Bleaching and Purifying Fats and Oils: Theory and Practice, Second Edition

Bleaching and Purifying Fats and Oils Theory and Practice Second Edition

Editor Gary R. List

Urbana, Illinois

AOCS Mission Statement To be a global forum to promote the exchange of ideas, information, and experience, to enhance personal excellence, and to provide high standards of quality among those with a professional interest in the science and technology of fats, oils, surfactants, and related materials. AOCS Books and Special Publications Committee M. Mossoba, Chairperson, U.S. Food and Drug Administration, College Park, Maryland R. Adlof, USDA, ARS, NCAUR-Retired, Peoria, Illinois M.L. Besemer, Besemer Consulting, Rancho Santa, Margarita, California P. Dutta, Swedish University of Agricultural Sciences, Uppsala, Sweden T. Foglia, ARS, USDA, ERRC, Wyndmoor, Pennsylvania V. Huang, Yuanpei University of Science and Technology, Taiwan L. Johnson, Iowa State University, Ames, Iowa H. Knapp, DBC Research Center, Billings, Montana D. Kodali, Global Agritech Inc., Minneapolis, Minnesota G.R. List, USDA, NCAUR-Retired, Consulting, Peoria, Illinois J.V. Makowski, Windsor Laboratories, Mechanicsburg, Pennsylvania T. McKeon, USDA, ARS, WRRC, Albany, California R. Moreau, USDA, ARS, ERRC, Wyndoor, Pennsylvania A. Sinclair, RMIT University, Melbourne, Victoria, Australia P. White, Iowa State University, Ames, Iowa R. Wilson, USDA, REE, ARS, NPS, CPPVS-Retired, Beltsville, Maryland Copyright © 2009 by the American Oil Chemists’ Society. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means without written permission of the publisher. The paper used in this book is acid-free, and falls within the guidelines established to ensure permanence and durability. Library of Congress Cataloging-in-Publication Data Patterson, H. B. W. (Henry Basil Wilberforce) Bleaching and purifying fats and oils : theory and practice / H.B.W. Patterson. p. cm. Includes bibliographical references and index. ISBN 978-1-893997-91-2 (alk. paper) 1. Oils and fats--Purification. 2. Bleaching. I. Title. TP673.P38 2009 665’.3--dc22 2008055038 Printed in the United States of America 00 99 98 97 96 95 94 5432

Contents Preface to the Second Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi Preface to the First Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .viii 1 Basic Components and Procedures H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Adsorption H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3 Adsorbents Dennis Taylor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4 Bleaching of Important Fats and Oils H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5 Bleachers H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 6 Filtration and Filters H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 7 Oil Recovery H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 8 Safety, Security, and the Prevention of Error H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 9 Important Tests Relating to Bleaching H.B.W. Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 10 The Freundlich Isotherm in Studying Adsorption in Oil Processing Andy Proctor and J.F. Toro-Vazquez . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 11 Enzymatic Degumming of Edible Oils and Fats David Cowan and Per Munk Nielsen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 v

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Preface to the Second Edition Since the publication of this book in 1992, the bleaching process has continued to attract the attention of researchers and the edible-oil industry, and a number of excellent reviews appeared in the literature. They include: Hoadson, 1996; Taylor, 2005; Proctor and Brooks, 2005; Zschau, 1998, 2000; DeGreyt and Kellens, 2000; Erickson, 1995; and Nueman and Dunford, 2004. Although Dr. Patterson briefly discusses silica refining, much progress has been made in this area, and is found in both the open and patent literature: Brooks et al., 1991, 1992; Toeneboehn, 1994; and Siew et al., 1994. In addition, silicate refining was described by Hernandez and Rathbone. Many of the references to trace-metal analysis are obsolete, and the reader is directed to more modern techniques such as flame-atomic absorption, graphite furnace atomic adsorption, and atomic emission spectrometry involving direct current plasma (DCP) and inductively coupled Plasma (ICP) (Dijkstra & Meert, 1982; Holm, 1967; List et al., 1971; Mertins et al., 1971; Otijko, 1976). Similarly, phosphorus in edible oils can be quickly and easily measured by a turbidometer introduced by Sinram, and forms one basis for an Official AOCS Method. Sleeter (1985) discusses the pros and cons of these methods. During the 1990s, a considerable amount of work was done using the Freundlich Equation to study the bleaching of edible oils, which was summarized by Proctor and Foro-Vasquez (1996, 2005) and Dijskstra (2002). The recovery of oil from spent bleaching clay has attracted the attention of researchers. Among the techniques reported are high-temperature water extraction (Penninger, 1979), high-temperature oxidative aqueous regeneration (Kalim & Joshi, (1985), and extraction with supercritical CO2 (King et al., 1992; Waldmann & Eggers). Over the past several decades, improved degumming methods were developed. They include enzymatic degumming (Anon., 1992), total degumming (Dijsktra & Van Opstal, 1989), and soft degumming (Gibon & Tirtiaux, 1997)—all with the objective of the removal of phosphatides to very low levels as a prerequisite for physical refining. Compared to water degumming (100–250 ppm of phosphorus), these processes will remove phosphorus to levels in the 5–10-ppm range. Another process developed in the past decade is that of using membrane filtrates (Iwama, 1989; Lin Lan & Koseoglu, 1996). Press Effect in Bleaching The continuous bleaching process in a plant or in a series of batch filtrations can result in a substantial cake buildup on the filter cloth. This phenomenon is termed “press effect,” and refers to additional bleaching that can take place in the press cake. In effect, the filter press cake acts as a fixed-bed adsorption column. When the clay has additional capacity (i.e., it is not in total equilibrium with the oil it comes in contact with), it can continue to remove impurities and color bodies from the oil. The press effect was studied in the laboratory (Henderson, 1993). Gary R. List Washington, Illinois

