Frying J Pokorny, Prague Institute of Chemical Technology 12.1 Introduction Frying, especially deep fat frying, has become the most popular food prepara tion technology during the last five decades. The reason is that the preparation is easy even for less experienced cooks, the procedure is rapid, and the finished product is highly palatable. In the frying procedure, fat is the medium of heat transfer. Two main frying methods exist, namely shallow frying and deep frying. In case of shallow frying, the layer of frying oil is about 1-10mm thick in a pan and the fried food is only partially immersed; it has to be turned during the process to obtain an evenly cooked product. The frying takes about 5-10 min, and frying oil is used for greasing the food as it is cooking. The oil is not reused In case of deep frying, the layer of frying oil is 20-200mm thick or greater and the fried material is immersed in oil or floats on the surface. The frying again takes about 5-10min, depending on the dimensions of the food being fried and on the temperature. After frying, the food is removed and the frying oil is used again for the next frying. The duration of use depends mainly on the fryin medium, the technical equipment and on the food The increasing consumption of fried foods contributes to a high intake of fats and oils Because consumers wish to reduce their consumption of fats and oils pans are offered on the market that do not require any fat. When these are used the heat transfer medium is not oil and therefore the process should not be regarded as frying but as roasting. During frying, fat or oil is preheated to tem- peratures of 150-180C In contact with oil, fried food is heated rapidly in the surface layers to the temperature of the frying oil. The temperature reaches only 80-100C in inner layers
12 Frying J. Pokorny, Prague Institute of Chemical Technology ´ 12.1 Introduction Frying, especially deep fat frying, has become the most popular food preparation technology during the last five decades. The reason is that the preparation is easy even for less experienced cooks, the procedure is rapid, and the finished product is highly palatable. In the frying procedure, fat is the medium of heat transfer. Two main frying methods exist, namely shallow frying and deep frying. In case of shallow frying, the layer of frying oil is about 1–10 mm thick in a pan and the fried food is only partially immersed; it has to be turned during the process to obtain an evenly cooked product. The frying takes about 5–10 min, and frying oil is used for greasing the food as it is cooking. The oil is not reused. In case of deep frying, the layer of frying oil is 20–200 mm thick or greater and the fried material is immersed in oil or floats on the surface. The frying again takes about 5–10 min, depending on the dimensions of the food being fried and on the temperature. After frying, the food is removed and the frying oil is used again for the next frying. The duration of use depends mainly on the frying medium, the technical equipment and on the food. The increasing consumption of fried foods contributes to a high intake of fats and oils. Because consumers wish to reduce their consumption of fats and oils pans are offered on the market that do not require any fat. When these are used the heat transfer medium is not oil and therefore the process should not be regarded as frying but as roasting. During frying, fat or oil is preheated to temperatures of 150–180 °C. In contact with oil, fried food is heated rapidly in the surface layers to the temperature of the frying oil. The temperature reaches only 80–100 °C in inner layers
294 The nutrition handbook for food essons 12.2 Changes in frying oil 12.2.1 Types of reaction The oil is subject to three types of reaction during deep frying hydrolytic reactions oxidation reactions pyrolysis of oxidation products Triacylglycerols in frying oil are hydrolysed by steam produced from water in the fried product when it is in contact with the hot frying oil. As the two react ing partners are not miscible, the reaction is relatively slow, resulting in the for- mation of diacylglycerols and free fatty acids. Diacylglycerols are more polar and therefore their contact with water vapour is better; monoacylglycerols and free fatty acids are formed by further hydrolysis. Monoacylglycerols are rapidly hydrolysed into fatty acid and glycerol. Under deep frying conditions, glycerol is dehydrated into acrolein, which is very volatile and its vapours irritate the eyes The rate of oxidation reactions depends on the concentration of oxygen Oxygen present in the original frying oil is rapidly consumed, usually before the temperature of oil reaches the frying temperature. Additional oxygen can enter frying oil only through diffusion from air(Fujisaki et al, 2000). When contact with air is moderate the oxidation of the frying oil is slow. It is consumed for the destruction of natural antioxidants, and only when they are destroyed, tria- cylglycerols are oxidised, too. Hydroperoxides are formed as primary reaction products, but they are very unstable at high temperature so that their content rarely exceeds 1%0 Some components present in fried food affect the oxidation rate of frying oil (Pokorny, 1998). The oxidation rate could be reduced by addition of antioxidants even when they are less efficient than under storage conditions. Most synthetic antioxidants, such as BHT and BHA, are too volatile under frying so that they have only moderate activity. Gallates are more efficient in frying oils. Currently it is considered preferable to use natural antioxidants. Tocopherols are present in most frying oils, and their addition is efficient (Gordon and Kourimska, 1995) Ascorbyl palmitate, citric acid and its esters are useful as synergists. Rosemary and sage resins were also found to be active in frying oils( Che Man and Tan, 1999). Oxidation reactions can be inhibited by polysiloxanes, which form a very thin layer on the surface of the frying oil, preventing the access of oxygen(Ohta et al, 1988). Because they are not resorbed in the intestines they are considered safe for human consumption The third group of reactions are secondary reactions of hydroperoxides. They are decomposed in three ways during frying Decomposition into nonvolatile products with the same number of carbon atoms, such as epoxides, ketones or hydroxylic compounds. When the con-
