Module 13: Changes occurring in oils and fats during frying Rajan Sharma and Bimlesh Mann Dairy Chemistry Division National Dairy Research Institute, Karnal rajansharma21@gmail.com Frying is a process of immersing food in hot oil with a contact among oil, air, and food at a high temperature of 150 C to 190 C. The simultaneous heat and mass transfer of oil, food, and air during deep-fat frying produces the desirable and unique quality of fried foods. Frying oil acts as a heat transfer medium and contributes to the texture and flavor of fried food. Deep-fat frying is one of the oldest and popular food preparations. The economy of commercial deep-fat frying has been estimated to be $83 billion in the United States and at least twice the amount for the rest of the world (Choe and Min, 2009). Frying time, food surface area, moisture content of food, types of breading or battering materials, and frying oil influence the amount of absorbed oil to foods. The oil contents of potato chips, corn chips, tortilla chips, doughnuts, French fries, and fried noodle (ramyon) are 33% to 38%, 30% to 38%, 23% to 30%, 20% to 25%, 10% to 15%, and 14%, respectively. The absorbed oil tends to accumulate on the surface of fried food during frying in most cases and moves into the interior of foods during cooling. Deep-fat frying produces desirable or undesirable flavor compounds, changes the flavor stability and quality, color, and texture of fried foods, and nutritional quality of foods. The hydrolysis, oxidation, and polymerization of oil are common chemical reactions in frying oil and produce volatile or nonvolatile compounds. Frying temperature and time, frying oil, antioxidants, and the type of fryer affect the hydrolysis, oxidation, and polymerization of the oil during frying. Changes in Oils During Heating and Frying A. Physical changes in oil during heating and frying Deep-fat frying is a process of cooking and drying in hot oil with simultaneous heat and mass transfer. As heat is transferred from the oil to the food, water is evaporated from the food and oil is absorbed by the food (Fig. 1). Many factors affect heat and mass transfer, including thermal and physical properties of the food and oil, shape and size of the food, and oil temperature. Table 1 lists some of changes in some physical parameters of oils/fat during deep-frying and the reasons for these changes. 1
Figure 1. Physical and chemical reactions that occur during frying. Source: Choe, E. and Min, D.B. (2009) Chemistry of deep-fat frying oils. J. Food Sci. 72: R77-R86 Table 1. Changes in some physical parameters of oils/fat during deep-frying and the reasons for these changes Physical Parameter Changes during deep-frying Caused by Refractive index/uv Increases Accumulation of conjugated fatty acids Density Increases Polymerized Triacylglycerols Dielectric coefficient Decreases Polar-oxidized components Colour Becomes more intensive & darker (Maillard) reaction products Conductivity Increases Polar compounds Surface tension Decreases Polar compounds Smoke point Decreases volatile oxidized decomposition products Specific Heat Increases Polar compounds Viscosity Increases Polymerized Triacylglycerols No B. Chemical Changes in oil during heating and frying Frying oils not only transfer heat to cook foods but also help to produce distinctive fried-food flavor and, particularly, undesirable off-flavors if deteriorated oil is used. During deep-fat frying various deteriorative chemical processes (e.g., hydrolysis, oxidation, and polymerization) take place, and oils decompose to form volatile products and nonvolatile monomeric and polymeric compounds (Figure. 1). With continued heating and frying, these compounds decompose further until breakdown products accumulate to levels that produce 2
off-flavors and potentially toxic effects, rendering the oil unsuitable for frying. The amounts of these compounds that are formed and their chemical structures depend on many factors, including oil and food types, frying conditions, and oxygen availability. In addition, these chemical reactions hydrolysis, oxidation, and polymerization are interrelated producing a complex mixture of products such as generation of free fatty acids, small molecular alcohol, aldehyde, ketone, acid, lactone and hydrocarbon, diglyceride and monoglyceride, cyclic and epoxy compounds, trans isomers, triacylglycerol monomer, dimmer, oligomer etc. The individual processes of hydrolysis, oxidation, and polymerization and their degradation products are described below. Table 2. Changes in some chemical parameters of oils/fat during deep-frying and the reasons for these changes Chemical Parameter Changes during deepfrying Caused by Anisidine value Increases Secondary oxidation products Iodine value Decreases Formation of oxidized fat products Peroxide value Increases but can also decrease Primary oxidation products Petrolether insoluble oxidized fatty acids Increase Oxidized polymerization products Polar compounds Increase Oxidized and polymerized degradation products including unchanged polar fat components Polymerized Oxidized and not oxidized polymerized Increase Triacylglycerols Triacylglycerols Acid value Increases Formation of oxidation products with free carboxyl groups 1. Hydrolysis of oil When food is fried in heated oil, the moisture forms steam, which evaporates with a bubbling action and gradually subsides as the foods are fried. Water, steam, and oxygen initiate the chemical reactions in the frying oil and food. Water, a weak nucleophile, attacks the ester linkage of triacylglycerols and produces di- and monoacylglycerols, glycerol, and free fatty acids. The basic reaction occurring during hydrolysis of oil is depicted in Figure 2. Free fatty acids contents in frying oil increase with the number of fryings (Figure 2). Free fatty acid value is used to monitor the quality of frying oil. 3
