regular use in central California, Oregon, and Washington for drying fruits. A few plants gradually built up a steady business in dehydrated onions, garlic, peppers, and several other vegetables. A number of years later Eidt (1938) described two-stage tunnels which had been designed and built in the Canadian Maritime Provinces for use in dehydrating apples. With the outbreak of the European war in the following year the British Ministry of Foods (1946), after intensive investigation, decided upon a two-stage tunnel dehydrator for its emergency vegetable dehydration plants. The same pattern was followed in most of the British Commonwealth countries. When the United States entered upon its own wartime dehydration program, individual operators were left free t o select the dehydration system they thought most suitable. Some of them adapted existing fruit dehydrating tunnels t o the faster evaporation rates obtainable from cut vegetables. Many new tunnels of the same basically simple design were built. Several of the larger plants installed two-stage tunnels, others purchased the more elaborate multistage transverse-flow “compartment tunnels.” At the end of the war, all three of these types were operating successfully in the United States. Today simple counterflow tunnels handle most of the prune and raisin dehydration and a considerable proportion of the apple dehydration; counterflow or two-stage tunnels are used for most of the current vegetable dehydration. The authors do not know of any count of the dehydration tunnels now in use in the United States, but the number must be several thousand. The vast majority of these are used for drying fruit. 2. Production of Dehydrated Fruits and Vegetables

Raisins, prunes, and apples make up by far the bulk of the dried fruits. Although most raisins are still sun-dried in California, golden-bleached raisins are dehydrated, and during the war there was extensive building of dehydrators for the Thompson Seedless grapes which are the main source of ordinary raisins. The annual production of raisins varies from about 150,000 t o about 400,000 tons.

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Substantially all of the American prunes are dehydrated. The production has declined somewhat since the early 1930’s, but is fairly steady around 200,000 tons. The production of dehydrated apples, peaches, and pears has gradually declined from peaks reached in the 1920’s and ’30’s. About 15,00020,000 tons of dried apples, something less than 10,000 tons of dried peaches, and 1000-2000 tons of dried pears have been produced during recent years. All of the dried apricots, and most of the dried peaches, figs, and pears are processed by sun-drying, rather than dehydrating; on the other hand, all of the dried apples are dehydrated. The production of dehydrated vegetables, in contrast to the production of dried fruits, has fluctuated widely in response to demands brought about by wartime emergencies. Total United States production in 1941 is estimated by Rasmussen and Shaw (1953) to have been 13,000,000 lb.; only 3 years later it had increased 15-fold1to 209,000,000 lb. In another 2 years it dropped back to 55,000,000 lb. The European crisis of 1948 boosted it steeply, and 3 years later the Korean war produced another upsurge. The production of 60,000,000 lb. in 1950 was composed 36 of potatoes, 15% of onions and garlic, 13 % of peppers, and the remainder distributed between many vegetables. Except for the fairly large proportion of mashed potato powder (“potato granules”) in this 1950 production, nearly all of the product was made in tunnel dehydrators. A growing demand for high-quality dehydrated vegetables in a variety of processed foods (canned hashes and stew, catsup, cottage cheese, “ a la king” products, meat pies, etc.) has resulted in a steady and diversified post-war growth of the civilian market. 11. CLASSIFICATION OF TUNNEL DEHYDRATORS

I. General Discussion, Characteristics, and Arrangements The fruit and vegetable dehydration industries, both in the United States and the British Commonwealth countries, have used the tunnel drier far more extensively than any other type of dehydrator. Tunnel dehydrators, as a class, are frequently called tunnel-and-truck, truck-andtray, or simply tunnel driers. Principles pertaining to their use have been discussed by Van Arsdel (195la,b) and Perry and associates (1946). Abstracts from these sources have been freely incorporated into the material contained in this part of the chapter. Likewise, Figures 1through 7 have been reproduced from these same sources. All tunnel-and-truck dehydrators for fruits and vegetables have a common feature which distinguishes them from other kinds of drier. This characteristic is the method of handling the commodity. Normally, the

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prepared commodity in piece form is spread thinly on special trays fabricated of wood or metal. Depending on the commodity, tray loading for vegetables may range from 1 t o 3 lbs. per sq. ft.; for fruit, theloading may be in the range of 1 t o 5 lbs. per sq. ft. Loaded trays are then stacked, one above another, on a portable low-bed truck or dolly. Height of the stacked trays may range from about 5 t o 7 ft., depending on operating conditions.

I WET TRUCKS INSERTED

DIRECTION OF AIR FLOW

,FOR

SIDE EXIT DRY TRUCKS

TRUCKS PROGRESS I N THIS DIRECTION

BLOWER’

FIG. 1. Simple counterflow tunnel (elevation) (from Iran Arsdel, 1951b, Fig. I).

The trays are so designed that, when loaded and stacked, there is a clear air passage left between successive trays. The loaded trucks are pushed, one a t a time, into one end (usually called the “wet end” or loading end) of the dehydrator’s drying section. The drying section or “tunnel” is a straight passageway with a cross-section just large enough t o accommodate the loaded trucks. Tunnel lengths vary; some may hold only 4 or 5 SIDE ENTRANCE FOR WET TRU

EXHAUST

TRUCKS PROGRESS IN THIS DIRECTION

FIG.2. Simple parallel-flow tunnel (elevation) (from Van Arsdel, 1951b, Fig. 2).

trucks, whereas others may contain as many as 15 t o 20. During operation, a truck of dried material is removed from the “dry end” of the tunnel, the remaining trucks are pushed forward one truck length, and a truck of wet material is rolled into the vacant space a t the “wet end” of the tunnel. It is obvious that operation is only quasi-continuous (not truly continuous as it would be in the case of most conveyer driers), and this is known as “progressive” operation. Primarily, the flow of hot air used for drying is across the horizontal surface of the layer of wet material. Very little air circulates through the layer of wet material while it is in a tunnel drier. This is‘distinctly different from the air flow in most conveyer driers in which the air flows up (or down) through the product layer. I n commercial use, there are 3 basic arrangements of tunnel driers,

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plus several different combinations of these basic arrangements. The essential difference between the various types is mainly the direction of air flow relative t o truck movement through the tunnel. The three basic types are illustrated by the simplified sketches shown in Figs. 1 to 3, and will be referred t o later. Actually, all three types are much more complicated than indicated, since working units have provisions for recirculating H,E ATE TS

0

WET TRUCK ___.

0-

-HEATER

EXHAUST AIR

TRUCKS PROGRESS I N T H I S DIRECTION

- F R E S H AIR INLET

‘BLOWER

FIG. 3. Simple combination compartment and tunnel (plan view) (from Van Arsdel, 1951b, Fig. 3).

part of the drying air. The paragraphs which follow describe the counterflow, parallel-flow, and compartment tunnels, as well as combinations of the parallel and counterflow units (two-stage tunnels) and other tunnel arrangements. 2. Longitudinal A i r Circulation a. CounterJlow Circulation. I n the counterflow tunnel (Fig. l ) ,the hot drying air is blown into the dry end of the tunnel and moves straight through it, in a direction opposite t o the movement of material being dried. The (‘wet” air is discharged a t the wet end of the tunnel where the prepared fruit or vegetable enters. I n actual operation, in order t o increase fuel economy, or to raise the air humidity in the tunnel, provisions are made for recirculating a part of the air discharged from the wet end. As the hot air passes through the line of loaded trucks, it picks u p moisture from the fruit or vegetables on the trays, and in so doing the air becomes cooler. I n the counterflow tunnel, the warmest, driest air comes in contact with the nearly dry product while the cooler, more humid air is in contact with the wet material entering the tunnel. The maximum air temperature which can be used is determined by the commodity being dried, and is that temperature which the nearly dried product will tolerate for several hours without perceptible damage. I n the counterflow tunnel, the best conditions for drying are a t the end of the tunnel where the product is nearly dry. Reasonably good drying conditions can be secured at the wet

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end of the tunnel if “light tunnel loading” is used. (Light tunnel loading refers t o a suitable balance between mass air flow rate, air temperature, and total water evaporated per unit of time, so that air a t the wet end of the tunnel has a reasonably high evaporative capacity, i.e., a wet-bulb depression of a t least 15 t o 25” F. for most commodities.) Industry has used several different arrangements of the counterflow tunnel. If production capacity requires more than one such tunnel, the TRUCKS

J

RECIRCULATION DAMPERS

I

TRUCKS PROGRESS IN THIS DIRECTION

FIG. 4. Direct-fired twin counterflow tunnels (plan view) (from Van Arsdel, 1951b, Fig. 4). FRESH AIR

\SIDE ENTRANCE FOR WET TRUCKS

\SIDE E X I T FOR DRY TRUCKS

FIG. 5. Side-entrance counterflow tunnel (elevation) (from Van Arsdel, 1951b, Fig. 5 ) .

initial investment in tunnel cost can be kept a t a minimum by using a common blower, heater, and recirculation return for two tunnels. Such an arrangement (see Fig. 4) is known as the “twin tunnel.” Arrangements such as the one illustrated in Fig. 4 sometimes prove unsatisfactory because of uneven air distribution. To correct this difficulty, a further modification of the basic counterflow design (using doors in the side walls of the dehydrator near the tunnel ends through which the trucks are pushed) is shown in Fig. 5. This side entry principle was

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used for many two-stage dehydrators operating in Great Britain during World War 11. However, the side entrance and exit method adds t o the complexity of truck movement, requires additional tunnel length, and increased floor space. b. Parallel-Flow Circulation.. I n the parallel-flow tunnel (Fig. 2), the air moves straight through the tunnel in the same direction as that of truck movement or progression. The name “concurrent,” instead of parallel-flow, is usually used in the British Commonwealth countries for this type of tunnel arrangement. The parallel-flow tunnel is very similar to the counterflow unit in the general arrangement and layout. Basically, the only difference is that the product loading and unloading ends are interchanged, resulting in a reversal of the direction of truck travel with respect to air movement. With these changes in mind, Figs. 4 and 5 are applicable t o either parallelflow or counterflow tunnels. There are marked differences in the behavior of parallel-flow and counterflow tunnels. For example, if prepared vegetables are dehydrated in a parallel-flow tunnel, difficulty may be encountered in drying the product sufficiently t o assure satisfactory stability in subsequent storage. On the other hand, in dehydrating a whole fruit, for example prunes, a parallel-flow tunnel may cause cracking of the skins and excessive loss of juice. These problems do not arise in proper counterflow tunnel operation. Nevertheless, the parallel-flow tunnel can be and is used very satisfactorily if operated in conjunction with an auxiliary or finishing drier. The following facts explain why it is impractical t o use the parallel-flow tunnel by itself for the dehydration of fruits and vegetables. The hot drying air entering the parallel-flow tunnel comes in contact with the very wet product at the loading end. As drying progresses, the wet product is warmed up by contact with the hot air. At the discharge end of the tunnel, the relatively dry product is in contact with moisture laden air which has been greatly cooled and has a very low evaporative capacity. Thus it is difficult t o dry a commodity to reasonably low moisture levels in a parallel-flow tunnel. Let us now re-examine the loading end of the parallel-flow tunnel. The hot drying air which enters the tunnel will always have a wet-bulb temperature very much lower than its dry-bulb temperature. If the commodity being dehydrated is a prepared vegetable, the temperature of the wet material (near the loading end) will not exceed the wet-bulb temperature of the hot drying air. This condition will prevail for an appreciable length of time. During this initial drying period, the evaporation process is quite similar to that which would occur if a wick were moistened with water and placed in the air stream. As the prepared vegetable dries down

