CHANGES IN EDIBLE FATS AND OILS DURING PROCESSINGF. SHAHIDI', P.K.J.P.D. WANASUNDARA and U.N. WANASUNDARADepartment of Biochemistry Memorial University of Newfoundland St. John's, NF, Canada, A I B 3x9Received for Publication February 28, 1997 Accepted for Publication August 27, 1997

ABSTRACTFats and oils are essential nutrientsfor humans and animals. They are the most concentratedsource o energy by contributing 9 kcaVg lipid serve as carrier offatf soluble vitamins andprovide essentialfat@ acids. Fats and oils give flavor and taste to thefoods, and also serve as a heat transfer medium. The lipid molecules undergo different chemical reactions during processing startingfrom separation f o m their raw sources to the storage. Chemical changes of lipids occurring during processing are numerous and can be desirable, undesirable, of questionable consequence, or a combination thereox This review discusses the chemical changes of lipids that occur during various types ofprocessing. Specific changes in the minor constituents of lipids are also included.

INTRODUCTIONLipids provide a concentrated source of energy and essential fatty acids through daily dietary intake. They also serve as important contituents of cell walls and carrier of fat-soluble vitamins. In addition, lipids provide flavor, texture and mouthfeel to the food. Food lipids may originate from plant, animal or possibly algal sources. Edible oils are mainly composed of triacylglycerols (TAG) with phospho- and glycolipids (PL and GL, respectively) comprising a small fraction. Sterols, waxes, lipid soluble vitamins, phenolics and other minor constituents comprise the unsaponifiable matter of oils. During processing and storage, food lipids undergo chemical and physical changes. Often, process-derived changes of lipids are necessary to manifest specific characters of the food, however, these changes should not exceed a desirable limit. Isolation of lipids from source material is often required for edible purposes; steps involved transform crude lipids to edible products. This paper summarizesprocess-induced changes in edible oils.

'Corresponding author Journal of Food Lipids 4 (1 997) 199-231. All Rights Reserved. oCopyright 1997 by Food & Nutrition Press, Inc., Trumbull, Connecticut 199

200

F. SHAHIDI, P.WANASUNDARA and U. WANASUNDARA

Changes During Edible Oil Refining The refining process removes or reduces the contaminants of the crude oil that could potentially affect the quality of the end product and the efficiency of lipid modification. At the same time these processes should not have any detrimental effects on the nutritional value of the oil or lead to the formation of undesirable artifacts. Impurities that negatively affect product quality or processing efficiency are moisture, dust, protein degradation products, free fatty acids, partial acylglycerols, phosphatides, oxidation products, pigments and trace elements (e.g., copper, iron, sulfur and halogens), polysaccharides and chlorinated pesticide residues (Young et ul. 1994). Figure 1 summarizes the major refhing stages of edible oil processing and the type of impurities removed during each step. Extraction of Oil. Oils are first separated from tissues by mechanical or chemical means or their combination. The raw material is subjected to cooking followed by mechanical separationof the oil, especially when dealing with animal tissues (rendering). Wet screw pressing or expelling (e.g., olive, palm), high pressure screw pressing for seeds with high oil contents (e.g., copra, palm kernel, peanut, olive), pre-pressing followed by solvent extraction (e.g., copra, palm kernel, peanut, sunflower, canola, cottonseed) and solvent extraction alone for primarily seeds with low oil content (e.g., soybean, rice bran) and for some highly permeable seeds with high oil contents (e.g., palm kernel) are practiced (Young et ul. 1994). Heat treatment is necessary to denature the protein and to break cell walls so that oil and water can be easily removed from animal tissues (Bimbo 1989). The raw material is first cooked with water in a continuous cooker at 50-70C. The denatured protein in stickwater may then be separated from the oil by centrifugation or removal of liquors (oil and water) by applying pressure in a screw-type continuous press in which oil can be separated by centrifugation (Lee 1963). The separated crude oil is then used for further processinglrefming. Degumming. Degumming is a pretreatment designed to remove phosphatides, polysaccharides,pigments and trace metals from crude oils. In this process, crude oils are treated with water or dilute acids such as phosphoric acid to remove these impurities. According to List et ul. (1978a) phosphoric acid degumming produces a better quality soybean oil in terms of flavor and oxidative stability than the waterdegummed oil (degummed without phosphoricacid). Removal of prooxidants such as iron has been partly responsible for the high oxidative stability and inhibition of off-flavor development in food lipids (List et ul. 1978b). Phosphoric acid may convert nonhydratable phosphatides into a form that can be removed. Gums left

Refining stage-t

Impurities removed or reduced Phospholipids, trace metals, pigments, carbohydrates, proteins

Nuetralization-4

Fatty acids, phospholipids, pigments, trace metals, sulfur, insoluble matter

~~

Washing-t

I+Water

ISoap

Drying

I-+-t

Bleaching Spent bleaching earth

I

I I-t

Pigments, oxidation products, trace metals, traces of soap

Deodrization

Fatty acids, mono- and diacylglycerols, oxidation products, pigment decomposition products, pesticides

Physical refining--t

Fatty acids, mono- and diacylglycerols, oxidation products, pigment decomposition products, pesticides