12.2 Changes in frying oil 12.2.1 Types of reaction The oil is subject to three types of reaction during deep frying: • hydrolytic reactions; • oxidation reactions; • pyrolysis of oxidation products. Triacylglycerols in frying oil are hydrolysed by steam produced from water in the fried product when it is in contact with the hot frying oil. As the two reacting partners are not miscible, the reaction is relatively slow, resulting in the formation of diacylglycerols and free fatty acids. Diacylglycerols are more polar and therefore their contact with water vapour is better; monoacylglycerols and free fatty acids are formed by further hydrolysis. Monoacylglycerols are rapidly hydrolysed into fatty acid and glycerol. Under deep frying conditions, glycerol is dehydrated into acrolein, which is very volatile and its vapours irritate the eyes and mucosa. The rate of oxidation reactions depends on the concentration of oxygen. Oxygen present in the original frying oil is rapidly consumed, usually before the temperature of oil reaches the frying temperature. Additional oxygen can enter frying oil only through diffusion from air (Fujisaki et al, 2000). When contact with air is moderate the oxidation of the frying oil is slow. It is consumed for the destruction of natural antioxidants, and only when they are destroyed, triacylglycerols are oxidised, too. Hydroperoxides are formed as primary reaction products, but they are very unstable at high temperature so that their content rarely exceeds 1%. Some components present in fried food affect the oxidation rate of frying oil (Pokorny´, 1998). The oxidation rate could be reduced by addition of antioxidants even when they are less efficient than under storage conditions. Most synthetic antioxidants, such as BHT and BHA, are too volatile under frying so that they have only moderate activity. Gallates are more efficient in frying oils. Currently it is considered preferable to use natural antioxidants. Tocopherols are present in most frying oils, and their addition is efficient (Gordon and Kourimska, 1995). Ascorbyl palmitate, citric acid and its esters are useful as synergists. Rosemary and sage resins were also found to be active in frying oils (Che Man and Tan, 1999). Oxidation reactions can be inhibited by polysiloxanes, which form a very thin layer on the surface of the frying oil, preventing the access of oxygen (Ohta et al, 1988). Because they are not resorbed in the intestines they are considered safe for human consumption. The third group of reactions are secondary reactions of hydroperoxides. They are decomposed in three ways during frying: • Decomposition into nonvolatile products with the same number of carbon atoms, such as epoxides, ketones or hydroxylic compounds. When the con- 294 The nutrition handbook for food processors
Frying 295 entration of these products(known as polar products)exceeds 25-27%o, the frying oil has to be replaced by fresh oil. At still higher levels of polar prod ucts, foaming takes place, which increases the contact area of oil with air, and thus the rate of oxidation Decomposition into volatile low-molecular weight compounds, such as alde hydes, alcohols, ketones or hydrocarbons. Some products possess a typical fried flavour, e.g. 2, 4-decadienals or unsaturated lactones. They are formed from linoleic acid bound in frying oil. Decomposition into high molecular weight compounds, usually dimers or trimers with fatty acid chains bonded by C-C, C-o-C or C-0-o-C bonds. The content of polymers is a good indicator of the degree of frying oil degra- dation. When their content reaches 10%o, used oil should be replaced by fresh oil Several methods are used for monitoring oil degradation during frying(Wu and Nawar, 1986). Used oil can be analysed with use of HPLC (for polar cor pounds)or HPSEC (for polymers): this is best done in combination with column chromatography(Sanchez- Muniz et al, 1993). Among other methods, the spec- trophotometry, determination of permittivity(dielectric constant), specific gravity or different colour tests can be used(Xu, 2000) o Frying oil can be used for a longer time if it is purified from insoluble parti s and polar substances by using a suitable adsorbent, such as magnesium sil icate(Perkins and Lamboni, 1998). Commercial products for this pupose are available(Gertz et al, 2000). Their combination with antioxidants is recom- mended (Kochhar, 2000). Membrane processes have been proposed for purifica tion of frying oil(Miyagi et al, 2001). 12.2.2 Choice of frying oil Frying oil should contain some bound linoleic acid to generate a fried flavour (Warner et al, 1997). Some oils, such as soybean, sunflower or rapeseed oils are rich in linoleic acid, but are rather unstable under frying conditions and should be replaced very often by fresh oil, which is expensive( Gertz et al, 2000). Low polyunsaturated oils, such as olive oil, are highly priced Hydrogenated veget able oils are more stable but are objectionable because of the content of trans unsaturated fatty acids. Pork lard is an excellent frying medium from the standpoint of sensory value, but there are objections because of its high content of saturated fatty acids and of cholesterol. The best choice are high-oleic low polyenoic modified vegetable oils, such as fractionated palm oil, 1. e. the palm olein fraction( Che Man and Hussin, 1998), modified soybean, sunfower, rape seed, peanut, and even linseed oil. If they contain 3-10% linoleic acid, they still produce an attractive fried flavour and are sufficiently stable on frying. A problem is their availability on the market