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t o a moisture content somewhere in the region of 50 to 65%, the water has more and more difficulty in traversing the internal structure t o the outer surface of the pieces where i t vaporizes. The surface of the product then becomes relatively dry. At this stage, the temperature of the vegetable pieces slowly rises above the wet-bulb temperature of the air. Therefore, the hot air supplied to a parallel-flow tunnel can be raised t o a higher temperature than would be safe for counterflow tunnel operation. A different situation arises if the commodity being dehydrated is uncut fruit. I n this case, since the moisture diffuses slowly t o the surface, the temperature of the wet material will rapidly rise above the wet-bulb temperature of the air. Drying under excessively high temperature conditions will tend t o make the fruit crack and bleed, and there may be a n appreciable loss of juice, When in proper use, evaporation is very rapid in the wet-end zone of the parallel-flow tunnel drier. Compared t o the counterflow tunnel, evaporation in this zone is at least 3 times as fast. Therefore, while the material is still very wet, excellent drying conditions prevail in the parallel-flow tunnel. Due t o the evaporative cooling effect, it is possible t o use a relatively high hot-end temperature without scorching the product. Consequently, parallel-flow tunnels have a high potential evaporative capacity. On the other hand, if a very dry product must be produced without aid of an auxiliary drier or finisher, the parallel-flow tunnel must be operated in such manner that heat economy and capacity are very low. Nonuniformity of drying may also be a serious difficulty in the use of a parallel-flow t*unneldrier. The tray edge closest t o the hot-air end of any tunnel is always exposed to more severe drying conditions than the down-stream edge. I n the parallel-flow tunnel this condition is most pronounced. c. Two-Stage Tunnels. The fruit dehydration industry has sometimes modified the design of the standard counterflow tunnel to take advantage of the good characteristics of the parallel-flow unit. One of these modifications, called the "hot-center " arrangement, has been used successfully in the drying of prunes. I n a typical unit, the hot drying air is blown into the center of a long tunnel. The air stream divides and moves toward both ends. This modification provides counterflow operation in the wet half and parallel-flow in the dry half of the tunnel. However, it does not take full advantage of the high wet-end evaporative capacity of the parallelflow tunnel. Another tunnel arrangement, with a parallel-flow wet end followed by a counterflow dry end, capitalizes on both the high wet-end evaporative capacity of the parallel-flow tunnel and the good final drying characteristics of the counterflow unit. This arrangement is best suited for the

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dehydration of commodities which are able t o withstand the high wet-end temperature characteristics of a parallel-flow first stage. There is ample evidence t o indicate that most vegetables are in this category; also some cut fruits, for example, apples. I n general, the determining criterion is that the prepared commodity shall be a relatively fast-drying material. The dehydration industry has also used a n arrangement called the “ center-exhaust ” dehydrator, illustrated by the simplified diagrammatic sketch of Fig. 6. This type unit operates b y drawing heated air into both ends of the tunnel. The hot air (under a slight negative pressure) travels through the loaded trucks and is sucked out of the tunnel chamber (near FRESH AIR TO FIRST STAGE HEATER, I

I

EXHAUST AIR

EXHAUSTEI~

RECIRCULATION

I

DAMPERS

h l R EXIT BELOW TRUCK L E V E L

FRESH AIR TO STAGE

SEC\oND

,HEATER

TRUCK) DOOR c

TRUCKS PROGRESS IN THIS DIRECTION

FIG.6. Center-exhaust tunnel dehydrator (plan view) (from Van Arsdel, l95lb, Fig. 11).

its center) by a blower acting as an exhauster. Although called a centerexhaust arrangement, the exhaust port is usually located about one-third of the tunnel length from the loading end. An advantage of the “centerexhaust” system lies in the fact th at trucks can be pushed straight through the tunnel, and therefore do not require rehandling during the transition from first to second stage drying. However, there are some serious design and operating problems. It is hard t o balance the air flow through the two ends, particularly if the two sections contain a n unequal number of trucks. It is also difficult t o secure good air-flow distribution through the trucks as they approach and leave the vicinity of the air exhaust section of the tunnel. Another arrangement of the two-stage type of dehydrator is shown diagrammatically in Fig. 7. Essentially this consists of separate blowers and heaters a t each end of a single tunnel. A sliding partition near the tunnel center divides the parallel-flow section from the counter-flow section, but permits truck movement from one t o the other. I n this type of divided single-tunnel, the trucks are pushed straight through from the parallel-flow section to the counterflow section. Sometimes, to economize

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on labor, provisions are made for automatic synchronization of the truckadvancing and partition-moving mechanisms. I n the United States, a widely used two-stage arrangement has the parallel-flow and counterflow stages physically separated. This requires handling of the trucks during their transfer from the parallel-flow tunnel to the counterflow unit. The usual practice is t o have a bank of parallelflow tunnels arranged side by side. Special transfer tracks with turntables permit manual transfer of the trucks from the “dry end” of the parallelflow units t o the “wet end” of the counterflow tunnels. The latter are also arranged in banks side by side, and usually located behind the bank of BLOWER,

\ HEATER,

FRESH AIR INLETS

BLOWER ,HEATER /’

I

I

TRUCK) DOOR TRUCKS PROGRESS IN THIS DIRECTION

FIG.7. Two-stage, single-tunnel dehydrator (plan view) (from Van Arsdel, 1951b, Fig. 12).

first-stage driers. This arrangement permits considerable flexibility in the dehydration plant’s operation and drier load capacity. The number of first- and second-stage driers in operation ran be varied, so as to gear the plant’s drying capacity to the commodity being processed in the preparation line ahead of the driers. Trucks from any of the operating first stage tunnels can be routed to any of the counterflow second stage driers. Since most of the evaporative load takes place in the parallel-flow or first-stage tunnels, there is only a relatively light evaporative load requirement in the counterflow sections. The heater and blower capacities in the two sections will usually differ accordingly. The air exhausted from the counterflow stage is relatively warm and dry. For reasons of heat economy, i t is desirable t o use this air as a part of the supply t o the parallel-flow section. This requires duct-work of fairly large dimensions when the two drying stages are physically separated. The two-stage arrangement, standardized by the British Ministry of Foods (1946)) uses primary and secondary stage tunnels of equal length placed side by side. Trucks progress in one direction through the parallelflow tunnel. As they leave that section, they are turned around and pro-

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gress in the opposite direction through the counterflow tunnel. I n this arrangement, the loading and unloading of a group of dehydrators are done at the same end. Air recirculation is controllable in each stage. Normally, the exhaust air from the secondary tunnel furnishes the entire "fresh-air '' supply for the parallel-flow first-stage unit. Extensive performance data and plant experience are given in the bulletin issued by the Great Britain, Ministry of Foods (1946). The two-stage method of dehydration offers some distinct advantages over a single-stage drier. The reversal of air-flow direction, with respect to movement of material, tends to give a more uniformly dried product. Drying times are shorter, and good drying conditions prevail a t both ends of a two-stage unit. These conditions tend to favor product quality. The shorter drying time also allows an increased output from a dehydrator of a given size. Three-stage tunnels have been used successfully in a t least one plant, but the dehydration industry generally favors one- or two-stage driers. Perhaps the chief advantage of the three-stage unit is its flexibility which permits the drying (under nearly optimum conditions) of a large variety of different commodities. As the number of stages increases, control becomes rather complex, and more labor is needed for operation (unless truck handling is completely automatic). 3. Transverse Air Circulation

a. Combination Compartment and Tunnel. I n direct contrast to the dehydrators previously mentioned, the combination compartment and tunnel drier operates with the drying air moving back-and-forth through the trucks transversely to the axis of the tunnel. The principle is illustrated in simplified form by Fig. 3. As is evident, the material advancing through the tunnel is subjected to reversals of air-flow direction, an advantage which tends to equalize drying. There are many possible variations of the basic arrangement. The combination compartment and tunnel drier can be equipped with controlled air reheaters and provisions for air recirculation for each cornpartment (each truck position). The large number of independent controls makes the combination compartment tunnel very flexible in operation. As the material is passed through the tunnel, the commodity can be subjected to almost any desired time-temperature-humidity drying condition. Such units are also well suited as general-purpose, experimental, continuous driers, and can produce the relatively large quantities of dehydrated material necessary for storage studies. For example, at the Western Regional Research Laboratory such a unit has been used for this purpose,

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and counterflow, parallel-flow, and various combinations of two-stage drying conditions simulated. The design and construction of a compartment tunnel requires the greatest of care to avoid operating difficulties. Elaborate provisions must be made for straightening out and equalizing the air flow across the trayed material and through the trucks. If air is forced to make sharp turns, it tends t o hug the outside of the curve. The ordinary arrangement of guide vanes or “splitters” will not control this tendency sufficiently when the high velocity air stream is turned through 180”. The use of a system of perforated plates, the small holes acting as orifices or nozzles, has met with some success when properly designed and installed. However, this requires a substantial increase in the power necessary for air movement. Short-circuiting of air from one compartment t o another, without going through a truck, is another difficulty, unless provisions are made for a permanent and reasonably tight seal between the trucks and tunnel walls. The general drying characteristics of the compartment type tunnel equipped with auxiliary heaters may be briefly characterized as follows: Each time that the air passes through a heater, both the wet-bulb and drybulb temperatures increase. If general movement of the air is toward the loading end of the tunnel, the wet-bulb temperature will progressively rise, unless additional fresh air is introduced a t each of the reheating stages. I n this regard, the compartment unit differs from the simple counterflow tunnel, for in the latter, the wet-bulb air temperature remains substantially constant throughout the tunnel. The commercial fruit and vegetable dehydration industry has used comparatively few compartment type tunnel driers. Preference has been shown to the counterflow and two-stage tunnels because of their relative simplicity and comparative freedom from the difficulties inherent in the compartment type.

4. Other Tunnel Arrangements Guillou and Moses (1943) developed a modified form of cross-flow fruit dehydrator for farm use, and presented plans, construction details, and operating instructions. This is a modified, simple form of the compartment type drier and has been used in a number of California orchards for dehydration of prunes. There are other possibilities of tunnel arrangements. Only two will be mentioned briefly, and neither of these apparently has progressed beyond the pilot-plant study stage. The closed-cycle system dehydrator does not exhaust to the atmosphere. It operates by partially dehumidifying the exhaust air from the

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drier and this air is returned for reuse. Proposals have been made to use the system for the dehydration of onions and garlic in order to overcome the normally obnoxious exhaust from such dehydrators. The plan has also been used as a part of a pilot plant system for dehydrating food commodities in an atmosphere of oxygen-free gas. A combination blancher-dehydrator has received some study. Essentially the arrangement consists of an isolated compartment a t the wetend of a tunnel drier, with air temperature and humidity in the compartment under independent control. Humidity and temperature are controlled, so that the raw, wet commodity can be rapidly elevated t o temperatures in the range of 180 to 210" F. with little drying and no condensation taking place. The wet product is held at the high temperature for a short period, as determined by the time-temperature requirement to inactivate its enzyme system completely, and thereby securing a full blanch. The hot product then immediately enters the tunnel drier section. Initial drying of the hot product is extremely fast, and its temperature rapidly drops to the vicinity of the ambient wet-bulb air temperature. Further dehydration proceeds in the usual manner. One of the system's advantages is the minimized loss of nutrient material from the product. During the conventional blanching procedure, there is a loss of such material due to leaching. This is obvious if water blanching is used, but it is also true to somewhat a lesser extent, if steam is used as the heat transfer medium. In the latter case, steam condenses on the product as it is heated, and the hot condensate has a tendency to leach out soluble material rapidly from the product. Theoretically, it is possible to adjust the wet-bulb and dry-bulb temperature of the circulating air in the blanching compartment, so that the product temperature can be rapidly elevated without either condensation or evaporation of moisture taking place. However, a t the high temperatures involved, there is little margin between the conditions required for condensation and those for extremely rapid drying and scorching. Informal reports and observations made by the authors indicate that control is difficult. The method offers potential advantages, and perhaps some future investigator will develop a practical procedure. 111. MECHANICAL ELEMENTS O F TUNNEL CONSTRUCTIOX 1. Fans and Blowers

The introduction of forced air movement is probably the most important single contribution in the development of the modern dehydrator. Prior to the use of fans, air movement in dehydrators was entirely dependent upon natural circulation of a rising current of warm drying