Figure 2. Hydrolysis of oil producing free fatty acids Figure 2. Free fatty acid formation in soybean and seasame oil mixture during consecutive frying of flour dough at 160 C. Hydrolysis is more preferable in oil with short and unsaturated fatty acids than oil with long and saturated fatty acids because short and unsaturated fatty acids are more soluble in water than long and saturated fatty acids. Frequent replacement of frying oil with fresh oil slows down the hydrolysis of frying oil. This practice is most commonly used by commercial houses involved in the business of fried foods. Free fatty acids and their oxidized compounds produce off-flavor and make the oil less acceptable for deep-fat frying. Di- and monoacylglycerols, glycerol, and free fatty acids accelerate the further hydrolysis reaction of oil. Glycerol evaporates at 150 C and the remaining glycerol in oil promotes the production of free fatty acids by hydrolysis. 2. Oxidation of oil Oxygen, which is present in fresh oil and is introduced into the frying oil at the oil surface and by addition of food, activates a series of reactions involving formation of free radicals, hydroperoxides, and conjugated dienoic acids. The chemical reactions that occur during the oxidation process contribute to the formation of both volatile and nonvolatile decomposition products. The chemical mechanism of thermal oxidation is principally the same as the autoxidation mechanism. The thermal oxidation rate is faster than the autoxidation, but specific and detailed scientific information and comparisons of oxidation rates between thermal oxidation and autoxidation are not available. The mechanism of thermal oxidation involves the initiation, propagation, and termination of the reaction (Figure 3). 4
Figure 3. The initiation, propagation, and termination of thermal oxidation of oil The various strengths of hydrogen-carbon bond of fatty acids explain the differences of oxidation rates of stearic, oleic, linoleic, and linoleic acids during thermal oxidation or autoxidation. The polyvalent metals such as Fe 3+ and Cu 2+ remove hydrogen protons from oil to form alkyl radicals by oxidation-reduction mechanism of metals even at low temperatures. The site of radical formation in saturated fatty acids is different from those of unsaturated oleic or linoleic acids. The alkyl radical of saturated fatty acids is formed at α-position of the carboxyl group having electron-withdrawing property. The primary oxidation products - hydroperoxides decompose rapidly at 190 C into secondary oxidation products such as aldehydes and ketones (Figure 4). Secondary oxidation products that are volatile significantly contribute to the odor of the oil and flavor of the fried food. If the secondary oxidation products are unsaturated aldehydes, such as 2,4-decadienal, 2,4-nonadienal, 2,4-octadienal, 2-heptenal, or 2-octenal, they contribute to the characteristic fried flavor in oils that are not deteriorated and can be considered desirable. However, saturated and unsaturated aldehydes, such as hexanal, heptanal, octanal, nonanal, and 2- decenal, have distinctive off-odors in olfactometry analyses of heated oil. Fruity and plastic 5
are the predominate off-odors of heated high oleic oils and can be attributed primarily to heptanal, octanal, nonanal, and 2-decenal. Figure 4. Secondary oxidation products formed from primary oxidation products - hydroperoxides. These reactions occur in frying oils at more than 190 C. 3. Polymerization of oil Polymerization occurs during frying, producing a wide variety of chemical reactions that result in the formation of compounds with high molecular weight and polarity (Figure 5). Polymers can form from free radicals or triglycerides by the Diels Alder reaction. Cyclic fatty acids can form within one fatty acid; dimeric fatty acids can form between two fatty acids, either within or between triglycerides; and polymers with high molecular weight are obtained as these molecules continue to cross-link. Formation of dimers and polymers depends on the oil type, frying temperature, and number of fryings. As the number of fryings and the frying temperature increase, the amounts of polymers increased. The oil rich in linoleic acid is more easily polymerized during deep-fat frying than the oil rich in oleic acid. Polymers formed in deep-fat frying are rich in oxygen. Oxidized polymer compounds accelerated the oxidation of oil. Polymers accelerate further degradation of the oil, increase the oil viscosity, reduce the heat transfer, produce foam during deep-fat frying, and develop undesirable color in the food. Polymers also cause the high oil absorption to foods. 6
Figure 5. Polymerization reactions in frying oils. Factors Affecting the Quality of Oil during Deep-Fat Frying Understanding the mechanism of thermal degradation of a frying oil is difficult because it is affected by many variables, such as unsaturation of fatty acids, oil temperature, oxygen absorption, metals in substrates and in the oil, and nature of the food. list of factors that affect the processes of hydrolysis, oxidation, and polymerization and eventually frying oil deterioration are presented in Table 3. Frying oil degradation can be managed and even inhibited by controlling these factors. Table 3. Factors affecting frying oil degradation Oil/Food Unsaturation of fatty acids Type of oil Type of food Metals in oil/food Initial oil quality Degradation products in oil Antioxidants Antifoam additives Process Oil temperature Frying time Aeration/oxygen absorption Frying equipment Continuous or intermittent heating or frying Frying rate Heat transfer Turnover rate; addition of makeup oil Filtering of oil/fryer cleaning 7