centration of these products (known as polar products) exceeds 25–27%, the frying oil has to be replaced by fresh oil. At still higher levels of polar products, foaming takes place, which increases the contact area of oil with air, and thus the rate of oxidation. • Decomposition into volatile low-molecular weight compounds, such as aldehydes, alcohols, ketones or hydrocarbons. Some products possess a typical fried flavour, e.g. 2,4-decadienals or unsaturated lactones. They are formed from linoleic acid bound in frying oil. • Decomposition into high molecular weight compounds, usually dimers or trimers with fatty acid chains bonded by C–C, C–O–C or C–O–O–C bonds. The content of polymers is a good indicator of the degree of frying oil degradation. When their content reaches 10%, used oil should be replaced by fresh oil. Several methods are used for monitoring oil degradation during frying (Wu and Nawar, 1986). Used oil can be analysed with use of HPLC (for polar compounds) or HPSEC (for polymers); this is best done in combination with column chromatography (Sánchez-Muniz et al, 1993). Among other methods, the spectrophotometry, determination of permittivity (dielectric constant), specific gravity or different colour tests can be used (Xu, 2000). Frying oil can be used for a longer time if it is purified from insoluble particles and polar substances by using a suitable adsorbent, such as magnesium silicate (Perkins and Lamboni, 1998). Commercial products for this pupose are available (Gertz et al, 2000). Their combination with antioxidants is recommended (Kochhar, 2000). Membrane processes have been proposed for purification of frying oil (Miyagi et al, 2001). 12.2.2 Choice of frying oil Frying oil should contain some bound linoleic acid to generate a fried flavour (Warner et al, 1997). Some oils, such as soybean, sunflower or rapeseed oils are rich in linoleic acid, but are rather unstable under frying conditions and should be replaced very often by fresh oil, which is expensive (Gertz et al, 2000). Low polyunsaturated oils, such as olive oil, are highly priced. Hydrogenated vegetable oils are more stable but are objectionable because of the content of transunsaturated fatty acids. Pork lard is an excellent frying medium from the standpoint of sensory value, but there are objections because of its high content of saturated fatty acids and of cholesterol. The best choice are high-oleic lowpolyenoic modified vegetable oils, such as fractionated palm oil, i.e. the palm olein fraction (Che Man and Hussin, 1998), modified soybean, sunflower, rapeseed, peanut, and even linseed oil. If they contain 3–10% linoleic acid, they still produce an attractive fried flavour and are sufficiently stable on frying. A problem is their availability on the market. Frying 295
96 The nutrition handbook for food processors 12.3 Impact of deep frying on nutrients The following main changes occur in the frying process(Fillion and Henry, 1998) Mass transfer between frying oil and fried food Thermal decomposition of nutrients and antinutritional substances in fried Interaction between fried food components and oxidation products of fried oil (Dobarganes et al, 2000) 12.3.1 Impact of frying on main nutrients The main change in the food composition during frying is the loss of water and its replacement with frying oil. Most foods(other than nuts) contain water as their major component. In contact with hot frying oil, water is rapidly converted into steam, at least in the surface layer of fried material. The temperature of inner layers does not exceed the boiling point of water so that water losses are only moderate paces left in fried food after water evaporation are filled with frying oil (Pinthus et al, 1995). This process increases the available energy content of the product and because the energy intake in the diet is too high in many countries, it is desirable to reduce the absorption of frying oil. This may be achieved by drying pieces of food on the surface before immersion into oil(Baumann and Escher, 1995). Another way is to produce a crust on the surface of fried pieces, which prevents water losses and oil uptake, and preserves juiciness in fried mate- rial(Ateba and Mittal, 1994). It is possible to cover the surface with batter or various other preparations, such as cellulose derivatives(Priya et al, 1996). The oil absorption can be reduced by half using these procedures. The oil removed by absorption into fried food should be replenished by fresh oil from time to time in order to keep the volume of frying oil constant hanges in nutritional value depend not only on the amount of absorbed frying oil, but also on its composition. If fresh edible oil is used, the contents of essen- tial fatty acids and tocopherols in fried food rise. If food is fried in oil used for a longer time, the content of essential fatty acids and tocopherols becomes low so that the increase in nutritional value, due to absorbed oil, is not significant. On the contrary, such antinutritional products as polar lipids and polymers are absorbed with used frying oil. fried food is stored, even under refrigeration, the thin layer of frying oil on the surface is autoxidized, especially in case of oil rich in pe acids(Warner et al, 1994). Fried food should be stored either in vacuum or an inert gas or protected by antioxidants. If fat-rich food is fried such as bacon, sausages or fat fishes, some fat origi nally present in food is released into frying oil. Eicosapentaenoic and docosa- hexaenoic acids were detected in oils used for frying fish like sardines ( Sanchez-Muniz et al, 1992). Cholesterol may also be extracted into frying oil