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air. Drying was slow and drier performance was poor, with the net result that a large number of driers was required for a given plant capacity. The modern dehydrator is a high-performance unit primarily because powerdriven fans make possible t,he movement of very large quantities of drying air. The performance of a drier is affected in two different ways by the velocity of the air moving through it. First, because an increase in air velocity past a moist body increases the rates of heat transfer and mass transfer, the rate of drying of the moist material increases. Second, and of greater practical importance, the mass of air moving through the drier is proportional to the velocity of the air, and the evaporative capacity of the drier is proportional t o this mass velocity. The effect of air velocity on drying rate is complex. The rate of evaporation from a free water surface is known to be proportional to the 0.8 power of the air velocity across the surface, but Guillou (1942) found that the drying rate of prunes increases only as the 0.2 power of the air velocity. Brown and Kilpatrick (1943) showed that the effect of air velocity on the drying rate of vegetables gradually decreases as the moisture content falls; below about 15 to 20% moisture content the drying rate is substantially independent of air velocity. High air velocity is effective in accelerating evaporation near the wet end of a vegetable dehydrator, but not near the dry end. Increasing the mass flow-rate of air through the tunnel increases the evaporative capacity of the tunnel, essentially by supplying additional heat which is available t o produce more evaporation. As is shown in a later section of this article (equation 2, p. 349) the temperature of the air falls and its humidity rises as it passes over the moist material in the drier, but the extent of these changes is inversely proportional to the massvelocity of the air. At a very high rate of air flow a good drying potential can be maintained even near the cool end of a very long tunnel. Air velocities in commercial fruit or vegetable tunnel dehydrators range from about 300 to over 1000 ft. per min., based on the entire crosssection of the tunnel, empty of trucks and trays; actual lineal velocity across the material in the trays will be from 50% to 100% greater. This is a range established by practical experience. To the writers’ knowledge, the only effort that has been made t o arrive a t an economic optimum air movement through application of information about the drying characteristics of a specific product was the design of Guillou and Moses (1943) of a farm fruit dehydrator. I n this case the drying characteristics of prunes and the importance of keeping capital costs and operating costs low led to choice of a relatively low air velocity. In vegetable dehydrators, on the other hand, the air velocity in the empty cross-

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section usually is as much as 600 t o 800 ft. per min. For tunnels of dimensions which are widely used (about 6.5 ft. wide by 7 f t . high) the air-handling requirement of the fan or blower ranges from about 10,000 cu. ft. per min. to about 40,000 cu. ft. per min.; if a single fan supplies air to two tunnels, these figures are, of course, doubled. I n considering fan performance it is customary t o standardize the conditions to air weighing 0.075 lb. per cu. ft., corresponding to dry air at a temperature of 70“ F. and a barometric pressure of very nearly 29.90 inches. The performance of a fan whose rating is known in terms of this “standard air” can be readily computed for other temperatures and pressures by means of the well-known fan laws given in engineering handbooks. Increasing the air flow in a dehydrator by increasing the speed of rotation of the fan is subject to a very drastic law of diminishing returns, because the power absorbed by a fan of given size varies as the cube of its rotational speed. For example, under otherwise identical conditions the time required for drying potato half-dice to 16.7% moisture content in a parallel-flow tunnel can be reduced from 3% hr. to 3 hr. by increasing the air velocity from 400 ft. per min. to 600 f t . per min. If the same size fan is used to obtain the higher air velocity in the same size dehydrator, the power consumed by the fan will increase 3.38 times. Whether the increase in power cost will offset the decreases in other costs occasioned by the 17% increase in output can be determined only by an analysis of all the other cost items. The resistance to air flow, or static pressure drop in tunnel driers ranges from a minimum of about 56 inch water gauge to a maximum of about 145 inches water gauge (standard air conditions), the magnitude of the resistance depending upon the length of the drier, the number of trucks in the drier, the air velocity, and the air-flow path. I n some dehydrators, especially those of earlier vintage, the air-flow resistance is lower than might be expected because much of the air flows around instead of through the space between the trays. This happens when there is excessive clearance between the trucks in the drier and the walls, floor, and ceiling of the dehydrator. The air, seeking the path of least resistance, tends to bypass the drying trays by traveling down the clearance paths. I n spite of a high rate of air circulation drying is slow because the air bypassing the trays has little influence on the drying. The size and type of fan used in a tunnel dehydrator depend upon a number of factors, the air-handling requirement, the air-flow resistance, the permissible noise level in the plant, the space available for mounting the fan, the need for a fan with nonoverloading characteristics, and last but not least, the relative importance of minimum equipment cost as compared with minimum operating cost.

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Space and cost play more important roles in the selection of a fan for use in a tunnel dehydrator than are generally recognized. For a given type of fan, there is usually more than one fan size t h a t can be used for a given air volume and air-flow resistance. Of the several sizes involved, one size will be more efficient than the others, and therefore would be the size selected unless space requirements or the importance of keeping equipment cost a t a minimum dictate the use of a smaller, less efficient fan. Selecting a fan on a compromise basis is not at all uncommon, especially when large size fans are involved. The need for a fan with nonoverloading characteristics depends a great deal upon the manner in which the tunnel is operated. If the drier is

k FIG.8. Siniple propeller-type fan.

operated so that the fan is at times discharging against considerably less than normal resistance pressure, then a fan with nonoverloadiiig characteristics is needed t o prevent the fan motor from being temporarily overloaded. On the other hand, if the fan is always discharging against a fixed resistance, then the nonoverloading characteristic is not essential. Nonoverloading type fans are used in most tunnel dehydrators because the flow resistance is considerably less than normal when the drier is only partly loaded, or when the end doors are open. Three different types of fans or blowers are commonly used in food dehydrators, namely propeller, axial flow, and centrifugal fans. I n general, simple propeller fans of the type shown in Fig. 8 are seldom found in tunnel dehydrators because they are used only in applications involving very low air-flow resistance, usually under $4 inch water gauge static pressure. T o improve their ability t o discharge against pressure, propeller-type fans are equipped with a special ring housing (see Figure 9*). When so equipped, they are used in tunnel dehydrators with moderate

* Mention of a specific manufacturer in the caption in Fig. 9, and at other places in the text, does not imply that the equipment shown or mentioned is recommended by the U. S. Department of Agriculture over similar equipment of other manufacture not mentioned or shown.

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air-handling requirement and nominal flow resistance (e.g. 20,000 cu. ft. per min. against a resistance pressure of 1% inches water gauge). Operating efficiency is improved if the fan discharge is equipped with an expanding conical duct connection to convert the velocity energy of the

FIG.9. Propeller-type fan with special ring housing (courtesy of Hartzell Propeller Fan C o . ) .

fan to pressure energy. Propeller fans equipped with the special ring housing are usually nonoverloading. The chief advantages of using propeller-type fans are simplicity of installation due to the compactness inherent in a piece of equipment i n which the air enters and leaves in the same direction, and comparatively

UPEL LER WHEEL OR

,/

AIR FLOW

VANES FIG.10. Cut-away view of vaneaxial fan (courtesy of Hartzell Propeller Fan

CO.).

low equipment or first cost. The principal disadvantage is the high operating noise level, an important factor where driers are located in populated areas. Axial flow fans can be divided into two general classifications, tubeaxial and vaneaxial (see Fig. 10). Both types resemble a propeller fan in that a rotating impeller moves air through the fan, with the air entering and leaving the fan in the same direction.

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A tubeaxial fan consists essentially of an impeller or wheel with airfoil blades, rotating within a cylinder. Tubeaxial fans are used to move air over a wide range of volumes a t medium pressures. In tunnel driers, for example, tubeaxial fans are used to move from 30,000 to 75,000 cu. ft. of air per min. against a resistance pressure of from 1 t o 1% inches water gauge. Both tubeaxial and propeller-type fans discharge air traveling with a rotating or screw motion. When the rotating air stream enters the stack of drying trays in the drier, some of the tray surfaces will be exposed t o an air stream approaching from the top while other tray surfaces will be exposed to an air stream approaching from the bottom, below the wood slats. The result is that drying will not be uniform because of differences in the air velocity over the trays. To correct this difficulty, air straighteners of the egg-crate type are often used in tunnel driers equipped with tubeaxial or propeller-type fans. A vaneaxial fan is essentially a tubeaxial fan with air-guide vanes located either before or after the impeller. The guide vanes improve the performance of the axial flow fan, especially when discharging against pressure. When used in tunnel dehydrators, for example, vaneaxial fans are generally somewhat more efficient than tubeaxial fans of equivalent size and rating. The vanes also straighten the air leaving the fan, eliminating the rotating or screw motion characteristic of the air stream leaving a tubeaxial or propeller-type fan. Vaneaxial fans are capable of delivering against higher pressures than tubeaxial fans, a factor important in drier applications only if the air-flow resistance in the drier is abnormally high. Both tubeaxial and vaneaxial fans are available with nonoverloading characteristics. Like propeller-type fans, tubeaxial fans and vaneaxial fans are more efficient when the fan discharge is equipped with an expanding conical duct connection to convert the velocity energy of the fan t o pressure energy. A centrifugal fan consists essentially of a fan rotor or wheel rotating within a scroll shaped housing. Centrifugal fans are capable of moving air over a wide range of volumes and pressures, and are commonly used in tunnel dehydrators of all types and sizes. When equipped with backwardly inclined wheel blades, they are nonoverloading. Because the air enters from the side, centrifugal fans must be installed with ample air-flow clearance on one or both sides of the fan, depending upon whether the fan is single or double entry. This requirement, combined with the fact that centrifugal fans are, in general, relatively large in size, results in very large space requirements within the drier to accommodate the fan. This is especially true in high-performance driers using centrifugal fans

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of the most efficientsize. To conserve space in such cases, fan size is often compromised by using a smaller fan of lower efficiency. Axial flow fans are competitive with centrifugal fans in most tunnel dehydrator applications. When compared with centrifugal fans, axial flow fans are more compact, easier, and therefore less expensive to install and, in general, lower in first cost. On the other hand, they are much noisier in operation and are somewhat less efficient if the fan is selected for maximum efficiency, independent of space requirements. 2. Heating Systems

Heating systems used in tunnel dehydrators are of two basic types, direct combustion heating and indirect heating. I n a direct combustion heating system the gaseous products of combustion are mixed and circulated with the drying air and hence come in direct contact with the product in the drier. An open flame in the main air stream of the dehydrator is an example of this type of heating system. I n an indirect heating system, the products of combustion are not circulated with the drying air. Heating surfaces are used to transfer the heat from the primary source to the drying air. A dehydrator using steam-air heating coils is an example of this type of heating system. Direct combustion heaters are widely used in tunnel dehydrators. Because there are no transmission losses, heat efficiency is at a maximum. The fuel used is usually either natural or manufactured gas, fuel oil, or bottled gas such as butane. A gaseous fuel is usually preferred to fuel oil because of the simplicity of the control equipment, the ease of handling, and the fact that the products of combustion are unlikely t o affect the quality of the dried fruit or vegetable. Gas burners are almost always of the “premixed” type, installed directly in the drier air stream with the flame shielded from the cooling effect of the surrounding air currents by a simple unlined sheet metal combustion chamber. Although not essential when using gaseous fuels, refractory-lined combustion chambers are sometimes used with gas burners to insure complete and therefore more efficient burning of the fuel. Oil burners are of many types-rotary, atomizing, centrifugal, pressure, etc. A basic rule in connection with the use of oil is that combustion must occur in a relatively high temperature zone. If the flame is chilled so that some of the oil particles are cooled below their ignition point, smoke and soot will be formed which will contaminate the product in the dehydrator. A common way of burning fuel oil in food dehydrators is in a refractory-lined steel sheet combustion chamber such as that shown in Fig. 11. The combustion chamber is divided into two zones by a refractory

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checker brick partition, the partition serving t o confine the radiant heat t o the primary zone. I n operation, this primary zone becomes incandescent so t ha t combustion of the fuel occurs a t incandescent temperatures. The checker screen also serves as a baffle t o prevent the escape of unburned oil droplets since impingement of the droplets on the incandescent screen results in surface combustion of the fuel. The secondary zone is a n added precaution against smoking due to incomplete combustion. Large unburned particles of oil that escape from the primary zone will burn a t an accelerated rate when they come into contact with the high-velocity, high-temperature gases flowing through the checker wall restrictions. The particles are then given additional time in the secondary zone t o burn completely before coming in contact with /CYLINDRICAL

