Antioxidant Activity of Skim Milk: Effect of Sonication

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Abstract
The antioxidant activity of skim milk was evaluated in a linoleate emulsion system with Hb as a pro-oxidant. Sonication greatly increased the antioxidant activity of skim milk. The antioxidant activity of the casein fraction of milk was increased most by sonication; this increase was nearly as great as that for skim milk, indicating that casein was almost totally responsible for the antioxidant effect of sonication. Total SH groups of skim milk decreased upon prolonged sonication, probably the result of the heat evolved in the process. Reactive SH content was unchanged by sonication. Sonication had no effect on antioxidant activity of .beta.-lactoglobulin, reduced urease or reduced RNase, proteins with free SH groups. Apparently, SH groups were not involved in the increased antioxidant activity of sonicated skim milk. Homogenization at 281.5 kg/cm2 did not increase the antioxidant effect of skim milk. Sonication probably disrupted casein micelles, increasing the effective concentration of casein, which could account for the increased antioxidant activity in the system.