12.3 Impact of deep frying on nutrients The following main changes occur in the frying process (Fillion and Henry, 1998): • Mass transfer between frying oil and fried food; • Thermal decomposition of nutrients and antinutritional substances in fried food; • Interaction between fried food components and oxidation products of fried oil (Dobarganes et al, 2000). 12.3.1 Impact of frying on main nutrients The main change in the food composition during frying is the loss of water and its replacement with frying oil. Most foods (other than nuts) contain water as their major component. In contact with hot frying oil, water is rapidly converted into steam, at least in the surface layer of fried material. The temperature of inner layers does not exceed the boiling point of water so that water losses are only moderate. Spaces left in fried food after water evaporation are filled with frying oil (Pinthus et al, 1995). This process increases the available energy content of the product and because the energy intake in the diet is too high in many countries, it is desirable to reduce the absorption of frying oil. This may be achieved by drying pieces of food on the surface before immersion into oil (Baumann and Escher, 1995). Another way is to produce a crust on the surface of fried pieces, which prevents water losses and oil uptake, and preserves juiciness in fried material (Ateba and Mittal, 1994). It is possible to cover the surface with batter or various other preparations, such as cellulose derivatives (Priya et al, 1996). The oil absorption can be reduced by half using these procedures. The oil removed by absorption into fried food should be replenished by fresh oil from time to time in order to keep the volume of frying oil constant. Changes in nutritional value depend not only on the amount of absorbed frying oil, but also on its composition. If fresh edible oil is used, the contents of essential fatty acids and tocopherols in fried food rise. If food is fried in oil used for a longer time, the content of essential fatty acids and tocopherols becomes low, so that the increase in nutritional value, due to absorbed oil, is not significant. On the contrary, such antinutritional products as polar lipids and polymers are absorbed with used frying oil. If fried food is stored, even under refrigeration, the thin layer of frying oil on the surface is autoxidized, especially in case of oil rich in polyunsaturated fatty acids (Warner et al, 1994). Fried food should be stored either in vacuum or an inert gas or protected by antioxidants. If fat-rich food is fried, such as bacon, sausages or fat fishes, some fat originally present in food is released into frying oil. Eicosapentaenoic and docosahexaenoic acids were detected in oils used for frying fish like sardines (Sánchez-Muniz et al, 1992). Cholesterol may also be extracted into frying oil. 296 The nutrition handbook for food processors
Frying 297 If plant foods are subsequently fried in the same oil, cholesterol or fish fatty acid may be absorbed Lipids present in food are decomposed only to a small extent, including high unsaturated fish oils. It is due to short frying time and limited access of oxygen Based on dry matter content, the concentration of most nutrients is reduced dur- ing frying, as the original nutrients are diluted with absorbed frying oil. Starch and non-starch carbohydrates are partially destroyed during frying, and starch lipid complexes are formed(Thed and Phillips, 1995). The fraction of resistant (undigestible) starch changes during the operation(Parchure and Kulkarni, 1997) Sucrose is hydrolysed into glucose and fructose, which are destroyed by heating. mostly by Maillard or caramelisation reactions. Proteins are rapidly denaturated in surface layers of food particles, more slowly in inner layers than on the surface. Enzymes get nearly completely deactivated. The availability of proteins in humans is usually reduced by fryin (Fukuda et al, 1989), especially on the surface(Pokorny et al, 1992). Some essen- tial amino acids are destroyed, such as lysine or tryptophan(Ribarova et al, 1994) If protein comes into contact with the hot walls of the frying pan above the oil level, it is dehydrated and pyrolysed into polycyclic aromatic compounds (Overvik et al, 1989) 12.3.2 Impact of frying on micronutrients Vitamins are relatively labile substances. Tocopherols are decomposed by oxi dation reactions so that frying oil used for repeated frying contains only traces of tocopherols. Ascorbic acid is also destroyed by mechanisms similar to those of reducing sugars. The Vitamin B complex is also substantially damaged by frying(Kimura et al, 1991; Olds et al, 1993). Carotenes and carotenoid pigments e easily oxidised and polymerised(Speek et al, 1988), which is visible from colour changes. Mineral components are also affected. Iron and other heavy metals are mostly bound in complexes, which are partially