STEEL SHELL\

BURNER PRIMARY ZONE

\FIREBRICK

LINING)

[FIREBRICK CHECKER WALL’

FIG.11. Refractory-lined combustion chamber.

the drying air. Without the secondary zone, combustion may not be complete enough t o eliminate smoking. Considerable care must be exercised in the selection of a fuel oil for use in a direct combustion food drier. I n most cases, oils with high sulfur content cannot be used satisfactorily because the product absorbs a n excessive amount of the sulfur dioxide liberated during combustion. Indirect heating systems for dehydrators usually involve steam-to-air heaters although combustion gas-to-air heaters are used, particularly in apple dehydrators. The principal advantage of using an indirect heating system is t ha t there is no possibility of contaminating the material being dried with the products of combustion. The principal disadvantages are the additional equipment required and the lower heat economy. 3 . Instrumentation

The dry-bulb temperature of the air entering the drying tunnel is automatically controlled in virtually all tunnel dehydrators. I n a very few plants the wet-bulb temperature of the entering air is also automatically controlled. Dry-bulb temperature is controlled by regulating the flow of heat into the drier, ordinarily by means of a valve in the fuel or steam supply

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line. Wet-bulb temperature is controlled by regulating the position of the recirculating air damper. The instruments used to control dry-bulb temperature in tunnel dehydrators are of two basic types, on-off and modulating or proportional control. The latter is by far the most common. Wet-bulb temperature can be controlled satisfactorily only with modulating or proportional type instruments. On-off instruments range in complexity from simple thermostatic switches to industrial type controllers that indicate and, if so desired, also record the temperature. On-off control of dry-bulb temperature is practical only if the air-heating system in the drier involves a large amount of thermal capacitance or heat inertia. I n a tunnel dehydrator this usually means the use of a combustion chamber large enough to serve as a heat reservoir, storing heat while the burners are on and releasing the stored heat to the air when the burners are off. With an on-off control, air temperature will fluctuate to some extent, the amplitude and frequency of fluctuation depending upon the instrument, its adjustment, the size of the thermal capacitance, and the size of the heating load. On-off control without thermal capacitance is unsatisfactory because air temperature will fluctuate excessively. Fluctuations can be minimized by by-passing the control valve with a manually operated valve adjusted to maintain, without help from the controller, an air temperature slightly lower than the correct temperature, and depending upon the controller to supply only the additional heat required to bring the air to the proper temperature. The practice is not a good one, however, because the air temperature can rise to damaging if not dangerous levels if the heating load is reduced much below normal, for example, by a slackening in the rate of supply of wet material to the dehydrator. The simplest on-off temperature control system would consist of a thermostatic switch opening and closing an electric solenoid valve in the fuel supply line. A more elaborate system would consist of a pneumatic or air-operated controller opening and closing an air-operated control valve. Modulating or proportional control is best exemplified by an ordinary float valve, wherein the valve opening is a function of the liquid level, the lower the level the greater the valve opening. In a modulating or proportional temperature control system, valve opening or damper position is a function of temperature, either dry- or wet-bulb. The control valve or damper is normally neither fully open nor fully closed but is modulating a t some intermediate position. As a result, the controlled temperature does not cycle between limits as it does with the on-off control, but, remains steady once the system is stabilized.

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Modulating control is applicable t o all types of heating systems commonly used in tunnel dehydrators. Proportional controllers are either electrically or air operated or self-acting. Fig. 12 shows a simple potentiometer type used in conjunction with a n electrically energized control valve. A self-acting controller is shown in Fig. 13. I n most plants the dry-bulb temperature is recorded continuously, either by use of a recording type controller or a n independent recording thermometer. Ordinary mercury thermometers are frequently used t o indicate both wet- and dry-bulb temperatures.

adjustment spring

y)Be'lows assembly

FIG. 12. Simple electrically-operated (potentiometer type) proportional temperature controller (courtesy of Minneapolis-Honeywell Regulator Co.).

4. Materials

of Construction

For obvious reasons, modern tunnel dehydrators heated by direct combustion are almost invariably built of fire-proof material such as hollow concrete block, hollow tile, sheet metal, or asbestos-cement sheeting. Most of the tunnels built on the Wrest Coast in recent years have been of hollow concrete or pumice block construction. Some of the early direct-fired driers still in use are built of wood, but they are fast disappearing. As might be expected, the use of wood and other flammable materials of construction increases the cost of insuring the structure against loss due t o fire.

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F I G . 13. Self-acting temperature controller (courtesy of Taylor Instr'ument Companies).

Indirect-fired dehydrators are less vulnerable to damage by fire! and, therefore, can be built from a wider variety of materials. Usually, however, the materials of construction used are the same as those used in their direct-fired counterpart. Compartment type tunnel dehydrators are frequently of panel. construction, with wood or metal structure frame, and wood, asbestos-ce ,merit

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board, or sheet-metal panels. The panels are usually insulated, with the thermal insulation applied between the two faces of the panels. 5. Trays and Trucks

Despite numerous attempts by designers and operators of tunnel dehydrators to find a better material of construction, drying trays used in most fruit and vegetable dehydration plants in this country are still made from wooden slats. The reasons are rather simple. A good drying tray

FIG.14. Wooden drying trays (court,esy of Gentry Division, Consolidated Fo as

Corp.).

must be easy t o fabricate, inexpensive (in terms of its probable USE ul life), easily scraped clear of adhering dried material, and light but strong and rigid. Furthermore, the tray material must not contaminate the product. Few materials besides wood can be used t o build trays which will possess a majority of the desired characteristics. All-metal trays, for example, are expensive, not easily fabricated, heavy in order t o be strong and rigid, and if fabricated with wire-mesh drying surfaces, tend t o develop a permanent sag with use. In some plants wooden trays enjoy an added advantage in that they can be readily fabricated by plant personnel during the off season and a t other times when the permanent members of the operating staff would otherwise be idle. A typical dehydrator tray is shown in Fig. 14. Most trays are 3 ft. by

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6 ft., made from Ponderosa Pine, Douglas Fir, or a combination of both woods. Wooden trays suffer from one serious drawback. Material drying on the trays tends to stick t o the wood surface. During the de-traying operation some wooden splinters may pull loose and stick to the product as it is scraped off of the trays. Most of the splinters are removed during final inspection of the finished product, but unless this inspection is painstaking, enough may remain in the dried material to pose a serious contamination problem. Elimination is especially difficult in the case of leafy

F

;ed

vegetables such as cabbage. Tooden slivers and pieces of dry produce with splinters adhering to them are usually removed from the dried product by hand, an expensive operation. To minimize product sticking and consequent pulling off of splinters, some operators oil or wax their wooden trays. Leafy vegetables such as cabbage are blanched on the drying trays. Moisture absorbed by the trays during the blanching operation increases the drying load in the dehydrator. To minimize the amount of water absorbed, some plants use wooden frame trays with wire-mesh drying surfaces for the blanching-drying operation. Drying trays are conveyed through the tunnels and to the tray loading and unloading stations on either of two types of vehicles-a flanged-wheel type that runs on steel rails (see Fig. 15), or a caster-wheel type that runs either on flat surfaces or in channel irons through the tunnels.

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The rails for the flange-wheel trucks are laid flush with the concrete floor. To move trucks at right angles to their line of movement in the tunnels, turntables (see Fig. 16), or transfer cars and rails are used (see Fig. 17). Transfer rails are recessed so that the rails on the top of the transfer cars are flush with the tunnel rails. To change direction of movement, tray trucks are pushdd onto the transfer cars and moved at right angles to their previous direction of travel.

FIG. 16. Turntable for tray trucks (courtesy of Gentry Division, Consolidated Foods Corp.).

IV. TYPICAL COMMERCIAL TUNNELDEHYDRATORS 1. Twin-Tunnel CounterJlow Dehydrator

Popularly known as a Puccinelli dehydrator (R. L. Puccinelli, prominent in the development of prune dehydration in California during the 1920's and still actively engaged in the business), the simple twin-tunnel counterflow drier is widely used for drying both fruits and vegetables, par-

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ticularly on the West Coast. The basic elements of the drier are shown diagrammatically in Fig. 18, consisting essentially of a direct-fired combustion chamber and a blower, both located in an air passage between the two drying tunnels. Air enters the drier through openings surrounding the front of the combustion chamber, is heated to the proper drying temperature by the direct combustion heater, and is discharged by the blower into the two drying tunnels. Upon leaving the tunnels, part of the air is recirculated via the central air passage while the balance is exhausted to the atmosphere via overhead discharge ducts.

FIG.17. Transfer car and tracks (courtesy of Gentry Division, Consolidated Foolds Gorp.).

A variation of this twin-tunnel arrangement places the direct-fired heater and the blower in an air passage located above the two drying tunnels, which are arranged side by side. Being custom-built in most cases, Puccinelli-type dehydrators vary somewhat in size. A typical unit would have drying tunnels about 6 ft. 4 in. wide by 7 f t . high (inside dimensions), with a central air passage of equivalent height, and a width of 9 f t . A drier accommodating in each drying tunnel 12 truckloads of trays measuring 3 ft. by 6 ft., would have an over-all length of approximately 50 ft. This allows about 7 f t . for the air stream t o straighten out before entering the trays, after making the turn from the central air passage into the drying tunnels. About 5 f t . of space is left a t the opposite end of each tunnel for the air to enter the

P

3 /-FIG.18. Twin-tunnel counterflow dehydrator.

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exhaust duct or t o be recirculated back into the central air passage. A shutter or sliding door is usually installed a t each dry-end air opening t o shut off the flow of hot drying air into the tunnels during loading and unloading operations. Total air movement and heating capacity vary from about 30,000 cu. ft. per min. and 3,000,000 B.t.u. per hr. for a fruit drier, t o 50,000 cu. f t . per min. and 5,000,000 B.t.u. per hr. for a vegetable drier. A typical up-to-date Puccinelli-type dehydrator would have hollow concrete or pumice-block walls, prestressed hollow concrete block roof slabs, and wooden-frame, metal-clad, center opening end doors. The drier would be equipped with a natural gas burner complete with a simple unlined sheet-metal combustion chamber, a direct drive tubeaxial blower with the motor cooled by a suction duct, wooden trays measuring 3 ft,. by 6 ft., flanged-wheel tray trucks running on rails set flush with the concrete floor, and a modulating dry-bulb temperature recording-controller. 2. The Miller Tunnel Dehydrator

The Miller dehydrator (L. N. Miller Dehydrator Company, Eugene, Oregon) is widely used in the Pacific Northwest for drying fruits such as

FIG.19. Miller tunnel dehydrator (elevation).

apples and prunes. The drier (see Fig. 19) is basically an indirect-fired, counterflow tunnel dehydrator, but of a special type. To create conditions believed t o be desirable for fruit drying, the Miller dehydrator is equipped with shutters or louvers located above the tray trucks in the middle third of the drier. The shutters are adjustable, t o vary the amount of air bypassing successive truckloads of product. The adjustable shutters make possible some measure of control of the humidity of the drying air in the various parts of the drying tunnel. By opening the shutters, for example, the relative humidity of the air a t the wet end of the drier can be increased. The resulting decrease in the rate of drying prevents the prune skins from

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cracking due t o “case-hardening,” with consequent loss of juice from the fruit. Drying trays are usually of wood, measuring 3 ft. by 3 ft., and stacked 24 t o a truck. Truck arrangement within the drier varies. Depending upon drier capacity, trucks are either 2 abreast or 3 abreast, with 10 t o 12 trucks per row. A typical tunnel would, for example, hold 36 trucks, 3 wide by 12 long. The cross-sectional dimensions of such a tunnel would be 7 ft. 7 in. high by 9 ft. 8 in. wide. Gross air velocity in the empty tunnel, with the shutters closed, is usually not more than 500 ft. per min. Trucks are of the caster-wheel type, running on steel tracks while inside the drier and directly on the concrete floor when outside. The drier is of panel construction, with the insulated, galvanized, sheet-metal-clad panels bolted t o an angle iron frame. The indirect air-heating system is commonly of the combustion gasto-drying air type, consisting of a combustion chamber and flue pipes t o transfer the heat from the combustion gases t o the drying air. Steamto-air heaters are used, but are less common. 3. The Carrier Compartment Drier

During World War 11, the Carrier Corporation of Syracuse, New York manufactured a vegetable drier which in many ways is typical of compartment-type tunnel dehydrators used commercially. The drier consists essentially of a steam-to-air preheater section, 6 drying compartments or sections each equipped with its own blower and steam-to-air heater, and a n exhaust air fan section, arranged so that truckloads of trayed material are progressively pushed through the tunnel formed by the 6 compartments in series (see Fig. 20). Although the air flow across the trays is in a direction transverse t o the direction of truck movement, the drier is essentially a counterflow unit, with the wet material entering the drier a t the end where the exhaust air is discharged from the tunnel. Fresh air enters a t the opposite end, through the steam-to-air preheater. From the preheater, the partially heated air enters the steam-to-air heater and blower units of the individual drying sections. Perforated baffles progressively decrease the amount of fresh air taken in a t each compartment (thereby progressively increasing the amount of air recirculated a t each drying section), starting a t the dry end of the dehydrator and ending a t the wet end. The mixture of fresh and recirculated air is reheated a t each stage of drying, the amount of reheating automatically controlled by a separate dry-bulb temperature controller a t each compartment. The amount of air discharged from the drier by the exhaust fan, and consequently the amount of fresh make-up air entering the drier, is controlled by the wet-bulb temperature of the exhaust air.