Bleaching and Purifying of Fats and Oils, Second Edition

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Preface to the First Edition The title of this book was chosen to emphasize that some major operations, traditionally named “bleaching,” are much wider in scope than the mere removal of color. The selective removal of unwanted nonfat minor components from the parent fat is a form of separation which may be understood to include, in some cases, their destruction or chemical modification. Sometimes the process step is directed principally at the removal of pigment, but removes other minor components as a bonus. In other cases, the removal of components—such as remaining traces of gums, soaps, poisons of hydrogenation catalysts, and prooxidant metals—is the prime consideration; hence, we speak of purifying. Damage to the parent fat has to be avoided, and this relates to its intended use. A procedure employed in preparation of a technical or nonedible product may be quite unacceptable for edible material. The different physical and chemical operations now in use for bleaching and purifying are described in detail from a practical standpoint. Adsorption is a major technique; the way it works has come to be better understood since the instruments and methods of analysis have grown more sensitive. The theory of their operation and the structure and manufacture of adsorbent earths and carbons are simply explained for the benefit of processors already working in the fats and oils industry and for those entering it. The special use of adsorbent silica is also described, as well as other techniques. The chemical nature of the unwanted components is given; this serves to distinguish them from the fats in which they are found. Against this background, the processing of some twenty of the most important fats or oils is considered individually. Separate chapters deal with bleachers and filters—not forgetting the vitally important filter membranes. Finally, oil recovery, safety matters, and the significance of commonly used tests are considered where these have particular relevance to the subject. As with two earlier books, the author is much indebted to the painstaking work of Mrs. Marjorie Honor, who prepared the text during the years of its composition. H.B.W. Patterson, D.Sc. 9 The Wiend Bebington Merseyside L63 7RG England

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Acknowledgments Acknowledgments and sincere thanks are hereby extended to the following persons and organizations who have helped me with advice on their own specialties, whether of process detail or plant design. In a number of cases this has included ready agreement to the use of illustrations from their publications. K. Abe (Mizusawa Industrial Chemicals Ltd., Tokyo) R.F. Ariaansz (Engelhard De Meern BV) A. Athanassiadis (Extraction De Smet, SA) EMI Corporation R.C. Hastert (Consultant) P. Haynes (Schenk Filters GmbH) Y. Hoffmann (AB Pellerin/Zenith) S.S. Koseoglu (Texas A [amp] M) R. Leysen (American Soybean Assn., Brussels) Marcel Dekker, Inc., New York R.B. Morton (Grace GmbH) N. Phillips, M.E. Davies, and P.R. Shanks (Laporte Absorbents) J.B. Rossell (Leatherhead Food Research Assn.) J.R. Santhiapillai (PORIM) W.A. Starkie (P and S Filtration Ltd.) A. Sen Gupta (Walter Rau AG) F. Veldkamp (L.F.C. Lochem BV) A.J. Weir (Sutcliffe Speakman Carbons Ltd.) J.L.B. Wesselink and F. de Vooys (Norit NV) D.C. Wolfe (United Catalysts, Inc.) F.V.R. Young (Consultant) W. Zschau (Süd-Chemie AG)

A Note on In-text Reference Citations in the 2009 Edition: Bibliographic reference citations have been changed from the numbered format to the new AOCS standard (Author Name, Date) system. The reference list can be found at the end of this volume. To facilitate use of this revised Edition, not that all references to other sections of this book are indicated as follows: The section name appears, along with the chapter number if the section is from another chapter. It is our hope that these changes will allow you to navigate through this updated edition easily.

Chapter 1

Basic Components and Procedures H.B.W. Patterson

The Nature of Fats and Oils In the Preface, bleaching and purifying are shown to be forms of separation of unwanted minor components from oils which, ultimately, can mean the destruction of some of them. One must recognize that the method chosen must avoid damaging the oils themselves and, if possible, their beneficial minor components. Constraints on procedure are therefore inevitable, and compromises may have to be made: these are closely related to the intended use of the final product—whether edible or technical. Finally, one must always keep in mind the relative costs of different methods, as compared to the technical gains in quality achieved (James, 1958). Some description of the chemical and physical characteristics of fats and oils and of their common minor components will make any future discussion of the methods of bleaching or purifying more readily understandable. Triglycerides The basic unit of a fat consists of a molecule of glycerol combined with three molecules of fatty acid. When all the fatty acid molecules are of the same kind, the result is described as a simple triglyceride; if more than one kind is present, it is a mixed triglyceride (Fig. 1.1). When the melting point of a triglyceride is below ambient temperature, it is commonly called an oil, and if above, a fat. The same material may be referred to as one or the other depending on the zone in which it is being handled. Various classes of fatty acids exist, and have a marked influence on the triglyceride in which they occur. In a mixed triglyceride, the sequence in which different fatty acids are distributed over positions 1, 2, and 3 has an influence on its character, especially its melting behavior (Sonntag, 1979; Taylor, 1973). Positions 1 and 3 are slightly more

=

O H2C—OC—R1 =

O HC—OC—R2 =

O H2C—OC—R3 Fig. 1.1. Structure of triglycerides. 1

2

H.B.W. Patterson

exposed to chemical attack than position 2 (Drozdowski, 1977; Kaimal & Lakshminarayana, 1979; Paulose et al., 1978; Sebedio et al., 1981). These matters are not important in regard to bleaching. The size of the triglyceride molecule is 1.5–2.0 nm (15–20 Å, 0.0015–0.002 µmL), so it can pass readily into meso- and macropores (see Use of Carbon) of an adsorbent, but not into micropores (under 2 nm in width). When fats are partially hydrolyzed, mono- and diglycerides result. These are important for industrial use as emulsifiers, but normally are present in crude oils in very small amounts. Just as differing fatty acids markedly affect the character of an individual triglyceride in which they occur, so the presence of differing triglycerides affects the character of a natural oil or fat, which often contains several varieties.

