INFLUENCE OF FATTY-ACIDS OF DIFFERENT UNSATURATION IN THE OXIDATION OF PURIFIED VEGETABLE-OILS DURING MICROWAVE IRRADIATION

Purified vegetable oils (soya bean, safflower and rapeseed) containing stearic, oleic. linoleic or linolenic acid at different levels (0.5-10.0 g kg-1). were exposed to microwave irradiation for several periods of time to study the contribution of fatty acids of different unsaturation to the oxidative stability of these oils. The evaluation of the oils was based on peroxide value or carbonyl and anisidine values as an indicator for the primary or secondary oxidation products, respectively. A pro-oxidative effect of the added fatty acids was established. The extent of this effect depended on the degree of unsaturation of added fatty acids and/or purified vegetable oils. During microwave treatment the oxidative rate of purified vegetable oils was rapid in the presence of the unsaturated fatty acids. but comparatively lower for the corresponding saturated fatty acid or hydrocarbon. The oxidative state of the oils was lower in the rapeseed oil than in the soya bean oil or safflower oil. In assessing the propensity to oxidation particular attention should be paid to not only the type of free fatty acids, but also the difference of glyceride structure in the purified vegetable oils.