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I n operation, the product moving through the drier is subjected t o a different temperature condition at each compartment, and the direction of air flow over the material is reversed at each succeeding drying section. Although basically a counterflow unit, the Carrier dehydrator has some of the characteristics of a two-stage drier. Both air flow and drying temperature are higher at the 2 wet-end drying sections than they are a t the other 4 sections. This is made possible by using larger heater and blower units at the 2 wet-end compartments. EXHAUST

EXHAVST FAN UNIT

H

PLAN VIEW

STEAM-AIR P ~ E H E ~ T E R S

PERFORATED PLATE BAFFLE

TYPICAL CROSS SECTION FIG.20. Diagrammatic sketch of Carrier Compartment Drier.

Air velocities in the four dry-end and two wet-end sections are 700 and 1200 ft. per min., respectively, through the free area of the loaded trucks. When operated at maximum capacity under high ambient moisture conditions, fresh make-up air enters the drier a t a rate of approximately 20,000 cu. ft. per min. Steam consumption under these conditions is about 5500 Ib. per hr. Trays are either of wood or metal, measuring 36 in. by 36 in., and are stacked forty to a truck. The Carrier dehydrator has a nominal rating of 30 tons of wet vegetables per 24 hr., based on a tray loading of 1.2 lb. per sq. ft. and 2 hr. drying time to reduce the moisture content of the product t o a level suitable for bin finishing.

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V. CRITERIAFOR SELECTIONOF TUNNEL DEHYDRATORS Numerous factors govern the selection of the drying system, or type of dehydrator, chosen for a particular plant. Economic factors, such as initial and installation costs and operating labor costs, although important, are not the only criteria for proper selection. Some of the other important considerations are listed below : (1) Flexibility with respect t o integrated plant operation. (2) Ability t o handle a wide range of commodities, if required or if the current economic demand changes. (3) Ability t o dry the product to meet current specifications for moisture content, product damage tolerances, etc. (4) Adaptability t o meet future contract specification changes for the product. (5) Floor-space requirements. (6) Capacity requirements, current and future; and the possible use of other final driers, such as finishing bins. (7) Availability of critical materials of construction and precision machine parts in case of emergencies. (8) Mechanical reliability and foolproofness t o guard against complete plant shut-down. Truck-and-tray type tunnel dehydrators are generally satisfactory for drying most of the various fruit and vegetable commodities which are processed in piece form. This type of drier can be used t o dehydrate a wide range of products and can be operated continuously or intermittently as desired. These factors add t o the flexibility of plant operation. The type of drier used influences, to some extent, the characteristics of the finished product. I n general, multistage driers permit the use of higher temperatures during the initial part of the drying cycle when the product’s surface is still moist, and consequently the product dries faster. This combination of higher temperatures and shorter drying time often produces a more porous and bulky material. The greater porosity of the finished product makes reconstitution faster and easier. However, the greater bulk may cause difficulty in meeting some contract specifications, i.e., getting the required weight in the containers. The counterflow tunnel is, perhaps, the most versatile of the truckand-tunnel driers for the dehydration of fruits and vegetables. These units are relatively easy t o operate and are of comparatively simple design. I n the fruit dehydration industry, the counterflow truck-andtunnel drier is the type most widely used, and the design has been more or less standardized (Perry, 1947, and Perry and associates, 1946). Tunnel-and-truck dehydrators with one, two and three stages may

346

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

be found operating satisfactorily in the vegetable dehydration industry, and they have been built by many different people. As previously mentioned in section I1 (Classification of Tunnel Dehydrators), the design has not been universally standardized. Broadly speaking, two-stage tunnel driers are preferable t o singlestage units for vegetable dehydration ij" the commodity has a very high percentage of water that can be removed rapidly (such as cabbage), or, if the commodity cannot tolerate a high final drying temperature (onions, 135 to 140" F.). The choice between one-stage and two-stage tunnel driers becomes more or less an arbitrary decision for operators drying commodities which have a relatively low initial moisture content and which can tolerate a reasonably high final drying temperature, for example, carrots. For a given capacity, the single-stage drier would occupy substantially the same floor space as the two-stage unit, provided the stages in the latter were not physically separated. Multiple-stage construction permits the use of drying conditions which change in a predetermined manner as the material progresses through the tunnel. This flexibility offers distinct advantages in operation. Moreover, the multistage unit, when compared t o a single-sbage dehydrator, provides more rapid drying of the product and somewhat better heat economy. Against the advantages for the multistage unit, there should be weighed the higher capital cost, the increased labor cost (unless an additional investment is made for an automatic mechanism to handle the trucks between stages), and the increased complexity of operation. Some of the justification for two- or three-stage operation is. also being weakened by the increased reliance upon finishing bins to accomplish the late stages of drying. There are also certain general basic considerations which should govern the selection of the drying system. Driers installed in multiples are less likely to cause a complete plant shutdown due to mechanical failure. Drying systems of proved or unquestioned performance have advantages since there is a calculated risk for each installation and the individual must decide how much of a pioneer he can afford to be. Initial cost of the equipment is often not as important in determining production costs, as is the effectiveness of the equipment chosen. For example, assume that two drying systems are available, one of which involves a sizeable capital investment, but has been proved t o be efficient and foolproof, and the other involves a modest investment, but is of doubtful efficiency and surety of operation. There is little question but that, generally, the more expensive unit should be given preference. Comparison of the truck-and-tray tunnel dehydrator with the continuous belt-conveyer dehydrator for a specific drying operation should take into account a t least the following general considerations.

TUNNEL DEHYDRATORS FOR FRUITS AND VEGETABLES

347

Assuming both units have the same drying capacity under the specified conditions, the truck-and-tunnel arrangement has the following advantages : (1) The installed cost of the system will probably be less. (During the war, the cost of a simple tunnel was considerably less.) (2) Smaller quantities of critical materials are required for construction, and fewer precision-fabricated parts are needed (an important consideration in wartime). (3) Operation can be either intermittent or continuous. (4) A wider range of commodities can be dried satisfactorily. (For example, whole or halved fruit, such as prunes or peaches, are unsuited t o conveyer-belt drying because of the long drying time involved. Shredded cabbage is likewise unsuited because of the tendency for the blanched material t o mat and not dry uniformly.) On the other hand, the conveyor drier has certain advantages over the truck-and-tunnel dehydrator of' a similar drying capacity, for example: (a) Less floor space is needed. (b) The dryiiig time can be made shorter, and product quality may thereby be improved. (c) Less operating labor is required since the conveyer drier is fundamentally automatic in operation. OF TUNNEL DEHYDRATORS VI. BASIC THEORY

The quantitative theory of drying, applicable t o the design and control of dehydrators for fruits and vegetables, is the work of many investigators, nearly all within the past 50 years. Advances in development of the theory, especially pertinent t o the subject of this chapter, were made by Grosvenor, 1908; Carrier, 1911, 1921 ; Hausbrand, 1912; Tiemann, 1917; Lewis, 1921; Cruess and Christie, 1921b; Sherwood, 1929-1932, 1936; Newman, 1931a,b; McCready and McCabe, 1933; Bateman et al., 1939; Hougen et al., 1940; Van Arsdel, 1942, 1947, 1951a; Marshall, 1942, 1923; Brown and Kilpatrick, 1923; Cruess and hlackinney, 1943: Perry, 1944; Perry et al., 1916; Ede and Hales, 1948; Marshall and Friedman, 1950; Broughton and Mickley, 1953; and Hendel et al., 1954. Several of the earlier discussions of tunnel design, in the absence of quantitative information about the effects of temperature, humidity, air velocity, and other factors on the drying rates of specific commodities, simply assumed that the time required to dry a commodity was wellestablished by practice and on that basis computed the requisite air flow and heat input by methods established in the field of heating and ventilating engineering. This procedure of course forswore any adventure off well-trodden paths. Lewis (1921), followed by Sherwood and several of his other colleagues at the Massachusetts Institute of Technology and by a number of other chemical engineering investigators, noted that many wet materials exhibit two sharply distinct phases of drying be-

348

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

havior under constant drying conditions: an initial phase of constant rate of loss of water per unit surface exposed to the air, and a final phase during which the drying rate falls steadily toward zero. Generalized correlations of drying rate with vapor pressure relations and air velocity were derived, and these in turn were applied to the heat-balance and mass-balance equations characteristic of practical dehydrators. Good correspondence of prediction with experience was obtained in the drying of several industrial materials, and Perry (1944) and his co-workers used the same principles successfully in designing prune dehydrators. Van Arsdel (1942) and Brown and Kilpatrick (1943), concerned a t that time with the dehydration of vegetables to quite low levels of moisture content, found that drying rates determined experimentally could not be represented satisfactorily by any simple mathematical formula. They and their colleagues at the Western Regional Research Laboratory published a series of bulletins (AIC-3 1, I-VIII, 1943-1947) which summarized the drying behavior of a number of common vegetable materials in the form of nomographs readily applicable to dehydrator calculations. Broughton and Mickley (1953) have made the final step in the retreat from Lewis’ highly idealized picture of drying behavior by basing dehydrator design upon an actual analog” drying experiment in which the temperature and moisture-content history of the experimental material serves directly as the basis for the design. Their procedure obviates the difficulty, recognized but not fully overcome by earlier investigators, that the drying rate of a hydrophilic material at any instant depends to some extent upon the previous drying history of the sample (the internal distribution of moisture within the pieces is determined by that history). Some of the later AIC-31 nomographs contain a correction factor intended to deal approximately with this situation. Van Arsdel (1942), noting that fruit and vegetable dehydration presented a special case in which the heat absorbed in evaporation of water far outweighed all other causes of heat usage, proposed the following theorem: I n any section of a tunnel dehydrator where no reheating of the air takes place, the change in air temperature i s proportional to the change in moisture content of the material, if moisture content is expressed on the ,‘dry basis.” This is, expressed in differential form, dt dx

-

=

b -dw” dz

Under these special conditions the tunnel acts substantially like an adiabatic humidifier. It was already well-known that in such an adiabatic system the wet-bulb temperature of the air remains constant, and that, at

* Refer to list of symbols on p. 369.

