Antioxidant Activity of Skim Milk: Effect of Heat and Resultant Sulfhydryl Groups

Abstract

Skim milk was evaluated for antioxidant activity in a linoleate emulsion with hemoglobin as a prooxidant. Milk sulfhydryl groups were measured with Ellman's reagent in the presence or absence of a denaturant. Heat treatment of skim milk increased antioxidant activity, increased “reactive” sulfhydryl groups, but decreased “total” sulfhydryl content. Addition of iodoacetic acid to raw or heated skim milk decreased antioxidant activity and sulfhydryl content. However, the role of the sulfhydryl group in the antioxidant activity of skim milk was not clear. Cysteine had a marked optimum in antioxidant activity at pH 8.6, but the antioxidant activity of heated skim milk, which was optimal at pH 7.2, was much less pH dependent. The antioxidant activity of skim milk resided in both whey and casein proteins. Casein provided a larger portion of skim milk antioxidant activity, but casein had only a small amount of sulfhydryl groups. The antioxidant activity of casein was reduced but not eliminated by treatment with iodoacetic acid. After treatment with iodoacetic acid, whey lost all antioxidant activity, but whey sulfhydryl content was not eliminated fully. Skim milk supplemented with cysteine or treated with sodium borohydride had greatly increased sulfhydryl content but little increase in antioxidant activity. Sulfhydryl groups were apparently responsible for only part of the antioxidant activity of skim milk. © 1980, American Dairy Science Association. All rights reserved.