Fatty Acids When a hydrocarbon chain is oxidized, terminally, so as to contain a carboxylic group, CH3-CH2—CH2CH2COOH the product is described as a fatty acid because many varieties of such acids occur in fats. The three simplest acids, formic, acetic and proprionic, are not included since they do not show the immiscibility-with-water characteristic of higher members. Butyric acid (CH3-CH2-CH2-COOH) is included only because it is found combined with butter. From the six-membered carbon chain (caproic acid), immiscibility with water grows as the chain lengthens. Because of the mechanism of their biochemical synthesis, the quantity of fatty acids containing an even number of carbon atoms in an unbranched chain vastly exceeds the total amount of other types (Taylor, 1973). However, because analytical techniques have improved since the 1950s, small amounts of many fatty acids with branched chains and chains containing an odd number of carbon atoms have come to light. For example, we may have a fatty acid with a total of 18 carbon atoms but containing a straight chain of 17 carbon atoms and an additional methyl group at one point or another along its length. The structural variants are positional isomers of one another. The 17-carbon atom unbranched-chain fatty acid, heptadecanoic (margaric) acid, occurs widely in animal fats, such as tallow, in very small amounts (around 1%), but is virtually absent from vegetable oils. Besides these, fatty acids were identified which contain epoxy, keto and hydroxy groups, and some which include in their chain three- (propenoid) and five- (furanoid) membered rings (Sonntag, 1979). These minor varieties usually account for less than 1% of the amount of fatty acid in common fats; occasionally a rare seed oil or fish-liver oil is found in which over one-half of the fatty acids are of exceptional types.

Saturated Fatty Acids When all carbons in the chain hold their full complement of hydrogen, the fatty acids are saturated. These are the most stable fatty acids; whether free or in combina-

Basic Components and Procedures

3

tion, their molecules pack together when solid more easily because of their regular contour. This causes a higher melting point, and as chain length increases, so does the melting point. The melting point of a fatty acid with an uneven number of carbon atoms in the chain is slightly lower than that of the even-numbered one immediately preceding it. The hydrophobic character increases with chain length. This means their sodium salts (soaps) become less soluble, but will form a more stable lather. Lauric (C12), palmitic (C16), and stearic (C18) acids are the most common saturated fatty acids (SFAs); chain lengths of up to 32 carbon atoms are found. SFAs are the most resistant to oxidation and other forms of chemical attack. They are not open to the same form of destabilization when heated with activated earth as may occur with fatty acids where considerable unsaturation is present in their composition.

Unsaturated Fatty Acids If a hydrogen atom is missing from each of two adjoining carbon atoms in the fatty acid chain, a double bond forms between them. Those fatty acids which contain only one such double bond are described as monounsaturated. Double bonds are potential points of oxidation and other forms of chemical attack; the vulnerability increases rapidly as the number of double bonds increases. Double bonds introduce an uneven feature into the chain. When the remaining hydrogen atoms on the two adjoining carbon atoms lie on the same side of the chain, the latter is seen as assuming an arc at that point, with the two hydrogen atoms lying toward the outside of the arc; this condition or isomer is known as a cis double bond. When the remaining two hydrogen atoms lie on opposite sides of the chain, a slight kink or dog-leg effect arises and is described as the trans isomer. These two forms exist because the rotational freedom of a single bond is lost, and the double bond now introduces a restriction or spatial rigidity. Unsaturation decreases hydrophobic character in comparison with a SFA, but as with the latter, an increase in chain length increases the hydrophobic effect. The trans isomeric form of fatty acid has the higher melting point, seemingly because the carbon chains pack together more easily to form a stable structure in the solid state. Cis isomers are associated with softness or liquidity, and this form is markedly dominant in the natural unsaturated fats and oils. Cis and trans forms are geometric isomers. If the double bond is located at different positions in the chain, these are positional isomers, and they also may have distinctly different physical characteristics, as Fig. 1.2 makes clear. Oleic acid is the most common unsaturated fatty acid found in plant and animal fats. Since it is not especially vulnerable to oxidation, oils in which it is the predominant unsaturated fatty acid, such as olive oil and groundnut oil, have good flavor stability. Palmitoleic acid (a 16-member carbon chain with a double bond at the C-9 position) also occurs widely.

4

H.B.W. Patterson

m.p. 16°C Oleic acid (cis-9 octadecenoic acid) Positional isomers Petroselenic acid (cis-6 octadecenoic acid) m.p. 30°C

Geometric isomers

Geometric isomers

m.p. 42°C Elaidic acid (trans-9 octadecenoic acid) Positional isomers Petroselaidic acid (trans-6 octadecenoic acid) m.p. 51.9°C

Fig. 1.2. Effects of geometric and positional isomerism (m.p.: melting point).