TUNNEL DEHYDRATORS FOR FRUITS -4ND VEGETABLES

349

constant wet-bulb temperature, the fall in temperature of the air is very nearly a linear function of its rise in absolute humidity. A mass balance relating the loss of moisture by the material t o the uptake of water vapor by the air then led directly t o equation 1. Wet-bulb temperature lines on a humidity chart using temperatures as abscissas and absolute humidities as ordinates are slightly curved and differ slightly in slope. The actual variations within the general range of interest in fruit and vegetable dehydration are as follows: Wet-bulb temperature 90" F. : Fall in air temperature per 0.001 rise in absolute humidity: Air temperature 120' F., 4.28" Air temperature 180" F., 4.35" Wet-bulb temperature 120" F. : Fall in air temperature per 0.001 rise in absolute humidity: Air temperature 140" F., 3.81" Air temperature 200" F., 3.96" Van Arsdel (1942) suggested that for general exploratory computations a temperature change of 5" F. per 0.001 change in humidity be used; this would correspond t o a lumped total of about 20% for all heat losses; a t the same time, however, he proposed that wet-bulb temperature be taken as constant in spite of heat losses. If the 5" F. figure is accepted, the coefficient b in equation 1 can be evaluated readily, and the equation becomes,

The plus sign will be used for a parallel-flow arrangement, the minus sign for counterflow. Brown (1943) and Lazar (1944) examined this approximation critically, and concluded that for the range of conditions encountered in parallel-flow and counterflow tunnel dehydrators for vegetables the errors t o be expected from i t are smaller than the other inherent uncertainties of such systems. The wet-bulb temperature in practical tunnels will usually fall less than 1" F. between the hot end and the cool end of the tunnel. Perry (1947) computed the heat balance for a typical counterflow prune dehydrator, with a hot-end air temperature of 165" F . and wet-bulb temperature of 110" F . ; the wet-bulb temperature of the exhaust air was calculated t o be 109.6" F. The British Ministry of Foods, in its bulletin ((VegetableDehydration" (1946) described the performance of two-stage tunnels in potato dehydration; in the first stage the fall in air temperature * Refer to list of symbols on p. 369.

350

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

averaged 4.6" F. for each increase of 0.001 in absolute humidity, and the fall in wet-bulb temperature through the tunnel was somewhat less than 0.5" F. The following discussion of theoretical tunnel behavior is based on computations made by use of the approximate relationship of equation (2). I . Theoretical Tunnel Behavior Figure 21 shows the computed course of moisture content of prunes being dehydrated in a counterflow tunnel and the change in air temperature as i t passes through the tunnel. The conditions assumed were those 70 I-

z 0

60

(r:

W

a v, cn

c

3

I-

w

- I70

50

40

3

u

W

LT 3

0

30

20

2

130

10

0

6

12

18

TI ME, HOURS FIG.21. Moisture content of fruit and temperature of air in counterflow tunnel drying prunes, computed from drying-rate expression derived by Guillou (1942). given by Perry et al. (1946), in Figs. 14 and 15, and the drying rate expression used in the computation was t h a t published by Guillou (1942), which correlates the drying rate with air temperature, humidity, and velocity and with the size of the fruit. The computed curves agree well with the curves of Fig. 14 in the cited publication by Perry and his associates, which represent data secured from commercial counterflow tunnels. Figure 22 is a similar diagram showing computed conditions in a counterflow tunnel dehydrating white potato half-dice in. x in. x in. in the wet state). The conditions assumed for the example, which is taken from Van Arsdel's (1951b) Fig. 7, are substantially those followed in commercial dehydration. The drying rate expression used in the com-

x~

(x

TUNNEL DEHYDRATORS FOR FRUITS AND VEGETABLES

351

putation is given by the group of nomographs in AIC-31-VII, Western Regional Research Laboratory (1945). The pictures presented by Figs. 21 and 22 exhibit the characteristic behavior of counterflow tunnels: maintenance of a relatively high air temperature through much of the tunnel, somewhat slow initial drying of the wet material, and good drying conditions a t the dry end of the tunnel. If the drying rate characteristics of a product have been determined, a single composite diagram can be used t o summarize the behavior of that product in counterflow tunnels. Figure 23 is such a diagram for the drying of prunes, published by Perry et al. (1916), Fig. 15. Figure 24 is a similar L

AIR TEMPERATURE

150 140

130 h IT

c

120

W

a E

110 W

2 0

E

0' 0

I 2

4

6

8

TIME, HOURS FIG.22. Moisture content of material and temperature of air in counterflow tunnel drying potato half-dice, computed from drying-rate nomographs of AIC-31-VII.

diagram for the counterflow drying of potato half-dice, published by Van Arsdel (1951b), Fig. 8. The general similarity of the two diagrams is striking, only the time scales being substantially different. The characteristics of a parallel-flow tunnel are, of course, the reverse of those of a counterflow tunnel; a relatively low air temperature prevails throughout much of the tunnel, very rapid evaporation occurs a t the wet end, and drying conditions are poorest a t the dry end of the tunnel. Van Arsdel (1951b), Fig. 10, illustrates the contrast in a n example reproduced in Fig. 25, showing the behavior of counterflow and parallelAow tunnels of the same length and same air flow, operated in such a way as t o produce the same hourly output of dehydrated potato half-dice. Van Arsdel points out t h a t whereas the most striking contrast is in the heat consumption of the two arrangements (almost 50% greater for the parallel-flow tunnel) there will also certainly be differences in the quality characteristics of the products turned out. Material dried in the parallel-

352

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

Net tunnel length, ft.

FIG.23. Relation between air velocity and drying time, with final exhaust temperature also given, for counterflow prune dehydrators of various lengths. Initial air temperature 165" F., wet-bulb temperature not over 105" F. Initial prune moisture content 70%, final prune moisture content 16.7%. Prune size, dry count of 50 per pound (from Perry et al., 1946, Fig. 15).

4

5

6

7 8

9

10 I I

12 13 14 15

16

Number of active trucks in tunnel

FIG.24. Relation between air velocity and drying time, with final exhaust temperature also given, for counterflow potato half-dice dehydrators of various lengths. Initial air temperature 150" F., wet-bulb temperature 85" F. Initial moisture content of material 76% final moisture content 6%. Tray-loading, 1.50 lb. per sq. ft., trucks contain 540 sq. ft. active tray surface (adapted from Van Arsdel, 1951b, Fig. 8).

T U N N E L DEHYDRATORS FOR FRUITS AND VEGETABLES

353

flow tunnel will be the more bulky because shrinkage stresses will open up internal voids in the pieces. Some difference in the extent of "heat damage " suffered during dehydration may also be apparent, damage probably being slightly greater in the counterff ow-dried material. If only the first stage in a multiple-stage dehydration system is being considered, there can be no doubt that the parallel-flow arrangement offers substantial advantages over counterflow. I n such a system product I90

180 W'

5

5

5 5 IQ

170 160 150 140

130 120 110

80

cn v,

70

a

m

60

I-+

50

w z

40

30

I-

cn0

I

20 10 n

" 0

I

2

3

4

5

6

7

T IME, HOURS FIG.25. Comparison of counterflow and parallel-flow drying; moisture content of material and temperature of air in tunnels drying potato half-dice, under conditions chosen to make outputs equal. Twelve trucks in each tunnel, 540 sq. ft. per truck, loading 1.50 lb. per sq. ft. Air velocity 1000 ft. per min. between trays. In counterflow drying, initial air temperature 150" F., wet-bulb temperature 85" F. In parallel-flow drying, initial air temperature 185" F., wet-bulb temperature 90" F. (from Van Arsdel, 1951b, Fig. 10).

is discharged from the first-stage tunnel still somewhat moist; the relatively poor drying conditions a t the dry end do not matter so much. The evaporative capacity of the tunnel can be increased substantially by raising the temperature of the inlet air t o a point far above that which would be safe for a counterflow tunnel. Van Arsdel (1951b) presents an example of first-stage driers for potato half-dice in which a parallel-flow tunnel, operated a t an inlet temperature of 200' F., turns out almost 50%

354

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

more material dried to 16.7% moisture than a counterflow tunnel of the same length, operated a t an inlet temperature of 145" F. Two-stage tunnels now generally consist of a parallel-flow first stage and counterflow second stage, the arrangement first suggested by Eidt (1938) for the dehydration of apples. The course of moisture content and air temperature in such a two-stage tunnel, as used for the dehydration 220 0 -

2

200

180

.=I

160

5 n 2

120

I-

100

W

m

Lg

-0 3w W E

5: Is

0 I

140

80 80

70 60 50 40

30 20

10

0

0

1

2

3

4

5

6

7

TIME, HOURS FIG.26. Moisture content of material and temperature of air in a, two-stage tunnel drying potato half-dice. Eight trucks (540 sq. f t . of tray surface in each) in primary stage, 16 trucks in secondary. Tray loading, 1.50 Ib. per sq. ft. Air velocity between trays, 1000 ft. per min. (from Van Arsdel, 1951b, Fig. 15).

of potato half-dice, are shown in Fig. 26, taken from a n example computed by Van Arsdel (195lb), Fig. 15. I n this case the secondary tunnel is twice as long as the primary. Van Arsdel showed that the combination, carrying altogether 24 truckloads of material, should produce about 7 % more product dried t o 6 % moisture than two 12-truck counterflow tunnels and use about 5 % less fuel. T o offset these advantages, the capital cost of the two-stage dehydrator would be somewhat greater, especially if truck transfer between stages were handled automatically, and the control of the unit would be more complex and critical with respect to maintenance of product quality.

T U N N E L DEHYDRATORS FOR F R U I T S AND VEGETABLES

355

The ratio of lengths of the two stages of this type of dehydrator apparently can vary within rather wide limits without much effect on performance. Commercially they have ranged between equality in length and 2 t o 1. Theoretically, for the example of Fig. 26, maximum output would have been realized if the total of 24 trucks were divided 7 t o 1, that is, 3 trucks in the parallel-flow primary, 21 trucks in the counterflow secondary; however, the gain in output would have been negligible, and operation of the 3-truck primary would make extreme demands on control and scheduling of the system. None of the theories referred t o above is directly applicable t o study of the operation of transverse-flow compartment tunnels. I n commercial forms of these dehydrators there may be as many as 6 or 8 compartments in which the air temperature is individually controlled a t desired levels. A truckload of material standing a t any one of these positions is exposed t o unvarying drying conditions until i t is shifted t o the next position, where the direction of air flow is reversed and a new set of drying conditions is maintained. No satisfactory mathematical formulation has been proposed for this kind of system. Prediction of the performance of a transverse-flow dehydrator, with given values for the air flow and air temperature a t each compartment, could presumably be accomplished by means of a n analog type of drying experiment somewhat similar t o t h a t proposed by Broughton and Mickley (1953). 2. Optimum Tray Loading

Drying-rate experiments have invariably shown that the rate of drying of wet materials spread on trays decreases as the load of material on the trays increases, once the load exceeds appreciably that of a single layer of pieces on the tray. The drying time is therefore shortest for light tray loadings. But for a fixed area of tray surface in a dehydrator, the output of produce is proportional t o the tray loading. Net dehydrator capacity is the resultant of these two effects. Quantitative estimations have been made of the effect of tray loading on dehydrator output, for 2 quite different vegetable products, potato half-dice and cabbage shreds. The drying-rate data are those of the ATC-31-VII and AIC-31-IV nomographs. Trucks in the tunnel postulated for these examples each contain 400 sq. ft. of useful tray surface. The mass air flow through the tunnel is 2000 lb. of dry air per min. and the air velocity between trays a t the wet end of the tunnel will be 900-1000 ft. per minute. For cabbage, a hot-end temperature of 140" F. and a wet-bulb temperature of 90" F. are assumed in counterflow drying, 180" F. and 95" F. in parallel-flow drying; for potatoes, a hot-end temperature of 150' F. and a wet-bulb temperature of 90" F . in counterflow drying,

356

P.

w.

KILPATRICK,

20 -

E. LOWE, AND

w.

B. VAN ARSDEL

PARALLEL-

CABBAGE SHREDS

0.5 I.o 1.5 2.0 TRAY LOADING, LBS./S 0. FT. FIG. 27. Effect of tray-loading upon tunnel capacity in dehydrating cabbage shreds; counterflow tunnel, 9 trucks, drying to 4.75 % moisture; parallel-flow tunnel, 12 trucks, drying only to 9.1 % moisture. 0

35

t

I

PARALLEL-

-

POTATO HAL F-DICE

0" 0

I

2

3

T RAY L 0AD 1NG, L BS./ S 0.F T FIG.28. Effect of tray-loading upon tunnel capacity in dehydrating potato halfdice; counterflow tunnel, 10 trucks, drying to 6.55 % moisture; parallel-flow tunnel, 12 trucks, drying only to 16.7% moisture.

