During frying, moisture is continuously released from the food into the hot oil. This process is equivalent to steam distillation, in order to remove volatile oxidation products from the oil. The released water acts to agitate the oil and accelerate the hydrolysis. Some volatile substances are produced due to the interaction of the food itself or the food and oil, such as sulfur compounds and pyrazine derivatives in potatoes. When frying, the food absorbs different amounts of oil. The oil content of fried potato chips is 30% -40%. Therefore it is necessary to continuously add fresh oil to quickly reach stable conditions. Food can also release some of the inner fat (such as the fat of meat products) into the fried fat. The oxidative stability of the newly mixed fat is very different from the original fried fat.
During the frying process, the frying oil undergoes intense chemical and physical changes. Volatile and unsaturated aldehydes, ketones, hydrocarbons, lactones, alcohols, acids and esters are produced from the formation and decomposition of hydroperoxide. Dimerization and polyacids are produced by the combination of heat and oxidation of free radicals. As well as dimerization and polyglycerol, the viscosity of the frying oil is significantly increased as a result of the polymerization; the triacylglycerol is hydrolyzed to form free fatty acids, which increase the viscosity and free fatty acid content of the oil, darken the color. The tension decreased and the refractive index changes. The tendency to form foam increases. Therefore, both the oil and the food undergo great physical and chemical changes during the frying process, and some changes impart the desired sensory quality of the fried product, and some changes are harmful.
Excessive decomposition of oils and fats will destroy the nutritional and sensory qualities of fried foods. So it must be strictly controlled. The chemical and physical changes frying fats by these low temperature vacuum frying are subject to various frying parameters. Studying the safety evaluation of oil after repeated frying has become the focus of research by foreign scholars. Shyi-LiangShyuetal (1998) and others studied the stability of oil after repeated vacuum frying, and pointed out the difference in stability of different types of oil during low temperature vacuum frying and the rapid detection of acid value, peroxide value and carbonyl value of oil. Experiments show that vacuum fried oil has lower oxidative degradation rate than normal pressure frying. In the early stage of frying, soybean oil has similar oxidative stability to lard and palm oil. However after fried for 24 hours, the rapid degradation happened.
In the meanwhile, the peroxide value, carbonyl value and dielectric constant have a good correlation with frying time. At present, there is no corresponding systematic research in domestic.