Polyunsaturated Fatty Acids By convention, if a fatty acid contains two or more carbon-carbon double bonds within the hydrocarbon chain, it is classed as polyunsaturated. Almost all polyunsaturated fatty acids (PUFAs) contain a methylene-interrupted configuration (-CH=CH-CH2-CH=CH-). As stated above, the double bond is vulnerable to attack and hence the focus of chemical reactivity. As the number of double bonds increases, the reactivity increases rapidly. The marine oils (where 20- and 22-carbon atom chains are substantial parts of the fats) have fatty acids with up to five and six methylene-interrupted double bonds. Figure 1.3 shows the well-known arrangements of double bonds in which unsaturation occurs. The possibility of an interaction between PUFAs and activated earths makes them of interest when bleaching or purifying is to be performed. Obviously, one must avoid process conditions which encourage harmful changes in the fat, as is discussed in later sections. Where a -CH2- or methylene group immediately adjoins a double carbon-carbon bond in the chain, it is known as an D-methylene group, and shows distinctly enhanced activity, which becomes even more marked if a second carbon-carbon double bond lies on its other side (see Fig. 1.3, 1,4 unsaturation). Oxidation at these sites leads to aldehydes, ketones, and free fatty acids, which are the sources of off-flavors and rancidity in edible fats. Rapid oxidation of very unsaturated oils can also yield epoxy-, oxy- and peroxy-groupings, and hence the hard, water-resistant skins derived from the so-called drying oils for paints and varnishes. Very often the natural long-chain PUFAs show a repetition of the skipped (-CH2-CH=CH-) group; this is consistent with the biochemical mechanism of their formation. This nonconjugated 1,4 unsaturation grouping may be transformed to the conjugated 1,3 unsaturation grouping of alternate double and single bonds (-CH=CH-CH=CH-) in certain reaction sequences. This conjugated arrangement is a most reactive system, and must be present, for example, for copper to act as a catalyst in the hydrogenation of oils. In spite of its instability, various examples of conjugated unsaturation occur in seed oils. Most of these are of little importance. The best known is 9,11,13 octadecenoic acid, or eleostearic acid, an isomer of the well-known natural linolenic acid (cis,cis,cis9,12,15 octadecenoic acid). Eleostearic acid is an example of 1,3,5 type of unsaturation because of the double-bond sequence. Tung-oil fatty acid is predominantly a cis,trans,trans form known as D-eleostearic acid, which melts at 49°C. This form easily converts to the trans,trans,trans or E-form, melting

Basic Components and Procedures

5

– –

H H –CH2–CH2–C=C–CH2–CH2–

cis

–

H trans

–

–CH2–CH2–C=C–CH2–CH2– H –CH2–CH=CH–CH=CH–CH2–

conjugated (1,3 unsaturation)

–CH2–CH=CH–CH=CH–CH=CH–CH2–

conjugated (1,3,5 unsaturation)

–CH2–CH=CH–CH2–CH=CH–CH2–

nonconjugated (1,4 unsaturation or skipped)

–CH3–(CH2–CH=CH)n...COO...

very common in nature; cis predominates

–CH2–CH=C=CH–CH2–

unknown fats

Fig. 1.3. Carbon-carbon double-bond arrangements.

at 71°C (Sonntag, 1979). In view of what was said earlier regarding the packing together of fatty acid chains, noteworthy is that here again the all-trans isomer has the highest melting point. The most important role of polyunsaturated oils is to act as building blocks from which the human metabolism is able to synthesize more complex molecules such as prostaglandins. These compounds have a wide variety of functions of the highest importance relating to blood pressure, male fertility, uterine contraction, nerve fibers, and so forth. Their usefulness is still being explored (Duffy, 1984; Frankel, 1984; Gunstone, 1984). Particular PUFAs—which the human metabolism needs for these purposes, but which it cannot synthesize itself—are classed as essential fatty acids (EFAs) and must be provided in the diet. The most common and important EFA is linoleic acid (cis9,cis12,octadecadienoic acid), CH3-(CH2)4 CH=CH-CH2-CH=CH-(CH2)7-COOH (11-14).

Pigments Already well-known is that moisture, suspended inorganic dirt, gums, and waxes can markedly affect the appearance of an oil, making it dull and altering its perceived shade or tint. When such unwanted components are removed by settling, degumming, alkali neutralization and washing, what is then accepted as the natural characteristic color of the oil becomes visible. This color may be derived from several pigments present in very different concentrations, and minor ones may yet remain to be precisely identified. As the chemical natures of these pigments differ, one type of processing is not equally effective in removing them; consequently the shade and intensity of color vary throughout refining. If the process step reduces or removes one color in a blend more than another, this, of course, creates the illusion that a

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H.B.W. Patterson

fresh color is being generated in the product. Also important to add is that some color may also be derived from the breakdown of the principal pigments or their reaction with nonfatty components also present. This is often the cause of the brown discoloration and lack of clarity. Very often this defect stubbornly resists complete removal. It usually results from the mishandling and the neglect of seed or crude oil. To take precautions in harvesting and in expelling/extracting oil to minimize this degradation is prudent and economical. These processes are reviewed in some detail elsewhere (Patterson, 1989). Parcels of inferior seed are best kept separate to avoid jeopardizing the quality of one or more shipments. The important types of pigments present in edible oils are described in the following pages; some other minor components whose prior removal makes bleaching easier are also briefly described. For example, if the bleaching effect of heat is being used to destroy a particular pigment, one must first remove other materials which darken when the oil is heated; equally, if materials, such as gums, compete with pigments for a place on the surface of activated clays or carbon, they, too, are best first removed to enhance the efficiency of adsorptive decolorization (Andersen, 1962; Norris, 1982). Radiation absorption from one region in the spectrum gives rise to a complementary color associated with another region. In this connection, to consider this is convenient: Far ultraviolet = 220–320 nm. Near ultraviolet Blue = 320–500 nm. Blue green Remaining visible region = 500–720 nm.

