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 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 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, 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%. 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