Some of the factors affecting oil degradation are explained below 1. Replenishment of fresh oil: A high ratio of fresh oil to total oil provides better frying oil quality. Frequent replenishment of fresh oil decreases the formation of polar compounds, diacylglycerols, and free fatty acids and increases the frying life and quality of oils. 2. Frying time and temperature: Frying time increases the contents of free fatty acids, polar compounds such as triacylglycerol dimers and oxidized triacylglycerols, dimers, and polymers. High frying temperature accelerates thermal oxidation and polymerization of oils. High frying temperature decreased polymers with peroxide linkage and increased the polymers with ether linkage or carbon to carbon linkage. The intermittent heating and cooling of oils causes higher deterioration of oils than continuous heating due to the oxygen solubility increase in the oil when the oil cools down from the frying temperature 3. Quality of frying oil: Oxidation rate of oil increased as the content of unsaturated fatty acids of frying oil increased. This explains why corn oil with less unsaturated fatty acid is a better frying oil than soybean or canola oils with more unsaturated fatty acids. The content of linolenic acid is critical to the frying performance, the stability of oil, and the flavor quality of fried food. Low linolenic acid oil produces less free fatty acids and less polar compounds during. Hydrogenation increases the frying stability of oil. However, hydrogenation produces trans fatty acid or metallic flavor, and it does not additionally improve the quality of oil with low linolenic acid. Filtering of oil to decrease FFA. 4. Compositions of foods: A large amount of moisture in foods increases the oil hydrolysis during deep-fat frying. The more the moisture content of food, the higher the hydrolysis of oils. Lecithin from frying foods caused foam formation at the initial stage of deep-fat frying. Starch increases the degradation of oil and amino acids protect the oil from degradation during deep-fat frying. Transition metals such as iron, which is present in meat, were accumulated in the oil during frying and this increased the rates of oxidation and thermal degradation of oil. 5. Types of fryer: The types of fryer affect the frying oil deterioration. Even and fast heat transfer to the oil can prevent hot spots and the scorch of oil. Polymerized fat deposited on the fryer causes scale formation, the formation of foam, color darkening, and further deterioration of frying oil. A small surface-to-volume ratio of fryer for minimum contact of oil with air is recommended for deep-fat frying. Oxidation is slowed by modifying a fryer to have a ratio of oil depth (D) to oil area (A) with D/A 1/2 = 0.93. Copper or iron fryer accelerates the oxidation of frying. 6. Antioxidants: The naturally present or added antioxidants in oils and foods influence oil quality during deep-fat frying. Tocopherols, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tert-butylhydroquinone (TBHQ) slow down the oxidation of oil at room temperature. However, they become less effective at frying temperature due to losses through volatilization or decomposition 7. Dissolved oxygen contents in oil: Nitrogen or carbon dioxide flushing decreased the dissolved oxygen in oil and reduced the oxidation of oil during deep-fat frying. A minimum of 15 min of nitrogen or 5 min of carbon dioxide flushing prior to heating decreases the oxidation of oil during deep-fat frying. 8
Biological Effects of Used Frying Oil A slight depression in growth to very poor growth Diminished feed efficiency Increased liver, kidney and heart sizes Fatty tissues of liver, kidney and heart organs Liver enzymes such as thiokinase and succinyldehydrogenase had lower activity The evidence of carcinogenicity (in highly abused frying oil) Analytical Methods for assessment of oils Physical methods include Determination of the smoking point, Viscosity, Conductivity, Dielectric constant Lovibond colour index. All these procedures are not suitable to describe the quality of a deep-frying medium quantitatively, but provide rather rough reference points for its evaluation Chemical methods include Determination of free fatty acids (acid value) by acid-base titration, Polar compounds by means of chromatographic procedures, of polymer triacylglycerols and oxidized fatty acids. The acid value is dependent on the kind of fat and therefore not suitable for the objective determination of the degradation condition. Only the determination of the polar compounds and the polymer triacylglycerols permit an objective evaluation of the thermal load condition of deep-frying fats. References: Choe, E. and Min, D.B. (2009) Chemistry of deep-fat frying oils. J. Food Sci. 72: R77-R86 Zhang, Q.; Saleh, A.S.M; Chen, J. and Shen, Q. (2012) Chemical alterations taken place during deep-fat frying based on certain reaction products: A review. Chemistry and Physics of Lipids, 165: 662-681. Warner, K. (2002) Chemistry of frying fats. In: Food lipids: Chemistry, Nutrition, and Biotechnology, 2 nd edn (Eds. C.C. Akoh and D.B. Min). Marcel Dekker, Inc, New York, 9