Factors Affecting Oxidation

The rate of autoxidation in vegetable oils is affected by many factors including degree of unsaturation of fatty acids, metals, and antioxidants. For example, linoleic acid, which contains two double bonds, is oxidized at a faster rate than oleic acid, which contains only one double bond. The relative rates of autoxidation of oleic to linoleic to lin-olenic (3 double bonds) acid has been reported as 1:4050:100 on the basis of oxygen uptake (14). Reasons for differences in autoxidative susceptibility include the fact that the bond strength of linoleic acid (52 kcal/mole) is much lower than oleic acid (77 kcal/mole), and linoleic acid has a higher reactivity with oxygen than oleic acid. Metals such as copper and iron that exist in two different valence states play an important catalytic role in the oxidation of fats and oils. Metals differ in their ability to catalyze the autoxidation of fats and oils depending on the concentration, reaction temperature, and polarity of the reaction medium. The addition of chelating agents such as ethylene-diaminetetraacetic acid (EDTA), phospholipids, and/or free fatty acids to oils can improve their oxidative flavor stability by sequestering the potentially catalytic metal atoms (15).

Natural and synthetic compounds that directly reduce the oxidation of fats and oils in one of three ways: (1) by becoming oxidized themselves, (2) by donating hydrogen to fatty acid free radicals to terminate free-radical chemical chain reactions, or (3) by forming a complex between the lipid radical and the antioxidant radical. Examples of each type of reaction can be seen here:

Antioxidant (A) + 02 Oxidized Antioxidant (1) R- + AH -* RH + A- (2)

Antioxidants such as butylated hydroxyanisole (BHA), bu-tylated hydroxytoluene (BHT), tocopherols, and propyl gallate are commonly used in the food industry to minimize the oxidation of fats and oils. The presence of light, oxygen, and high storage temperatures should be controlled to help antioxidants most effectively minimize oxidative deterioration in fats and oils.

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