--t

Any residual traces of oil insolubles

FIG. 1. REFINING STAGES OF EIBLE OILS AND THE MAJOR IMPURITIES REMOVED

202

F. SHAHIDI, P. WANASUNDARA and U. WANASUNDARA

in the oil may cause high loss of oil during refining and also produce sediments in the storage tanks. In addition, phosphatides have a poisoning effect on catalysts used for hydrogenation. Marine oils are not usually degummed before being refined. The removal of phospholipids and mucilaginous materials in such oils, is concurrent with the removal of free fatty acids by alkali-refining.Alkali Refining and Neutralization. Alkali refining may cause both physical and chemical changes in the oil. The alkali (normally dilute sodium hydroxide) added to the oil reacts with free fatty acids present to form soap. Gums absorb alkali and are coagulated by hydration, much of the pigments are degraded, adsorbed on the gums or made water-soluble by the alkali and the insoluble matter is entrained with other coagulated materials (Bimbo and Crowther 1991). After adding alkali to crude oils, the mixture is slightly heated to break the emulsion and then the soapstock is removed by centrifugation (Cowan 1976; Carr 1978). The refined oil is washed with warm water to remove the last traces of soap (Kwon e f a .1984). Such impurities, if not removed, could affect the color, foam and smoke l characteristics and/or cloudiness of the oil in later stages of processing that involve high temperatures. The soap which is removed in the refining process is diluted with water and acidified to form fatty acids. The fatty acids are centrifuged to remove the aqueous phase, dried and used for production of fatty acids, soap or feed manufacturing. This product is called acid oil or acidulated soapstock by the oil industry (Bimbo and Crowther 1991). Wiedermann (1 98 1) has summarizedthe reactions involved in the degumming and alkali refining. The a-lipoids are referred to the phospholipids that react easily with water and readily precipitate as oil-insoluble hydrates. The p-lipoids are nonwater precipitable and require addition of alkali or acid for their removal from the oil. Bleaching. After alkali-refining, the oil is usually bleached. Bleaching is performed to improve the color, flavor and oxidative stability of the oil. Although the main objective of bleaching is to reduce the amount of colored compounds and natural pigments (e.g., carotenoids, chlorophyll, xanthophyll and polyphenols), some suspended mucilaginous and colloid-like matter are also removed (Chang 1967). The Lovibond color values of the oil during refining indicate that the red color of the original oil (7-8 R) is reduced gradually by bleaching (2-2.5 R), deodorizing (0.4-0.8 R) and finally heat bleaching (< 1.O R). Any traces of soap are also adsorbed by the bleaching clay. Commonly used bleaching materials are natural clay, activated clay and carbon (Cowan 1976). Activated carbon is normally used at 5-10% in combination with the clay. Natural clay is used if the oil is readily bleachable. However, acid-activated clay is usually used because its

EDIBLE FATS AND OILS

203

bleaching power is greater than that of the natural clay (Boki et al. 1989; Morgan et al. 1985; Richardson 1978). Winterization. Winterization involves chilling of the oil at a prescribed rate, allowing the high melting fraction, referred to as stearin, to crystallize and finally removed by filtration while still cold (Bimbo 1989). Winterization is an old practice that evolved from the observation that storage of oils in outdoor tanks during cold weather caused deposition of high-melting TAG at the bottom and clear liquid oil on the top; the clear oil was then decanted and used as a light oil. In other words, winterization removes the more saturated fraction of the oil from its unsaturatedpolyunsaturated fraction. Solvent winterization of oil has recently been developed. In this method, the viscosity of the oil is reduced by dissolving it in a solvent such as hexane. Crystals of saturated fats formed over a short period of time are readily separated from the low viscosity liquid phase. Crystallized and noncrystallized fractions are then separated by filtration. Nowadays, marine oils are winterized to remove (1) waxes and other non-TAG constituents, (2) naturallyoccurring high-melting TAG and (3) TAG formed during partial hydrogenation (List and Mounts 1980). Deodorization. Deodorization is the last major step in the refining of edible oils. This processing operation removes undesirable odors and flavors from the oil and ensures their shelf-life stability (Gavin 1978). Prior to deodorization, the oil may contain volatile odor and flavor-active components originally present in the crude, the soapy odor created by alkali-refining, the earthy odor generated by bleaching and the typical hydrogenation odor caused by hardening (Chang 1967). It should be noted that some compounds may impart undesirable odor or flavor to oils at concentrations of 1 to 10 ppm or even lower (Chang 1967). The deodorization is essentially a steam distillation process where the volatile compounds are stripped from the nonvolatile oil (Bimbo and Crowther 1991) and degrades peroxides in the oil and removes any aldehydes or other volatile products which might have resulted from atmospheric oxidation (Lin et al. 1990). This process also serves to decrease the free fatty acid content and improves the color of the oil. The conventional procedures used for deodorization (i.e. steam stripping at 200-24OC) are not suitable for marine oils since they may cause oxidation of polyunsaturated fatty acids (PUFA) (Bimbo and Crowther 1991). Therefore, modifications of the conventional process have been sought to deodorize marine oils. Dinamarca et al. (1990) have developed a pilot-scale process for deodorizing fish oil by high vacuum distillation at low temperatures (below 150C) and produced a bland oil without destroying its long-chain PUFA. Since environmental contaminants have been a concern in the marine oil industry, in certain parts of the world, short path distillation under high vacuum or supercritical fluid extraction may be suitable to prepare pesticide-free and high quality marine oils (Breivik 1992).

204

F. SHAHIDI, P. WANASUNDARA and U. WANASUNDARA

Changes in Minor Constituents of the Oil Due to Refining There are specific changes that may occur in minor constituents of edible oils during processing. In the process of preparation of extra virgin and virgin olive oil, fruits are pressed and the expelled oil is bottled without any further processing, except washing, decanting, centrifugation and filtration. The free fatty acid content is the major criterion used for categorization of the oil. Extra virgin oil contains