TUNNEL DEHYDRATORS FOR FRUITS AND VEGETABLES

357

200' F. and 100" F. in parallel-flow drying. Final moisture content, for counterflow operation, is 4.75% (wet basis) for cabbage, 6.55% for potatoes; for parallel-flow operation (used as the first stage in multiplestage dehydration), 9.1 % for cabbage, 16.7% for potatoes. Figures 27 and 28 show estimated tunnel capacity for dehydration of cabbage shreds and potato half-dice (tons of prepared material per 24-hr. day) as a function of tray loading, pounds of prepared material per square foot. Tunnel capacity reaches substantially a maximum at a loading of about 1.5 lb. per sq. ft. for the cabbage shreds, something over 3 lb. per sq. ft. for the potato half-dice. The figures for drying time marked on the curves indicate that, if the tunnel is being operated near its maximum loading, a material shortening of drying time may be accomplished with only minor sacrifice of capacity by lightening the tray loading. If heat damage is being experienced, this may be a n important remedial measure. 3 . Optimum Recirculation of

Air

The reasons for, and advantages and disadvantages of recirculation of air in a tunnel dehydrator have been discussed by Van ArsdeI (195la, pp. 80-84). Recirculation raises the wet-bulb temperature of the air, returns some heat t o the system and thereby saves fuel, but a t the same time reduces the drying rate of most materials. Purely from a drying cost standpoint, the saving in fuel cost must be balanced against the decrease in production and the attendant increases in other unit costs. Ramage and Rasmussen (1943) noted that there should be some proportion of recirculation t h a t would give the minimum drying cost per pound of product, and they computed this optimum for one simple set of conditions. So many combinations of the numerous variables are possible that no single general principle has been established. The following example illustrates the procedure and typifies the kind of results that map be computed A counterflow tunnel long enough t o hold'a maximum of eleven %foot trucks (400 sq. ft. of useful tray surface on each truck) is t o be used t o dehydrate potato half-dice t o 6.55% moisture (wet basis) or cabbage shreds t o 4.75% moisture. Trays will be loaded with 1.40 lb. of prepared cabbage or 2.50 lb. of prepared potato per sq. f t . The fan supplying the air flow through a pair of these tunnels is of the "limit-load" type, double width, with a standard air rating of 54,000 cu. ft. per min. against 1.5 in. static pressure, when operated at 423 r.p.m. and absorbing approximately 17 h.p. The hot-end temperature for the cabbage dehydration will be 140" F., for the potato dehydration 150" F. The power absorbed by the fan a t a temperature of 140-150" F. is approximately 15 h.p. The outside fresh air temperature is 60" F., absolute hu-midity is 0.0100 lb. of water

358

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

vapor per lb. dry air. The tunnel will be operated with a minimum of a 15"-wet-bulb depression a t the cool end, and, if a n increase in recirculation would result in a lower wet-bulb depression than that, the load on the tunnel will be reduced by decreasing the number of trucks in it. The cost of heat is taken as 35# per million B.t.u. transferred t o the air stream, the cost of power 26 per kw.-hr., operating labor cost for each side of the double tunnel $1.45 per hr. regardless of output within the range considered in the example, and all other costs (plant overhead and fixed charges) $1.35 per hr.

c

t

t

7 OTHER

LABOR

POWER

n "

00

90

100

W E T-BULB

0

Q20

0.40

0.60

0.00

1.00

PROPORTlON CF AIR REClRCULATED

FIG.29. Effect of air recirculation upon drying costs: shredded cabbage, counterflow tunnel.

The computations involve estimation of drying time for selected values of the wet-bulb temperature, using the AIC-31 nomographs, and then deriving the corresponding values of tunnel output, proportion of air recirculated, and necessary heat input. The approximations described by Van Arsdel (1951a) were used. The results of the computations are shown graphically in Figs. 29 and 30. It is evident from these curves that, for the combination of conditions chosen, the minimum total cost for drying both vegetables would be realized by employing little or no recirculation of the air; the saving in cost of heat through recirculation would be more than offset by the increases in other costs. Quite a different result would have appeared if a material like prunes were being dehydrated. According to Guillou (1942) and Perry (1944), the drying rate of prunes is substantially independent of the relative humidity of the drying air unless the relative humidity exceeds about 35 %. T ha t being the case, raising the wet-bulb temperature by increasing

359

TUNNEL DEHYDRATORS F O R F R U I T S AND VEGETABLES

the proportion of recirculation within reasonable limits will decrease the cost of heat materially without a t the same time increasing other costs.

/

8 TRUCKS

TOTAL

8 TOTAL

z

w o 0

I

POWER

80

100

120

WET- BULB TE MP ERAT URE

0

0.20

0.40

0.60

a00

1.00

PROPORTION OF AIR REClRCULATED

FIG.30. Effect of air recirculation upon drying costs: potato half-dice, counterflow tunnel.

No data are available t o indicate whether other fruits, such as grapes and sliced apples, will behave more like the cut vegetables than like prunes.

4. Product Temperature in the Dehydrator Perry (1944) and Perry et al. (1946) have published measurements of the internal temperature of prunes during dehydration. Figure 31, taken

I-

100 } ~

0

6

I2

18

24

TIME, HOURS FIG.31. Air temperature and fruit temperature in a typical counterflow prune tunnel (from Perry et al., 1946, Fig. 14).

from the second of these references, illustrates the typical course of fruit temperature during counterflow dehydration, with the air temperature

360

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

also shown for comparison. There appears to be no tendency for the fruit temperature t o hold for a period of time a t or near 105" F., the wet-bulb temperature of the air. No similar measurements of material temperature of vegetable pieces undergoing dehydration have been published, in spite of the obvious importance of the time-temperature relationship t o the quality of the dry product. The following procedure leads t o a rough approximation t o the material temperature under typical tunnel conditions. There is some evidence that during early stages of drying the shrinkage in volume of vegetable pieces very nearly equals the volume of water lost by evaporation, but in later stages the volume shrinkage is less, and no substantial further decrease in volume occurs as the pieces dry below about 15 t o 20% moisture. If the density of the dry substance is 1.25 g. per ml., the piece area during early stages should change as follows:

(3) Study of drying rates for potatoes shows th at this relationship holds down t o about w = 1.50 lb. of moisture per lb. dry solids (60% moisture) for potato pieces. A graph of area versus moisture content can be extrapolated with only moderate curvature t o an expected final dry area of 43 % of the original area. If it is now assumed that the drying rate in very early stages maintains the material a t the wet-bulb temperature of the air by convective heat transfer (radiative and conductive transfer being neglected), a convective heat transfer coefficient can be computed from the following relationship:

-H,

~.Lo

w,,+ 1

dw -

(4)

d6

The transfer coefficient k is expressed here in terms of unit area of tray surface. Now if it be further assumed th at this transfer coefficient will remain unchanged throughout the entire tunnel, allowance being made for the area shrinkage of the material, the piece temperature will be given approximately by the following equation : T = t f

HL,

dw*

-+ 1) '2Z

i%a(w,

(5)

Material temperatures computed in this way, and the corresponding * Refer to list of symbols on p. 369.

T U N N E L DEHYDRATORS FOR FRUITS AND VEGETABLES

160

g

L

,

4 120

E

220

I \

2oo-l \

PRIMARY

7

180 -

w

[L

3

160-

\ \ \

SECONDARY

STAGE

','

STAGE

AIR TEMPERATURE

WET-BULB I

TEMPERATURE

361

362

P. W. KILPATRICK, E. LOWE, AND W. B. VAN ARSDEL

can easily lead to the serious quality defect variously known as “heat damage,” ‘‘ caramelization,” L‘scorching,’l or simply “browning.” A product which has been damaged in this way by the drying step may be unacceptable as a food product because of off-color and off-flavor. The phenomenon, by whatever name called, is the result of a highly complex system of reactions in and among the natural constituents of the fruit or vegetable. Its chemistry is beyond the scope of this article. T h e two points of importance here are these: (a) the reactions leading t o (‘browning” or ‘(scorching” have a very large positive temperature coefficient; (b) the reaction rate increases as the concentration of the components increases (i.e., as the moisture content of the product is reduced), but water itself must be involved in the reactions in some way, because when the moisture content is reduced far enough the rate of browning becomes very low. The result is that, in some intermediate range of moisture content, the rate of browning a t any given temperature goes through a maximum. Studies of the kinetics of this system have been reported by Hendel et al. (1954). For white potato the maximum browning rate occurs in the range of 15 t o 20% moisture content. When sufficient quantitative information becomes available, it may for the first time be possible t o devise dehydrator-operating procedures for an optimum combination of high output and minimum heat damage. 5. Departures from Theory The foregoing discussion of tunnel theory ignores several complications of actual dehydrator operation. One of these is inherent in the nature of a tunnel-and-truck drier, whereas the others may be classed as inevibable imperfections of design and operation. The simple theory th at has been presented assumes that the tunnel is loaded continuously and operates in the steady state so that the material moisture content and the air temperature and humidity a t any given point along the length of the tunnel will remain unvarying. The fact that truckloads of material are introduced a t finite intervals of time and are advanced through the tunnel in discrete steps, introduces a complication for which no mathematical description has been proposed. The temperature t o which a product is exposed in its progress through the tunnel changes discontinuously, not along a smooth curve. This so-called “sawtooth effect” has been discussed by Van Arsdel (1951a). Practical experience suggests t hat its effect cannot be very great, except possibly in short parallel-flow tunnels. The imperfections of construction and operation, on the other hand, sometimes may have very serious effects. The commonest causes of trouble are poor air distribution, temperature stratification in the air,

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uneven loading of trays, and poorly designed or sagging trays. Even with good design and careful maintenance and operation, these troubles will always be present t o some extent. Since they obviously cannot be embodied in the theory in any quantitative way, the dehydrator operator must learn by experience how much of a safety factor he must apply t o the predictions of dehydrator theory.

VII. OPERATINGPROCEDURES FOR TUNNEL DEHYDRATORS Methods for dehydrating fruits and vegetables have been and are constantly being improved, for competition is keen and i t is necessary t o increase product quality and general efficiency. Although the dehydration a r t is well established, the drying characteristics of most vegetables and fruits are known only in rather general terms. Conditions and operation procedures differ from plant t o plant, and general recommendations may not apply t o a particular dehydrator. Therefore, only certain items of general interest will be brought t o the reader's attention, and details of operating a tunnel drier will be described only briefly. From the discussion which follows, it will become apparent that, t o achieve best results, each operator must take into consideration the physical setup and conditions present in his plant and use his initiative to secure the best combinations for high product quality, capacity, and over-all efficiency. As illustrated by the series of curves (Figs. 23 and 24), and as previously mentioned, if the tunnel length, air velocity, and initial temperature are fixed, then the characteristics of the material being dried will determine the drying time and final air temperature. The original authors have pointed out that even though the curves apply quantitatively only t o a particular set of conditions, the character of the curves is such that broad generalities can be deduced. Some of these generalized facts are pertinent to the operation and proper use of tunnel driers and merit discussion. If the commodity being dehydrated is a slow-drying material, the temperature drop per foot of tunnel length will be small and long tunnels can be used. If the tunnel is relatively short, the temperature drop through i t will be small, and unless air recirculation is used, the heat efficiency will be low. On the other hand, if the commodity is a fastdrying material, the temperature drop per foot of tunnel length will be large. I n this case, if a long tunnel is used, it should not be completely filled with trucks, otherwise drying conditions will be unsatisfactory a t the cool humid end. However, if the number of trucks in the tunnel is increased, the tunnel output capacity will be increased (assuming there is no change in the air velocity), but a t the possible expense of injury t o the product. The latter is particularly true, since both the drying time and

