Natural oils and fatty acids have many functional groups such as carboxyl groups, ester bonds, double bonds, carboxyl groups, and α-H of double bonds, and thus many reactions can occur.
(1) Hydrolysis of oils and fats
Under appropriate conditions, the reaction of oils and fats with water to form glycerol and fatty acids is called hydrolysis. This reaction is carried out stepwise and reversibly, first hydrolyzed to form a diglyceride, then hydrolyzed to form a monoglyceride, and finally glycerol. The triglyceride is insoluble in water, so its hydrolysis rate is very slow, but the hydrophilicity of the diacyl ester and the monoglyceride is sequentially increased, so that the hydrolysis rate is sequentially increased. The reaction can be accelerated by the high temperature and high pressure and a large amount of water, and the inorganic acid, alkali, enzyme, sulfonic acid and metal oxide can also accelerate the hydrolysis reaction.
The free fatty acids in foods can also catalyze the hydrolysis of oils and fats, especially in the high temperature of canned food heat sterilization or when the food is fried and heated, the oil hydrolysis reaction is quite rapid. Since the rate of the hydrolysis reaction is proportional to the amount of free fatty acids, the reaction produces more free fatty acids which tend to make the reaction more rapid.
Metal oxides can also catalyze the hydrolysis of oils and fats. Metal oxides such as ZnO, MgO, and CaO catalyze the hydrolysis of oils and fats at medium pressure and at 150-225 ° C to produce light-colored fatty acids that do not require distillation. Non-catalyzed high pressure hydrolysis is also employed in the industry, namely continuous countercurrent hydrolysis at high pressure and 240 to 260 °C. The method has high hydrolysis rate and is economical and effective, but is not suitable for hydrolysis of heat-sensitive greases, because heat-sensitive greases such as conjugated acid-containing fats and oils undergo thermal polymerization under high temperature and high pressure.
Lipolytic enzyme can catalyze the hydrolysis of oils and fats at normal temperature and pressure, so it is especially suitable for the hydrolysis of heat-sensitive oils. However, industrial development has not yet been carried out. The main reason is that the degree of reaction is incomplete, the speed is slow, and unit handling is difficult.
(2) The rancidity of fat
Oxidation of oils and fats is one of the main causes of food spoilage. It causes edible oils and fat-containing foods to produce various odors and odors, collectively known as rancidity. In addition, oxidation reactions can reduce the nutritional value of foods, and certain oxidation products may be toxic. Oxidation of oils and fats can be divided into automatic oxidation, photosensitive oxidation and enzymatic oxidation. The oxidized primary product hydroperoxide is extremely unstable and can be rapidly decomposed to form an alkoxy radical, and the alkoxy group can further form a hydrocarbon, an aldehyde, a ketone, an alcohol, an acid, and the like. Enzymatic oxidation (β-oxidation) produces keto-type rancidity in foods, and the β-oxidation intermediate β-ketoacyl-CoA can form methyl ketones under the action of decarboxylase or hydrolyze to form keto acids. Both methyl ketone and keto acid have unpleasant odors.
The polyunsaturated fatty acid having a cis, cis-pentadiene structure can be catalytically oxidized by the lipoxygenase pathway to form a hydroperoxide having a conjugated double bond. Finally, the decomposition product cis-3-hexenal has a grassy or green bean flavor, that is, the strange smell before the edible oil is rancid, which is called aftertaste. This oxidative pathway is very broad in food.
Oxidation of fats and oils is closely related to the composition of fatty acids, free fatty acids, oxygen, temperature, surface area, moisture, light and radiation, co-oxidants and antioxidants. Among them, transition metals, especially divalent or multivalent transition metals with suitable oxidation-reduction potentials are co-oxidants, and the oxidation catalysis of different metals is as follows: lead>copper>brass>tin>zinc>iron>stainless steel>silver. Oil antioxidants are a class of substances that can delay or slow the oxidation of oils. They are natural and synthetic, and can be classified into two types: fat-soluble water-soluble according to solubility. For example, there are butyl hydroxyanisole (BHA), dibutylhydroxytoluene (BHT), propyl gallate (PG), and the like, and natural tea polyphenols, tocopherols, and the like.
(3) Changes in fats and oils at high temperatures
1. Thermal polymerization and thermal oxidation polymerization
Under high temperature conditions such as atmospheric frying, the polymerization and decomposition of fats and oils are very prominent. The process of polymerizing into macromolecular compounds by Diels-Alder reaction is roughly as follows: cis-non-conjugated unsaturated fatty acids in natural oils undergo cis-trans isomerization and positional isomerization at high temperatures to form inverse, reverse-conjugated double bonds. A compound which no longer undergoes a Diels-Alder reaction with an unsaturated fatty acid to form a dimeric monocyclic compound. The reaction can be carried out either inside a triacylglycerol or between two triacylglycerol molecules.
There are many ways to polymerize at a high temperature of aerobic, for example, the oil is oxidized to be converted into a radical, and then polymerization into an oxidized polymer is another polymerization route. Thermal oxidative polymerization is faster and can be catalyzed by metals, and the products are more complex and diverse. For example, arachidonic acid can form a variety of cyclic compounds, and two molecules of triacylglycerol can form glycerolipid dimers. Studies have shown that some products are toxic substances, and ingestion can cause abnormal physiological reactions.
2, the condensation of oil
The oil undergoes a hydrolysis-dehydration-condensation reaction at a high temperature to form an ether type compound.
3. Thermal decomposition and thermal oxidative decomposition of oils and fats
The oil is thermally decomposed under high temperature and anaerobic conditions to form small molecule compounds such as acrolein, fatty acid, CO2, methyl ketone and propylene glycol diester. Under high temperature and aerobic conditions, the saturated oils are thermally oxidized and decomposed to form various hydrocarbons, aldehydes, methyl ketones and γ-lactones. In addition to the decomposition of unsaturated oils, the oxidative decomposition of fatty acid chains can produce aldehydes, ketones and acids. Small molecule compounds such as lactones, alcohols, CO2, short chain fatty acid esters, and hydrocarbons. When oxygen attacks the α, β, γ-methylene groups of the carbonyl group, the products formed are different. When oxygen attacks the α-methylene group adjacent to the alkenyl group on the fatty acid chain of the unsaturated oil, the reaction mechanism is similar to that of the automatic oxidation.