Natural tocopherols were removed from soybean oil with carbon-black. Partial stripping of the tocopherol from soybean oil markedly improves its oxidative stability, but results in only an occasional improvement in its flavor stability. Therefore, the high concentration of natural tocopherols in soybean oil (1500 ylg) appears to be excessive for maximal oxidative stability. Optimum tocopherol concentration varied from 400 to 600 y/g. u-Tocopherol, BHA '"+, B H T , and propyl gallate did not improve the oxidative and flavor stability of tocopherol-stripped soybean oil. Anhydo-piperidino-hexosereductone, ascorbic acid, Evanacid a n d citric acid increased markedly the oxidative stability of tocopherolfree soybean oil without, however, improving its flavor stability. The extent of oxidative "synergism" between tocopherol and citric acid decreased at high concentrations of tocopherol.
T h e noneffectiveness of antioxidants in soybean oil in controlling flavor reversion cannot be attributed to the high, initial concentration of tocopherols. Lack of activity of antioxidants, either in the presence or absence of tocopherol, indicates that tocopherol shows neither synergistic nor antagonistic effects. The failure of antioxidants in soybean oil is attributed to its highly unsaturated fatty acid composition. Hydroperoxide decompositions which contribute to flavor reversion in soybean oil m a y not b e controlled b y antioxidants.
Phenolic antioxidants a r e well known to b e ineffective in stabilizing highly unsaturated oils which a r e prone to flavor reversion. Since soybean oil contains a relatively high concentration of natural tocopherols, further addition of phenolic antioxidants would not b e expected to improve the oil. The early study of C. R.
Thompson a n d N. Sfeenbockl showed that very small additions of a-tocopherol (2-4 y l g ) had n o antioxidant effect on various fats unless they had been previously freed from natural antioxidants by adsorption through alumina.
Few studies have been reported on the effect of antioxidants in soybean oil probably because of its high concentration of tocopherols (1500 ylgj. Reductones were recently found to b e a notable exception in improving markedly the oxidative stability of vegetable oils, including soybean oil, when evaluated chemically, however, no improvement in organoleptic stability could b e shown with these additives. Other studies with refined vegetable oils 3 , have indicated that although phenolic anti-: > Dedicated to Prof.