decomposed during frying, and metal ions may contaminate frying oil by decreasing its resistance to oxidation. Ferric ions are less digestible than iron in haem complexes. Sodium and potassium chlo- rides present in food are very slightly dissociated, and sodium and potassium ions react with free fatty acids forming soaps(Blumenthal and Stockler, 1986). Soaps are surface active agents, increasing foaming and thus accelerating oxidation Volatile mineral components, such as selenium or mercury derivatives, are par tially lost at high frying temperatures. Many foods contain antinutritional or even toxic substances, which are often partially decomposed or evaporated during frying 12.3.3 Changes in sensory characteristics Frying imparts a distinctive flavour to fried products; some flavours are common to all fried foods and some are additional and, specific for particular products, e.g. french fries(Wagner and Grosch, 1998)
If plant foods are subsequently fried in the same oil, cholesterol or fish fatty acids may be absorbed. Lipids present in food are decomposed only to a small extent, including high unsaturated fish oils. It is due to short frying time and limited access of oxygen. Based on dry matter content, the concentration of most nutrients is reduced during frying, as the original nutrients are diluted with absorbed frying oil. Starch and non-starch carbohydrates are partially destroyed during frying, and starchlipid complexes are formed (Thed and Phillips, 1995). The fraction of resistant (undigestible) starch changes during the operation (Parchure and Kulkarni, 1997). Sucrose is hydrolysed into glucose and fructose, which are destroyed by heating, mostly by Maillard or caramelisation reactions. Proteins are rapidly denaturated in surface layers of food particles, more slowly in inner layers than on the surface. Enzymes get nearly completely deactivated. The availability of proteins in humans is usually reduced by frying (Fukuda et al, 1989), especially on the surface (Pokorny´ et al, 1992). Some essential amino acids are destroyed, such as lysine or tryptophan (Ribarova et al, 1994). If protein comes into contact with the hot walls of the frying pan above the oil level, it is dehydrated and pyrolysed into polycyclic aromatic compounds (Övervik et al, 1989). 12.3.2 Impact of frying on micronutrients Vitamins are relatively labile substances. Tocopherols are decomposed by oxidation reactions so that frying oil used for repeated frying contains only traces of tocopherols. Ascorbic acid is also destroyed by mechanisms similar to those of reducing sugars. The Vitamin B complex is also substantially damaged by frying (Kimura et al, 1991; Olds et al, 1993). Carotenes and carotenoid pigments are easily oxidised and polymerised (Speek et al, 1988), which is visible from colour changes. Mineral components are also affected. Iron and other heavy metals are mostly bound in complexes, which are partially decomposed during frying, and metal ions may contaminate frying oil by decreasing its resistance to oxidation. Ferric ions are less digestible than iron in haem complexes. Sodium and potassium chlorides present in food are very slightly dissociated, and sodium and potassium ions react with free fatty acids forming soaps (Blumenthal and Stockler, 1986). Soaps are surface active agents, increasing foaming and thus accelerating oxidation. Volatile mineral components, such as selenium or mercury derivatives, are partially lost at high frying temperatures. Many foods contain antinutritional or even toxic substances, which are often partially decomposed or evaporated during frying. 12.3.3 Changes in sensory characteristics Frying imparts a distinctive flavour to fried products; some flavours are common to all fried foods and some are additional and, specific for particular products, e.g. french fries (Wagner and Grosch, 1998). Frying 297
98 The nutrition handbook for food processors The colour of the fried product often differs substantially from that of the orig- al food material. The most important reactions are nonenzymic browning reac- tions between reducing sugars and free amino acids, called Maillard reactions. Colourless premelanoidins are a group of intermediary products with very low nutritional value. They are rapidly polymerised into macromolecular deep brown melanoidins, which are completely unavailable for human nutrition. To obtain light-coloured potato chips, it is necessary to adjust the concentration of reduc ing sugars to low values(Califano and Calvelo, 1987). A side reaction is the Strecker degradation of free amino acids under attack of dicarbonylic sugar degradation products. Heterocyclic products, such as pyrazines or furans, have typical fried or roasted flavour notes( Chun and Ho, 1997). Similar browning reactions are caused by interactions of frying oil oxidation products(mostly hydroperoxides or unsaturated aldehydes) with the free amine group of bound lysine(Pokorny, 1981). Lysine thus becomes unavailable for human nutrition Oxidised frying oil also contributes in significant degree to the flavour of fried ood( Chang et al, 1978). Moderate amounts are necessary to produce the typical fried flavour but greater amounts are objectionable. For this reason the frying oil should not be too fresh for high quality fried foods. The favour improves by repeated use for frying, but if use is too long then products are of lower quality The producer's aim is to maintain frying oil for the longest time possible at the stage of optimum performance(Blumenthal and Stier, 1991). 