Chlorophylls (Davies et al., 1989; Erickson, 1989; Rodd, 1959) A very important group of natural pigments arises from this basic porphin skeleton.

Four pyrrole nuclei (A, B, C, D) are linked by four methine groups (=CH-), together forming a 16-membered central ring. When the E-pyrrole positions are occupied by arrangements of methyl and ethyl groups, we have various porphyrin structures which show characteristic absorption bands in the visible region of the spectrum. Porphin rings form complex metallic derivatives with metals (e.g., Mg, Fe, Cu, Zn, etc.). The complexes containing magnesium (-phyllins) or ferrous iron

Basic Components and Procedures

7

(-haems) retain the metal less tenaciously. Chlorophylls contain a partially reduced porphin skeleton (the E positions of ring D) with additional groups as shown (Fig. 1.4). Chlorophylls constitute the green pigments of plants. The higher plants and green algae contain forms a (bluish green) and b (yellowish green) in proportions of about 3:1. Marine algae contain forms a and c. Parcels of Antarctic fish oil are found in which the marked green color shows resistance to refining and hydrogenation (Zschau, 1990). Red algae contain mainly a and d forms. These different chlorophylls have their own distinctive, but related, absorption spectra (Pfannkoch & Gill, 1990). The pure chlorophylls take the form of waxy blue-black crystals; in organic solvents, a red fluorescence is observed (Erickson, 1989). A loss of magnesium from chlorophylls a or b during extraction/processing operations generates pheophytins a or b, respectively. These are important olivegreen pigments; the magnesium is replaced by two hydrogen atoms. The loss of the carbomethoxy group experienced at high- process temperatures as well as the loss of the magnesium atom generates pyropheophytins a and b; the carbomethoxy group is replaced by a hydrogen atom. In contrast to the brilliant clear-green or yellowishgreen color of chlorophylls, the pheophytins present a duller brownish green. Chlorophyll acts as a photosensitizer for the production of singlet oxygen, and hence, possibly the initial oxidation of oils (Gunstone, 1984; Taufel et al., 1959; Usuki et al., 1984). After traces of chlorophyll (e.g., 4 ppm) are destroyed, free radical types of oxidation take over. Chlorophylls are reasonably unaffected by alkali refining and not especially thermolabile in deodorization. They are most diminished by acid-activated clay adsorption.

Carotenoids The carotenoids are easily the main source of yellow/red color in plant and animal fats, with the coloration of the latter being much affected by diet, hence, varying with season and location. Over 70 varieties of carotenoids are recognized. As a class, they are built up from isoprene units, and contain both cyclic and acyclic forma-

Fig. 1.4. Chlorophylls.

8

H.B.W. Patterson

tions; they also include derivatives of the polyisoprenic units thus obtained. The isomeric hydrocarbons—lycopene D-, E-, and J-carotene—are well-known, especially E-carotene. When the elements of water are added to each half of a E-carotene molecule that was divided at its central double bond, two molecules of vitamin A are produced (Fig. 1.5). An adult needs about 1 mg of vitamin A per day. Other noteworthy carotenoids are the ketonic or hydroxylic derivatives, the xanthophylls, such as lutein (Fig. 1.5). Absorption occurs in the near-ultraviolet and the bluegreen portions of the spectrum (420–475 nm), hence, the yellow/red appearance. The absorption is related to the extensive system of conjugated double bonds. Goodquality palm oils of different origins show peak absorptions in the region of 458 nm. Modest deviations from a precise absorption pattern are explicable by some variation in the carotenoids present in different species. Their outstandingly high-carotene contents are in the range of 500–2000 ppm. Where any particular palm oil has suffered damage, not only is the height of the peak (E 1%/1 cm 458 nm) diminished, but also a displacement toward 450 nm may be evident. Carotenoids, in general, are fat-soluble and water-insoluble, stable to alkali but unstable to heat, acids, and oxidation. Hydrogenation easily removes their color since the system of conjugated double bonds is attacked. Although in alkali neutralization some carotene may be occluded in the soapstock and removed, most of it concentrates preferentially in the oil layer. Unlike many other pigments, natural and synthetic, carotenes are singlet

Fig. 1.5. Carotenoids.

Basic Components and Procedures

9

oxygen quenchers, and therefore oppose the initiation of photooxidation (Carlsson et al., 1976; Gunstone, 1984; Patterson, 1989). In edible-oil processing, avoid oxidative bleaching since by the time substantial color is removed, the glycerides are well-advanced toward rancidity. Acid-activated clays and carbon readily adsorb carotenoids, but this is not true of synthetic silicas.