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possibility of other unfavorable drying conditions will be increased. Therefore, it is apparent there is an optimum balance for each different operating condition. The series of curves referred t o previously, also illustrate another fact. If the number of trucks is increased, and a t the same time the air velocity is increased just enough to keep the wet-bulb depression unchanged a t the wet end of the tunnel, two desirable things occur. The product drying time becomes shorter (a tendency that favors product quality), and the output of the tunnel increases. Thus the operator must use good judgment and balance the gains in increased capacity and better product against the substantial increase in the power required t o run the blower. It should be remembered that power consumption increases approximately eight-fold if the air velocity is doubled, even though the number of trucks remains the same (see pp. 326-332, Fans and Blowers). Power consumption will be even greater if the number of trucks is increased simultaneously. Although several tunnels in a dehydration plant may be of identical design and be drying the same commodity t o a given moisture content, the drying time will inevitably differ somewhat from tunnel to tunnel. Also there may be even a day t o day difference in the drying time for a given tunnel. Small variations in drying time can be caused by slight differences in the amount of recirculated air, effectiveness of air distribution in the tunnel, hot-air temperature, uniformity of tray loading, atmospheric conditions at the fresh air intake, and last, but not the least important, are the variations in the nature of the prepared commodity. The effect of these variables has been discussed in TJnited States Department of Agriculture Miscellaneous Publication No. 540 (1944), and by Van Arsdel (1951a). Although the drying times will differ as indicated, there will be a mean time interval for each truck handled. The preparation line must be geared t o that rate, and even then there will occasionally be either a shortage or an excess of loaded trucks at the dehydrators. Several tunnels may be needing trucks at the same time, and a little later other loaded trucks may begin t o accumulate because no tunnel is ready to accommodate them. Regardless of this situation, a crew a t the dry end of the tunnel must be ready t o remove the trucks of dried product whenever the cars are scheduled t o be pulled. It is necessary t o keep the dry-end foreman informed as t o the exact time each wet truck load is placed in a tunnel, so that he can add the probable cycle times and schedule the pull times. This calls for some kind of message or signal system plus a running log sheet a t the dry end of the tunnel on which the probable pulling times can be entered. If the preparation line must be slowed down for a number of hours, the drier foreman needs advance notice, for he may have t o change

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drier conditions to compensate for partly empty tunnels. A more complex scheduling system is needed in a plant which operates two-stage tunnel dehydrators, than in one which uses only counterflow units. Operation of a tunnel dehydrator under normal conditions presents few difficulties. At the beginning and end of a processing run, certain modifications of tunnel drying conditions are usually necessary t o prevent product injury during the starting-up and shutting-down periods. There are various procedures for starting and shutting down tunnel dehydrators, as discussed by Perry and associates (1946), and as explained in the United States Department of Agriculture Miscellaneous Publication No. 540 (1944). Exact procedures will, of course, vary from plant t o plant. Essentially, the methods consist of readjusting either the amount of recirculation air, the air temperature, or rate of tunnel loading, or adjusting and balancing combinations of these factors which affect the drying conditions within the tunnel. The procedures are a necessary operating requirement, not only t o minimize excessive heat damage t o the product, but also t o avoid other undesirable drying conditions, which may in turn, cause other types of quality deterioration before the product is dried. As a n example of the latter condition, let us assume th a t several trucks or cars of wet material are placed in a counterflow tunnel on start-up. The evaporation rate from the first truck would be great, and the drop in temperature of the air passing through th a t car would therefore be high. The air moving through the last truck would be nearly saturated. The product in th at car would be heated u p rapidly t o the wet-bulb temperature of the air, and little evaporation would take place until the first cars of material had lost a considerable part of their moisture content. Such drying conditions would be very undesirable, and probably adversely effect the product quality. For example, under some conditions, there might be a n excessive loss of ascorbic acid, or rapid growth of microorganisms and attendant spoilage. On the other hand, as the leading truckload of product progressed through the tunnel, it would be subjected t o a n abnormally hot, untempered blast of air during its entire time in the tunnel. If the product happened t o be heat sensitive, then heat damage might result. Furthermore, each succeeding truck would be subjected to nonuniform drying conditions as the tunnel was progressively filled. In order t o avoid the difficulties mentioned, one of the start-up procedures for counterflow tunnels uses a modified hot-end air temperature schedule, and loaded trucks are pushed into the tunnel a t the normal or regular scheduled rate. Another method fixes the hot-end air a t the normal operating temperature, and a t first the loaded trucks are rolled into the tunnel a t a faster rate than the normal schedule. The time interval between introduction of cars is progressively increased until normal

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operating conditions are reached. If the latter method is used, scheduling can become rather complex if there are numerous tunnels. Regardless of the starting method used, as the pull-time approaches for the first car, the dried material is usually examined and the drying schedule modified if necessary. Experience shows t h a t a trained operator can estimate the moisture content of the warm, nearly dry material rather closely b y “feel.” One of the authors, during the course of research work on vegetable dehydration, developed this knack and could readily estimate moisture contents of dehydrated products, such as cabbage, carrots, white potatoes, sweet potatoes, onions, spinach, corn, peas, and green beans. Normally, the uncertainty of the estimates, as shown by subsequent assays, was about 1% moisture content. Terminating a run in a counterflow dehydrator is relatively simple. As the evaporative load in the tunnel decreases, there will be a decline in the wet-bulb temperature of the air if recirculation is being used. Ordinarily this condition will not result in product injury, but may carry the material t o a lower moisture content than desired. To prevent this, some operators readjust the recirculation damper, and by doing so also save some fuel. Sometimes the tunnel temperature is lowered slightly, or the last trucks are removed ahead of schedule. I n the first stage tunnel (or parallel-flow section) of a two-stage dehydrator, drying conditions are rather critical during the start-up and shutdown periods. During normal operation, the controlled air temperature a t the hot end is usually above 180” F., and, in some cases, may be set in the region of 250” F. Most of the original moisture is removed from the product while it is in the first-stage drier. The material is usually dried t o a moisture content of about 50% in that section of the dehydrator. This corresponds t o a moisture removal of roughly 75 t o 90% for most commodities. However, the evaporative load in the tunnel is obviously light during the start-up period. To retard excessive drying of the product’s outer surface and prevent the consequent possibility of case hardening, heat damage, or even scorching, it is necessary t o keep the wet-bulb temperature of the air a t a reasonably high level. Therefore, during the starting-up period, it is necessary t o readjust the recirculation damper carefully, so as to maintain normal operating wet-bulb conditions in the parallel-flow tunnel section. Readjustment of the damper may be required after each truck enters that tunnel section, and until the section is full and normal operating conditions are reached. Likewise, when operations are being terminated, good judgment must be used in shutting down the first stage. Near the end of a run, as filled trucks are replaced by empties, the evaporative load decreases, and consequently, since recirculation is normally used, the wet-bulb temperature of the air will

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begin t o drop. Instrumentation usually controls the dry-bulb temperature of the air. Unless the instrument is manually reset, the air will remain at its constant high temperature level. If changes are not made, the last filled truck passing through the first stage will be subjected t o a constant high dry-bulb temperature and a continually decreasing wet-bulb temperature. Unless controlled, there will be a cumulative effect t h a t will produce higher and higher drying rates within each car of the last tunnel load. This may result in case hardening, discoloration, scorching, or other product injury. Corrective measures are relatively simple ; for example, the hot-end, dry-bulb temperature can be progressively decreased stepwise, as empty trucks replace full ones. Theoretically, each temperature drop should be equal t o the average temperature drop across a truck. The latter can be estimated from the known operating data for that particular first-stage tunnel (normal hot-end temperature minus normal cold-end temperature divided by the number of trucks in t h a t tunnel section). I n a similar manner, the wet-bulb can be kept a t a relatively constant level by adjusting the recirculation damper. VIII. RECENTTRENDS IN TUNNEL DEHYDRATION OF FRUITS AND VEGETABLES The vegetable dehydration industry has gradually come t o rely more and more on the use of bin-type finishing driers. Their operation, general use, and design are discussed in United States Department of Agriculture Miscellaneous Publication No. 540 (1944), and Bureau Publication AIC-15 (1943, Revised 1944). The advantages obtained through the use of finishing bin driers are numerous. The bins provide a low-cost method for removing the moisture from the product during the slow-drying stage near the end of the dehydrating process, and they permit close control of the product’s moisture content during the final stage of drying. The bins also provide a n economical means for holding the product and equalizing its moisture content before the material is packaged. Dried apples and prunes are commonly stored for some time in bins after drying for this same purpose. I n addition, a properly designed bin finishing system can materially increase tunnel capacity and improve operation flexibility. I n some plants the bins are made portable for convenience. additional flexibility of operation, and also, they reduce conveyer equipment requirements. Sometimes dehumidified air is supplied t o the bins to expedite the drying process. The theory of through-flow drying in the low-moisture region, as applied t o bin finishing driers, has been discussed by Van Arsdel (1953). I n order t o make the dehydration operation less critical with respect t o possible heat damage or other injury t o the product, morKattention is

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being directed to the quality and preparation of the raw material. For example, consideration is now being given to the control of the sugar content in potatoes. Campbell and Kilpatrick (1945) demonstrated that sensitivity to heat damage during dehydration is a function of the reducing sugar content in the potatoes. It has been shown that blanching is necessary during the preparation of most vegetables that are to be dehydrated. The two primary reasons for this requirement are: (I) to prevent or check the development of undesirable colors, flavors, and loss of vitamins which occur during dehydration and subsequent storage, and (2) to obtain a finished product which will rehydrate readily, cook rapidly, and yield a cooked product of desirable texture and flavor qualities. There are no data available which directly correlate the degree of enzyme inactivation and quality retention of dehydrated products during storage. However, most investigators agree that blanching to a degree sufficient to inactivate the peroxidase enzyme systems present in the various commodities is sufficient for quality retention. Blanching much beyond this point is undesirable due to the loss of soluble nutrients as a result of leaching caused by extended exposure of the product to the blanching medium. I n order to prevent undesirable changes during dehydration and storage, there is a recent trend to treat the raw prepared material with additives. Sulfur dioxide has, of course, been used for this purpose for many years. Some of the new additives may have certain advantages over sulfur dioxide. As examples of the new additives, starch has been used to coat prepared carrots. Compared with sulfur dioxide, the starch gives carrots equal or better protection from loss of carotene and color during subsequent storage, according to Masure and associates (1950). Treatment of potatoes with calcium chloride to control sloughing and reduce heat damage during dehydration was suggested by Campbell and Kilpatrick (1945). Experimental confirmation of the favorable results of this treatment has recently been presented by Simon et al. (1954). The same authors (1953) proposed the use of thinner pieces, treated with calcium chloride to control sloughing, in order to speed up the drying and thus reduce heat damage. In effect, most of these procedures increase the drying capacity of the dehydrators by permitting the use of higher drying temperatures. The mounting cost of labor has caused a trend toward the use of labor-saving devices in the operation of dehydrators. Semi-automatic and automatic tray loaders, stackers, and unloaders have been recently developed for use in conjunction with tunnel drier operations in the dehydration industry, and they have been used with apparent success. Automatic truck transfer equipment is being used to handle cars between staged tunnels. Reducing the complexity of operation tends to alleviate

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the labor problem. Therefore, there is a tendency t o favor the simple single-stage tunnels, rather than the more complex multistage units. Some vegetable dehydration plants even prefer t o operate their tunnels without recirculation of air, sacrificing fuel economy in order t o simplify the job for the tunnel crew. As pointed out in another section of this chapter (pp. 357-359), the drying characteristics of cut vegetables are such that this practice probably entails little or no increase in total drying cost. The vast majority of the fruit and vegetable dehydration plants in the United States use truck-and-tunnel driers in preference t o other type dehydrators. A small number of conveyer driers is in use, and the popularity of this type of dehydrator appears t o be increasing. However, for many years t o come, the tunnel drier will continue t o serve as the dependable workhorse of the fruit and vegetable dehydration industry. The authors believe it will continue t o occupy a foremost place in the further development of the industry.

IX. LIST OF SYMBOLS USED A-Evaporating

area of a piece, square feet.

A A 83XL, -Coefficient in tunnel equation, = G(w, + I ) . G--Mass air flow, pounds dry air per minute. 11-Latent heat of evaporation, B.t.u. per pound. /