12.4 Future trends Deep frying will probably become an even more important process of food prepa- ration, and its conditions and the equipment will be improved gradually. Fried foods contain high amounts of fat, and are therefore rich in available energy. Since energy intake is too high in most countries, frying technology will be modified in order to reduce the absorption of frying oil during the operation; however, the intensity and pleasantness of fried food should remain unaffected Frying oil is oxidised during heating. Criteria for replacing used oil by fresh oil are arbitrary(Firestone, 1993), e.g. 25% polar products and 10% polymers, and are not based on experimental evidence. The composition of oxidation prod ucts and their impact on food safety should be determined more precisely. Frying oil deterioration depends on its degree of unsaturation and on the content of natural and/or added antioxidants. New oils will be introduced on the market with better stability during frying and intervals between necessary replacing of used oil will get longer. The flavour quality of fried products should not be affected by new, more stable frying oils. New flavourings will be developed for flavour- ing products fried in oils free of polyunsaturated fatty acids, or obtained by heating in pans designed for frying without oil. The flavour of such products should be that of deep fried products
The colour of the fried product often differs substantially from that of the original food material. The most important reactions are nonenzymic browning reactions between reducing sugars and free amino acids, called Maillard reactions. Colourless premelanoidins are a group of intermediary products with very low nutritional value. They are rapidly polymerised into macromolecular deep brown melanoidins, which are completely unavailable for human nutrition. To obtain light-coloured potato chips, it is necessary to adjust the concentration of reducing sugars to low values (Califano and Calvelo, 1987). A side reaction is the Strecker degradation of free amino acids under attack of dicarbonylic sugar degradation products. Heterocyclic products, such as pyrazines or furans, have typical fried or roasted flavour notes (Chun and Ho, 1997). Similar browning reactions are caused by interactions of frying oil oxidation products (mostly hydroperoxides or unsaturated aldehydes) with the free amine group of bound lysine (Pokorny´, 1981). Lysine thus becomes unavailable for human nutrition. Oxidised frying oil also contributes in significant degree to the flavour of fried food (Chang et al, 1978). Moderate amounts are necessary to produce the typical fried flavour but greater amounts are objectionable. For this reason the frying oil should not be too fresh for high quality fried foods. The flavour improves by repeated use for frying, but if use is too long then products are of lower quality. The producer’s aim is to maintain frying oil for the longest time possible at the stage of optimum performance (Blumenthal and Stier, 1991). 12.4 Future trends Deep frying will probably become an even more important process of food preparation, and its conditions and the equipment will be improved gradually. Fried foods contain high amounts of fat, and are therefore rich in available energy. Since energy intake is too high in most countries, frying technology will be modified in order to reduce the absorption of frying oil during the operation; however, the intensity and pleasantness of fried food should remain unaffected. Frying oil is oxidised during heating. Criteria for replacing used oil by fresh oil are arbitrary (Firestone, 1993), e.g. 25% polar products and 10% polymers, and are not based on experimental evidence. The composition of oxidation products and their impact on food safety should be determined more precisely. Frying oil deterioration depends on its degree of unsaturation and on the content of natural and/or added antioxidants. New oils will be introduced on the market with better stability during frying and intervals between necessary replacing of used oil will get longer. The flavour quality of fried products should not be affected by new, more stable frying oils. New flavourings will be developed for flavouring products fried in oils free of polyunsaturated fatty acids, or obtained by heating in pans designed for frying without oil. The flavour of such products should be that of deep fried products. 298 The nutrition handbook for food processors
Frying 299 12.5 Sources of further information and advice BLUMENTHAL MM(1987), Optimum frying: theory and practice, Piscataway, NJ, Libra Laboratories BOSKOU D and ELMADFA I(1999), Frying of food, Lancaster, PA, Technomic Publishing PERKINS E G and ERICKSON M D(1996), Deep frying: chemistry, nutrition and practical application, Champaign, IL, AOCS CORNY J(1989), 'Flavor chemistry of deep fat frying in oil,, in Min D B, Smouse T H, ng(eds), Flavour Chemistry of Lipid Foods, Champaign, IL, AOCS, 113-55 special issue on deep frying(1998), Grasas Aceites, 49, No. 3/4 A special issue on deep frying(2000), Eur J Lipid Sci Technol, 102, No. 8/9 12.6 References ATEBA P and MITTAL G S(1994), 'Dynamics of crust formation and kinetics of quality changes during frying of meatballs, J Food Sci, 59, 1275-8, 1290 BAUMANN B and ESCHER F(1995), ' Mass and heat transfer during deep-fat frying of potato slices, I. Rate of drying and oil uptake, Lebensm Wiss Technol, 28, 395-403 BLUMENTHAL MM and sToCKlER JR(1986), ' lsolation and detection of alkaline contam- inant materials in used frying oils, J Am Oil Chem Soc, 63, 687-8 BLUMENTHAL MM and STIER R F(1991),Optimization of deep-fat frying operation Trends Food Sci Technol. 2. 