Flavines The basic flavinoid structure is shown in Fig. 1.6. When the 2,3 double bond is reduced, we have flavanones. Hydroxyl derivatives of the parent flavine are common in plants as yellow mordant pigments, and synthetic varieties are produced as dyes. Well-known flavine derivatives contain hydroxyl group(s) at the following position(s): 3; 5; 3,5,3°’,4°’; 3,5,7,3°’,4°’; and in the case of the ancient natural dye, luteolin, 5,7,3°’,4°’. Hydroxy flavines are frequently present in the plant as glycosides. Although many flavine colors possess absorption bands below 400 nm, some show distinct bands just above 400 nm, and this prompted the suggestion (O’Connor et al., 1949) that these may contribute, along with carotenoids, to the yellow part of the soybean pigment. Alkali refining does not affect the color, which is, however, readily removed by clay adsorption; hence, deodorizing brings no further change. The removal of flavinoid colors presents no problem in oil processing; interestingly, some flavines have a distinct antioxidant effect, evidently related to the phenolic-type hydroxyl groups present (Hudson & Lewis, 1983; Patterson, 1989). Anthocyanidins A great many of the familiar colors of flowers and berries come directly from the presence of anthocyanins. These result from a combination of anthocyanidins and a sugar. When the anthocyanin is hydrolyzed by an acid, the sugar and the salt of the anthocyanidin are produced. The basic structure of anthocyanidin is illustrated in Fig. 1.6. The overall similarity to the flavine structure is evident. The naturally occurring anthocyanins exhibit both phenolic and basic properties, forming salts with alkali or acid. Thus, a salt with an alkali may be blue (cornflower), or with an acid, red (rose, geranium), while the free anthocyanin is violet. Sugar units attach either

Fig. 1.6. Flavines and anthocyanidins.

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H.B.W. Patterson

at the 3-position alone or at both the 3- and 5-position. Anthocyanins show marked absorptions in the 480–550-nm region; they may make a minor contribution to the color of an oil, such as soybean (O’Connor et al., 1949), which is not important. Like the flavines, they are not affected by alkali refining, but are readily adsorbed in clay bleaching.

Gossypols Gossypol is a phenolic substance present in the glands of traditional types of cottonseed; hence, the crude cottonseed oil may contain up to about 0.2% of it (Kenar, 2006). As might be expected from its structure, it has appreciable antioxidant properties.

Unfortunately, oxidation leads to a marked darkening of the oil in which it is present, and this is related, in part, to the formation of the pigment gossypurpurin. Pigs and poultry are badly affected by free gossypol remaining in the meal or cake after cottonseed oil is extracted; so this aspect has also come under strict control. In mild oxidative conditions, J-tocopherol present in small amounts in refined cottonseed oil may gradually be converted to the dark red chroman 5,6-quinone. As the oxidative induction period is exceeded, the chroman 5,6-quinone itself begins to oxidize, and the color lightens as it is destroyed. This effect, of course, is far too late to be of use to the manufacturer of an edible product who is committed to flavor stability as well as light color. Fortunately, gossypol and kindred substances are amenable to the alkali refining of the crude oil, and this is performed without allowing the temperature to rise too high. The much lighter color of the resultant semirefined or washed cottonseed oil may be further lightened by an activated clay treatment. A huge amount of research was done on these topics (Anon, 1978; Patterson, 1989), from which, obviously, the old adage “prevention is better than cure” applies. Hence, precautions commence at harvesting in avoiding conditions which allow seed to rot or overheat. Freshly expelled crude oil must not remain hot and in contact with air; this applies equally to fresh solvent-extracted oil. In each case, the risk is that color could be fixed by oxidation and, therefore, its subsequent removal by alkali refining, washing, and clay adsorption would be less effective (Hutchins, 1976; Patterson, 1989). Plant breeders have succeeded in producing cotton plants bearing glandless seeds which yield superior oil, meal, and lecithin; an improvement in their resistance to insect pests is being sought (Cherry, 1983; Patterson, 1989).

Basic Components and Procedures

11

Tocopherols and Chromans Tocopherols are best known as antioxidants widely distributed among plants, but much less common in animal tissue. Their basic structure is shown in Fig.1.7, in which one can see that they are derivatives of tocol, itself derived from chroman. D-Tocopherol is identified with vitamin E. The ranking of antioxidant potency is generally accepted as G > J > E > D, but to generalize on this matter is misleading since factors such as temperature and concentration can affect tocopherols’ behavior. The absolute and relative amounts of different tocopherols in different plant species vary enormously (Bieri, 1984). They are most effective when present at the same concentration levels found in nature. At higher concentrations, their efficiency drops steadily, and may actually reverse to become prooxidant. Lauric, palm, and olive oils contain less than 100 ppm of total tocopherols; sunflower, cottonseed, and groundnut, several hundred ppm; soybean and maize, around 1000 ppm and above; wheat-germ oil ca. 0.5% and rice-bran oil, between 2 and 4%, this being a major part of the total unsaponifiable matter present, which can itself range from 3 to 8%. The long side-chain induces such a high solubility in oil that, in spite of the phenolic hydroxyl group, tocopherols are not lost to more than a very small percentage in the soapstock and washes of alkali refining. Adsorption on activated clay is also low, but activated carbon at its usual level of usage up to 0.5% of carbon/oil may take out about one-half of the tocopherols present, and in the highly unlikely use of around 2% of carbon, the greater part of the tocopherol is likely to be adsorbed. Deodorization, especially at temperatures of 235°C and above and at higher vacuum, does strip out substantial amounts of tocopherol (Klagge & Sen Gupta, 1990) so that deodorizer condensates are processed to extract them. At the same time, the products in which the deodorized oil is incorporated may have tocopherol concentrate added to them to restore their oxidative and hence, flavor stability. Hydrogenation of oil causes little or no loss of tocopherols. At worst, normal refining leaves more than one-half of the original tocopherol content. Pure tocopherols are colorless or light yellow. Comparatively, mild oxidation can eventually break them down. In the case of J-tocopherol, as shown in Fig. 1.7, this produces the dark-red chroman 5,6-quinone, which resists bleaching stubbornly. This feature is true for many colors produced by oxidative degradation (Rich, 1964). As was previously mentioned, the avoidance of oxidative conditions where feasible, from harvesting to the end of processing, is a worthwhile precaution. Phosphatides (Gums) Complex esters which contain phosphorus, nitrogen bases, sugars, and long-chain fatty acids are classed as phospholipids. The phosphatides in oils are fatty acid esters of glycerol—which, at the same time, are also esters of phosphoric acid. The phosphoric acid may also be linked with a nitrogen base or a sugar, and a cation such as magnesium, calcium, or sodium.

