Autoxidation And Antioxidants

The unsaturated bonds present in all oils and fats represent active centers, which may react with oxygen. This reaction leads to the formation of primary, secondary, and tertiary oxidation products which may result in the fat or fat-containing food becoming unfit for consumption. Oxidation of fats and oils may occur by a free-radical chain reaction known as autoxidation. Another process of oxidative deterioration involves the presence of a sensitizer and exposure to light. This is known as photosensitized oxidation (23,24).

The process of autoxidation and the resulting deterioration in flavor of fats and fatty foods are often described by the term rancidity. Lundberg (25) distinguishes several types of rancidity. When fats are exposed to oxygen, common oxidative rancidity will result in sweet but undesirable odors and flavors which will progressively become more intense and unpleasant. Flavor reversion is the term used for the objectionable flavors that develop in oils containing linolenic acid, especially soybean oil. This type of oxidation is produced with considerably less oxygen than common oxidation.

Among the many factors which affect the rate of oxidation are the following: amount of oxygen present, degree of unsaturation of the oil, presence of pro- and antioxidants, presence of heme-containing molecules and lipoxi-dase, light exposure, nature of packaging material, and storage temperature.

The autoxidation reaction can be divided into the following three parts: initiation, propagation, and termination. During the initiation, hydrogen is abstracted from an olefinic compound to yield a free radical:

The removal of hydrogen takes place at the carbon atom next to the double bond. The dissociation energy of hydrogen in various olefinic compounds has been listed by Ohloff (26). The value for an isolated double bond is 103 kcal/mol; for two double bonds separated by a methylene group it is only 65 kcal/mol. Once a free radical is formed, it will combine with oxygen to form a peroxy free radical, which can in turn abstract hydrogen from another unsaturated molecule to yield a peroxide and a new free radical, thus starting the propagation reaction. This may be repeated up to several thousand times and has the nature of a chain reaction:

The propagation reaction can be followed by termination if the free radicals react with themselves to yield non-active products:

The hydroperoxides formed in the propagation part of the reaction are the primary oxidation products. The hydroperoxide mechanism of autoxidation has been described (27) and reviewed (23). The primary oxidation products are unstable and decompose into secondary oxidation products, mostly carbonyls. The peroxides are not important in flavor deterioration, which is caused wholly by secondary oxidation products. In the initial stages of the reaction there is a slow increase in the amount of hydroperoxides formed; this is the induction period. At the end of the induction period there is a sudden and rapid increase in peroxide content. The induction period is measured in accelerated tests to determine the storage stability of a fat or oil.

The rate of oxidation depends greatly on the degree of unsaturation. In the series of 18-carbon-atom fatty acids 18:0, 18:1, 18:2, 18:3, the relative rates of oxidation have been reported to be in the ratio of 1:100:1200:2500. In addition to the degree of unsaturation, the position of the double bonds in a polyunsaturated fatty acid may affect its oxidation rate. Zhan and Chen (28) found that conjugated linolenic acid oxidized considerably faster than linolenic acid. The reaction of unsaturated compounds proceeds by the abstraction of hydrogen from a carbon a to the double bond, resulting in a free radical stabilized by resonance. These free radicals are then transformed into a number of isomeric hydroperoxides of the general structure

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