144-8 CALIFANO A N and CALVELO A(1987),'Adjustment of surface concentration of reducing sugars before frying of potato strips, J Food Process Preserv, HANG SS, PETERSON R J and HO C T(1978), Chemical reactions involved in the deep- fat frying of foods, J Am Oil Chem Soc, 55, 718-2 CHE MAN Y B and HUSSIN WRw(1998), 'Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut,, J Food Lipids, 5, 197-210 CHE MAN Y B and TAN CP(1999), 'Effects of natural and synthetic antioxidants on chang in refined, bleached and deodorized palm olein during deep-fat frying of potato chip AOCS,76.331-9 CHUN H K and HOC T(1997), Volatile nitrogen-containing compounds generated from Maillard reactions under simulated frying conditions, J Food Lipids, 4, 239-44 DOBARGANES C, MARQUEZ-RUIZ G and vELASco J(2000), 'Interactions between fat and food during deep frying, Eur J Lipid Sci Technol, 102, 521-8 FILLION L and HENRY C JK(1998), 'Nutrient losses and gains during frying, Int J Food ci Nutr,49,157-6 FIRESTONED(1993), Worldwide regulation of frying fats and oils, INFORM, 4, 1366-71 FUJISAKI M, MORI S, ENDO Y and FUJIMOTO K(2000), " The effect of oxygen concentration on oxidative deterioration in heated high-oleic safflower oil,. JAOCS. 77. 231-4 FUKUDA M I, KUMISADA Y and TOYOSAWA I(1989), 'Some properties and in vitro digestibi ity of fried and roasted soybean proteins, Nihon Eiyo Shokuryo Gakkaishi, 42, 305-11 GERTZ C, KLOSTERMANN s and KOCCHAR S P(2000), Testing and comparing oxidative tability of vegetable oils and fats at frying temperature, Eur J Lipid Sci Technol, 102 43-51 GORDON M H and KOURIMSKA L(1995),'Effect of antioxidants on losses of tocopherols during deep-fat frying, Food Chem, 52, 175-77 KIMURA M, ITOKAWA Y and FUJISAWA M(1991), 'Cooking losses of thiamin in food and its nutritional significance, J Nutr Sci Vitaminol Suppl, 36, 17-24 KOCHHAR S P(2000), 'Stabilization of frying oils with natural antioxidative compone Eur J Lipid Sci Techno, 102, 552-9
12.5 Sources of further information and advice blumenthal m m (1987), Optimum frying: theory and practice, Piscataway, NJ, Libra Laboratories boskou d and elmadfa i (1999), Frying of food, Lancaster, PA, Technomic Publishing perkins e g and erickson m d (1996), Deep frying: chemistry, nutrition and practical application, Champaign, IL, AOCS pokorny´ j (1989), ‘Flavor chemistry of deep fat frying in oil’, in Min D B, Smouse T H, (eds), Flavour Chemistry of Lipid Foods, Champaign, IL, AOCS, 113–55 A special issue on deep frying (1998), Grasas Aceites, 49, No. 3/4 A special issue on deep frying (2000), Eur J Lipid Sci Technol, 102, No. 8/9 12.6 References ateba p and mittal g s (1994), ‘Dynamics of crust formation and kinetics of quality changes during frying of meatballs’, J Food Sci, 59, 1275–8, 1290 baumann b and escher f (1995), ‘Mass and heat transfer during deep-fat frying of potato slices, I. Rate of drying and oil uptake’, Lebensm Wiss Technol, 28, 395–403 blumenthal m m and stockler j r (1986), ‘Isolation and detection of alkaline contaminant materials in used frying oils’, J Am Oil Chem Soc, 63, 687–8 blumenthal m m and stier r f (1991), ‘Optimization of deep-fat frying operations’, Trends Food Sci Technol, 2, 144–8 califano a n and calvelo a (1987), ‘Adjustment of surface concentration of reducing sugars before frying of potato strips’, J Food Process Preserv, 12, 1–9 chang s s, peterson r j and ho c t (1978), ‘Chemical reactions involved in the deepfat frying of foods’, J Am Oil Chem Soc, 55, 718–27 che man y b and hussin w r w (1998), ‘Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut’, J Food Lipids, 5, 197–210 che man y b and tan c p (1999), ‘Effects of natural and synthetic antioxidants on changes in refined, bleached and deodorized palm olein during deep-fat frying of potato chips’, JAOCS, 76, 331–9 chun h k and ho c t (1997), ‘Volatile nitrogen-containing compounds generated from Maillard reactions under simulated frying conditions’, J Food Lipids, 4, 239–44 dobarganes c, márquez-ruiz g and velasco j (2000), ‘Interactions between fat and food during deep frying’, Eur J Lipid Sci Technol, 102, 521–8 fillion l and henry c j k (1998), ‘Nutrient losses and gains during frying’, Int J Food Sci Nutr, 49, 157–68 firestone d (1993), ‘Worldwide regulation of frying fats and oils’, INFORM, 4, 1366–71 fujisaki m, mori s, endo y and fujimoto k (2000), ‘The effect of oxygen concentration on oxidative deterioration in heated high-oleic safflower oil’, JAOCS, 77, 231–4 fukuda m, kumisada y and toyosawa i (1989), ‘Some properties and in vitro digestibility of fried and roasted soybean proteins’, Nihon Eiyo Shokuryo Gakkaishi, 42, 305–11 gertz c, klostermann s and kocchar s p (2000), ‘Testing and comparing oxidative stability of vegetable oils and fats at frying temperature’, Eur J Lipid Sci Technol, 102, 543–51 gordon m h and kourimska l (1995), ‘Effect of antioxidants on losses of tocopherols during deep-fat frying’, Food Chem, 52, 175–77 kimura m, itokawa y and fujisawa m (1991), ‘Cooking losses of thiamin in food and its nutritional significance’, J Nutr Sci Vitaminol Suppl, 36, 17–24 kochhar s p (2000), ‘Stabilization of frying oils with natural antioxidative components’, Eur J Lipid Sci Technol, 102, 552–9 Frying 299