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H.B.W. Patterson

Fig. 1.7. The tocopherols.

Figure 1.8 shows a phosphatidic acid and the well-known ester, D-lecithin, in which the phosphatide group contains the base choline. If the phosphatide group is attached at the 2-glyceryl position, the compound is referred to as a E-lecithin. The D arrangement is more common. R1 and R2 are the long-chain hydrocarbon units of fatty acids. In natural phosphoglycerides, R2 is frequently unsaturated. The lysophosphatides are phosphatides in which one acyl group was removed by hydrolysis. Chemical hydrolysis is random, but enzymes are specific (e.g., phospholipase A removes the 2-acyl group). Their general behavior is comparable with phosphatides, notably regarding solubility in oil or water. Both D- and E-cephalin are other wellknown phosphoglycerides; they contain ethanolamine as the nitrogen base. The presence of phosphatides in many fats and oils is one of the most important reasons why the term “bleaching” is recognized as inadequate when describing the improvement in the quality of a crude oil which is being sought. Mere removal of pigment is not the only requirement; it may not even be the most important.

Basic Components and Procedures

13

Fig. 1.8. Typical phosphatides.

Phosphatides, as the principal constituent of gums in the crude oil, severely interfere with the efficiency of subsequent process steps if allowed to remain. Thus, in alkali neutralization, phosphatides’ presence causes increased amounts of neutral oil to be emulsified and hence, lost in the soapstock; diminishes the adsorptive action of clay and carbon by adhering to their surfaces; similarly poisons nickel catalyst; darkens the color of an oil if they become broken down by heat; and can lead to impaired flavor stability. The main responsibility for this last ill-effect, however, is linked with traces of prooxidant iron liable to persist in refined oil along with the gummy material rather than phosphatides themselves (Dijkstra & Van Opstal, 1989). A wide variation exists in the typical phosphatide content of different vegetable oils. For a long time, crude oils—such as olive, palm, palm kernel, babassu, and coconut—were known to contain a very small amount, perhaps a small fraction of 1%, of phosphatides; soybean, maize, sunflower, linseed, rapeseed, and cottonseed hold substantial amounts, best dealt with early in their processing; fats prepared from the specialized animal tissue containing large deposits of fat (tallow, lard, blubber) are low in phosphatides, but this is less certain since the whole carcass was processed. Fortunately, the composition and behavior of phosphatides have become much better understood since the 1950s; thus, effective and economical ways were discov-

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H.B.W. Patterson

ered of getting rid of them before neutralization, bleaching, hydrogenation, or other oil-modification steps are attempted. Briefly, phosphatidic acid in its undissociated form is soluble in oils but not in water; if it can be caused to dissociate, it hydrates, flocculates to micelles or liquid crystals, and can be encouraged to pass to the aqueous phase by settling or centrifuging. Similarly, when the magnesium or calcium salts, which are insoluble in water, are treated with an acid or cation precipitant to remove the metal, they too will hydrate and can be separated (Zschau, 1990). This process we know as degumming. The water-binding capability increases by increasing the degree of dissociation (Braae et al., 1957; Hvolby, 1971). The relevant techniques are further described in the section on degumming (see section Degumming). Relatedly, more efficient and selective adsorbents of phosphatides are now attracting attention; these are the synthetic silicas. Preliminary aqueous degumming remains the obvious first step. This process has the further advantage of removing other unwanted minor components such as protein, sugars, and iron soap trapped in the micelles or liquid crystals (Segers, 1983, 1985).

Sterols Sterols are high-melting polycyclic alcohols of the general structure:

They are major components of the unsaponifiable portion of many vegetable and animal fats. They may occur in the free form, or the hydroxyl group may be esterified by a fatty acid such as linoleic acid, yielding a wax-like arrangement. They may also be present as glucosides. A very common sterol in animal fats is cholesterol: R = -CH(CH3) (CH2)3 CH(CH3)2. The sterols of vegetable oils are classed as phytosterols; one of the best known, E-sitosterol: R = -CH(CH3)CH2CH2CH(C2H5) CH(CH3)2, is prominent in the unsaponifiable portion of cottonseed oil and its isomer, J-sitosterol, in soybean oil (Sonntag, 1979). Some sterols occur in both animal and vegetable fats. In alkali refining, an appreciable loss of sterols is entrained in the soapstock. Deodorization at higher temperatures also removes substantial amounts of sterols, but the adsorption in clay during normal bleaching of oils is small. However, activated clay treatment may induce some chemical modification.

