Chemical changes in cooking oil during frying

      Frying technology is an ancient way of cooking food, which uses fat as a heat transfer medium to heat food from the outside to the inside until it is mature. However, the high temperature, water and oxygen-rich environment of the frying process will cause various chemical reactions such as oxidation, hydrolysis, polymerization or cracking of edible oils.

1. Oxidation reaction

Oil deterioration is a key factor that causes the toxicity of edible oil, and lipid oxidation is one of the main reasons for oil deterioration. Deterioration of oils and fats can cause undesirable flavors and odors in edible oils and fat-containing foods. For example, the odor experienced by consumers when tasting fried foods is often caused by deterioration of oils and fats. Once food contains havoc, it will cause dissatisfaction among consumers. Therefore, how to prevent lipid oxidation is a key issue that the food industry needs to overcome. Under normal circumstances, the deterioration process of oil will produce two major types of decomposition products: one type mainly affects the sensory quality of fried food. Because it is volatile, most of the components will disappear with the moisture during the frying process. Evaporate and escape from food; the other type is not volatile. The most abundant decomposition products of this type are polar substances - by reducing the surface tension of oil and fat, its surface activity is increased, thus seriously endangering human health.

1.1. Oxidation reaction of unsaturated oil components

The unsaturated fatty acids in oil will spontaneously undergo oxidation reactions due to higher frying temperatures. The process mainly includes three stages. Induction period: Because only a small amount of oxygen is absorbed, the quality of the food does not change much; Development period: The quality of fried food is significantly deteriorated due to the large amount of oxygen absorption; Termination period: Oxygen gradually becomes saturated and the absorption of oxygen gradually stops. During the oxidation process of oil, the first chemical reaction of unsaturated fatty acid free radicals is carried out. The speed at this stage is relatively slow and mainly generates peroxide (primary product); subsequently, the peroxide is decomposed and rearranged to form Aldehydes, alcohols, hydrocarbons (secondary products); then aldehydes undergo oxidation to generate carboxylic acids. The absorption of oxygen increases significantly at this stage, the peroxide value increases, and the content of volatile substances also increases significantly, causing the oil to undergo Violent oxidation, the quality of oil begins to decline.

1.2. Oxidation reaction of saturated oil components

Saturated oils generally do not oxidize, but they will undergo oxidation reactions when the oil temperature exceeds a certain level. When oil is heated to above 150°C in the presence of oxygen, the oxidation products of oil molecules contain a certain amount of alkanes and fatty acids, a small amount of alcohols and γ-lactones, and methyl ketones and aldehydes of different relative molecular masses. In this case, oxygen will preferentially attack the α, β, and γ carbons closer to carbonyl, and then further decompose to form hydroperoxides.

2. Hydrolysis reaction

During the frying process, the moisture contained in the food raw materials itself comes into contact with the oil, and the ester bonds of the oil will be broken due to hydrolysis, producing glycerin and free fatty acids. Thereafter, free fatty acids continue to undergo thermal oxidation reactions to produce lipid peroxides, which are eventually decomposed into a variety of small-molecule compounds. At higher temperatures, the loss of water from glycerol will produce easily volatile acrolein. When frying, the more water there is in the food, the more water will enter the oil, which will cause the food debris to increase as the oil temperature rises, causing the free fatty acids to generate oil faster. Under the same frying conditions, if the oil renewal rate is high and the oil quality is relatively high, the production rate of free fatty acids will be relatively slow.

3. Thermal decomposition

Various chemical reactions will occur during the heating process of food, some of which will affect the appearance, flavor, nutritional value of the food, and even cause toxicity. During the frying process, the nutrients in the food will decompose with the oil, and complex chemical reactions will also occur between these nutrients to generate a large number of new compounds. During this process, saturated fatty acids will generate a small amount of lactones, hydrocarbons, and carbonyl compounds with shorter chain lengths; unsaturated fatty acids mainly generate dimers when heated under anaerobic conditions, and under aerobic high-temperature conditions The products are mainly oxidized dimers, epoxides, hydroperoxides, hydroxides or carbonyl groups, ethers, etc.

4. Thermal polymerization reaction

    During high-temperature frying, the unsaturated fatty acid double bonds in the oil will undergo two reactions: reverse addition and polymerization, forming polymers that deepen the color of the oil and increase the viscosity of the oil. The main factors that affect the oxidation rate include oil temperature, surface area temperature of food contact grease, unsaturated fatty acids, and the content of metal ions such as copper and iron also has a great impact on the oxidation rate. In addition, fried foods absorb a large amount of oil, the slow renewal frequency of cooking oil, and the presence of ultraviolet rays can accelerate the oxidation of oil.