300 The nutrition handbook for food processors MIYAGI A, NAKAJIMA M, NABETANI H and SUBRAMANIAN R(2001), ity of recycling ed frying oil using membrane process, Eur J Lipi 208-1 OHTA S, NAGANO S, YASUSHI A, HONDA K and HARA Y(1988), 'Preventive effects of silicone oil on the thermal deterioration of oils heated at wide surface, Yukagaku, 37 185-9 OLDS S I, VANDERSLICE J T and BROCHETTiD(1993), Vitamin B6 in raw and fried chicken by HPLC, J Food Sci, 58, 505-7, 561 RG I and GUSTAFSSON J A(1989), 'Influence of creatine, amino acids and water on the formation of the mutagenic heterocyclic amines found in cooked meat, Carcinogenesis, 10, 2293-301 NI PR(1997), 'Effect of food processing treatments on gen- eration of resistant starch.Int J Food Sci Nutr. 48 257-60 PERKINS E G and LAMBoNiC(1998), Magnesium silicate treatment of dietary heated fat effects on rat liver enzyme activity, Lipids, 33, 683-7 PINTHUS E J, WEINBERG P and SAGUY I S(1995), Oil uptake in deep fat frying as affected by porosity, J Food Sci, 60, 767-9 POKORNY J(1981), ' Browning from lipid-protein interactions,, Proc Food Nutr Sci, 5, POKORNY J(1998),"Substrate influence on the frying process', Grasas Aceites, 49, 265-70 POKORNY J. REBlova Z KOURIMSKA L. PUDIL F and KWIATKo (1992),“ fect of interactions with oxidized lipids on structure change and properties of food proteins in Schwenke K D, Mothes R, (eds ), Food Proteins, Weiheim, Chemie, 232-5 PRIYA R, SINGHAL R s and KULKARNI PR(1996), ' Carboxymethylcellulose and hydroxy propylmethylcellulose as additives in reduction of oil content in batter based deep-fat fried boonies, Carbohydrate Polymers. 29, 333-5 RIBAROVA F, YURUKOV H and SHISHKov s(1994), ' Stability of tryptophan during heat treat ment of meat products', Hranit Prom, 43, 9-11 ANCHEZ-MUNIZ F J, VIEJO J M and MEDINA R(1992), 'Deep frying of sardines in differ ent culinary fats. Changes in the fatty acid composition of sardines and frying fats J Agric Food Chem, 40, 2252-6 SANCHEZ-MUNIZ F J, CUESTA C and GARRIDO-POLONIo C(1993), Sunflower oil used for a frying: Combination of column, gas and HPSEC for its evaluation, JAOCS, 70, 235-40 SPEEK A J, SPEEK-SAICHUA S and SCHREURS W HP(1988), " Total carotenoid and B-carotene contents in Thai vegetables and the effect of pre ng. Food Chem. 27. 245-57 THED S T and PHILLIPS R D(1995), Changes of dietary fiber and starch composition of processed potato products during domestic cooking, J Food Sci, 52, 301-4 VAGNER R K and GROSCH w(1998), 'Key odorants of french fries, JAOCS, 75, 1385-92 VARNER K, ORR P, PARROTT L and GLYNN M(1994), ' Effects of frying oil composition on potato chip stability,, JAOCS, 71, 1117-21 ARNER K, ORR P and stability of fried foods, JAOCS, 74, 347-56 WU P F and NAWAR ww(1986),'A technique for monitoring the quality of use J Am Oil Chem Soc. 63 XU xQ(2000). 'A new spectrophotometric method for the rapid assessment of deep oil quality, JAOCS, 77, 1083-6
miyagi a, nakajima m, nabetani h and subramanian r (2001), ‘Feasibility of recycling used frying oil using membrane process’, Eur J Lipid Sci Technol, 103, 208–15 ohta s, nagano s, yasushi a, honda k and hara y (1988), ‘Preventive effects of silicone oil on the thermal deterioration of oils heated at wide surface’, Yukagaku, 37, 185–9 olds s i, vanderslice j t and brochetti d (1993), ‘Vitamin B6 in raw and fried chicken by HPLC’, J Food Sci, 58, 505–7, 561 övervik e, kleman m, berg i and gustafsson j a (1989), ‘Influence of creatine, amino acids and water on the formation of the mutagenic heterocyclic amines found in cooked meat’, Carcinogenesis, 10, 2293–301 parchure a a and kulkarni p r (1997), ‘Effect of food processing treatments on generation of resistant starch’, Int J Food Sci Nutr, 48, 257–60 perkins e g and lamboni c (1998), ‘Magnesium silicate treatment of dietary heated fats: effects on rat liver enzyme activity’, Lipids, 33, 683–7 pinthus e j, weinberg p and saguy i s (1995), ‘Oil uptake in deep fat frying as affected by porosity’, J Food Sci, 60, 767–9 pokorny´ j (1981), ‘Browning from lipid-protein interactions’, Proc Food Nutr Sci, 5, 421–8 pokorny´ j (1998), ‘Substrate influence on the frying process’, Grasas Aceites, 49, 265–70 pokorny´ j, réblová z, kourˇimská l, pudil f and kwiatkowska a (1992), ‘Effect of interactions with oxidized lipids on structure change and properties of food proteins’, in Schwenke K D, Mothes R, (eds), Food Proteins, Weiheim, Chemie, 232–5 priya r, singhal r s and kulkarni p r (1996), ‘Carboxymethylcellulose and hydroxypropylmethylcellulose as additives in reduction of oil content in batter based deep-fat fried boondies’, Carbohydrate Polymers, 29, 333–5 ribarova f, yurukov h and shishkov s (1994), ‘Stability of tryptophan during heat treatment of meat products’, Hranit Prom, 43, 9–11 sánchez-muniz f j, viejo j m and medina r (1992), ‘Deep frying of sardines in different culinary fats. Changes in the fatty acid composition of sardines and frying fats’, J Agric Food Chem, 40, 2252–6 sánchez-muniz f j, cuesta c and garrido-polonio c (1993), ‘Sunflower oil used for frying: Combination of column, gas and HPSEC for its evaluation’, JAOCS, 70, 235–40 speek a j, speek-saichua s and schreurs w h p (1988), ‘Total carotenoid and b-carotene contents in Thai vegetables and the effect of processing’, Food Chem, 27, 245–57 thed s t and phillips r d (1995), ‘Changes of dietary fiber and starch composition of processed potato products during domestic cooking’, J Food Sci, 52, 301–4 wagner r k and grosch w (1998), ‘Key odorants of french fries’, JAOCS, 75, 1385–92 warner k, orr p, parrott l and glynn m (1994), ‘Effects of frying oil composition on potato chip stability’, JAOCS, 71, 1117–21 warner k, orr p and glynn m (1997), ‘Effect of fatty acid composition of oils on flavor and stability of fried foods’, JAOCS, 74, 347–56 wu p f and nawar w w (1986), ‘A technique for monitoring the quality of used frying oil’, J Am Oil Chem Soc, 63, 1363–7 xu x q (2000), ‘A new spectrophotometric method for the rapid assessment of deep frying oil quality’, JAOCS, 77, 1083–6 300 The nutrition handbook for food processors