Basic Components and Procedures

15

Waxes Waxes are typically the high-melting fatty acid esters of long-chain fatty alcohols with a generally low solubility in oils. Other esters, such as certain sterols (as discussed in the previous section), where the alcohol is cyclic, conform broadly to wax characteristics. Several processing procedures to remove waxes are used, depending on how much wax is present and whether or not achieving a very low wax content in the final product is important (e.g., clear salad oil or solid margarine). The typical wax content of many vegetable oils may be only a few hundred ppm, rising in the case of a few oils to 2000 ppm and above. Much depends on whether the relatively high wax content of the seed shell was allowed to pass to the extracted oil. Sunflower seed is a good example since the dehulling of the seed prior to extraction is an obvious way of lowering the wax content of the final extracted crude oil. In practice, therefore, we have a situation where oils of peanut, rape, sesame, soy, palm, palm kernel, fish, tallow, and lard call for no special dewaxing step in processing. Sunflower and maize oils almost certainly will require dewaxing; crude rice-bran oil is notorious for a wax content of ca. 5% but sometimes exceeding 9%. The oil of the sperm whale contains about 75% of long-chain fatty alcohols, mainly C14–C20, saturated and unsaturated, combined with long-chain fatty acids. These waxes provide the basis of high-class durable lubricants (Patterson, 1983). For use as salad oils, oils need to remain clear for some minimal number of hours at a cold temperature, and this is generally achieved when the wax content is reduced to around 10 ppm. Chilling and filtering oils after bleaching or deodorizing them suffice for many seed oils which, in the crude state, have only a small wax content anyway. For those with a higher wax content (e.g., sunflower), a preliminary and substantial reduction of wax content is advantageous; this may be preceded by degumming, or degumming and dewaxing may be combined (see section Degumming). After neutralization, a final and more rigorous dewaxing may then follow bleaching or deodorizing, according to the intended end use (Fedeli, 1983; Forster & Harper, 1983; Haraldsson, 1983; Latondress, 1983; Pritchard, 1983; Sullivan, 1980). In the filtration steps at the reduced temperature of dewaxing, a siliceous precoat and body feed may often be used, but this is to hasten filtration and is not related to pigment removal. Trace Metals Many metals of the transition groups in the periodic table are active catalysts in promoting the breakdown of lipid hydroperoxy free radicals and, therefore, in accelerating the oxidative deterioration of fats and oils. Even in Roman times, holding olive oil in copper vessels was likely to shorten its useful life. Now that the whole topic of the stabilization of fats and oils is much better understood, naturally, particular attention is paid to traces of prooxidant metals as well as organic pigments and enzymes capable of hastening damage via oxidation or hydrolysis. Copper and iron attract the most attention because: (i) they occur widely as natural components at a very low level in various fats and oils and (ii) noncautious handling during

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H.B.W. Patterson

harvesting/processing/shipment can raise their levels to a disastrous extent. While metals such as cobalt and manganese are sufficiently active to be chosen for use as prooxidants in the paint industry, their occurrence is too low and infrequent to be a significant hazard (Waters, 1971). Thomas (1982) makes a similar point in a comprehensive survey of other undesirable minor components to be removed from edible fats and oils. Obviously, the removal of trace metals to an acceptable level must depend on steps taken before deodorization. Table 1.1 illustrates this. Bearing in mind the current permissible upper limits (in Germany) for contents of these metals in fats and vegetable oils (As, 0.1; Pb, 0.25; Cd, 0.05; Hg, 0.05, ppm), evidently, further reduction is no longer required. Where oils were hydrogenated, one must monitor the final nickel content. The prooxidant effects of various metals (Cu, Mn, Fe, Cr, Ni, V, Zn, and Al) were compared quantitatively (Sonntag, 1979). The effectiveness of any prooxidant is limited by the type of fat against which it is acting and the conditions of its encounter (such as temperature, acidity or alkalinity, moisture, light, and catalyst concentration). As is being discovered, some organic catalysts can even exert a pro- or antioxidant effect according to conditions, especially concentration. The more highly unsaturated oils are potentially the most at risk. Next, the commercial issue is how much and what kind of harm is done. This could relate to flavor instability, worsening in color, or failure to bleach adequately when the oils are processed. Fortunately, a better understanding of the effect of trace metals has brought about improvements in the handling and, consequently, the quality of crude oils since the early 1970s. Whereas, in 1970, crude palm oil at a maximal content of 10 ppm of iron and 0.2 ppm of copper was commonly offered, by 1983, good crude palm oil at a maximum of 3 ppm of Fe and 0.02 ppm of Cu was available; some parcels at 0.1 ppm of Cu were still to be found. Keeping Cu below 0.05 ppm is desirable because even at levels as low as 0.02 ppm, one can detect some prooxidant influence. A deodorized palm oil is safest below 0.2 ppm of Fe (Patterson, 1989). List and Erickson (1980) reviewed the effects of iron and copper in crude and fully refined soybean oil. Normal beans yield oil up to 3 ppm of Fe and 0.05 ppm of Cu. For safety, the deodorized oil should not exceed 0.1 ppm of Fe and 0.02 ppm of Cu. Iron, even as low as 0.3 ppm, has a distinctly bad influence on flavor TABLE 1.1. Influence of Alkali Refining, Washing, and Clay Bleaching on the Content of Hazardous Metals (ppm) in Soybean Oil Refining step

As

Pb

Cd

Hg

Crude oil

0.02

0.06

0.005