ANTIOXIDATIVE MECHANISM OF MAIZE ZEIN IN POWDER MODEL SYSTEMS AGAINST METHYL LINOLEATE - EFFECT OF WATER ACTIVITY AND COEXISTENCE OF ANTIOXIDANTS

Food proteins such as soybean protein, milk casein, maize gluten, and zein were examined against methyl linoleate autoxidation in powder model systems. A gas chromatographic (GC) method was employed to estimate antioxidant activity by measuring oxygen consumption of head-space gas. Although strong antioxidant activities were observed for gluten and zein, stored at 60-degrees-C without humidity regulation, they resulted from phenol compounds contaminating in the proteins such as tocopherols and not from the antioxidant action of proteins themselves. Water activity affected the antioxidative efficacy when alpha-tocopherol was added to the proteins: the tendency that the higher the A(w), the stronger was the antioxidant activity was found. But the prominent antioxidant activity of defatted zein at A(w) 0.9 could not be contributed only to the activity of traces of unextracted phenolics contained in zein, as zein's hydrolysate almost lost activity at the same condition. A remarkable decrease in hexane-extractable efficiency of substrate oil at A(w) 0.9 compared to that at A(w) 0.3 implied a possible mechanism which suggested a delayed binding to protein or physical shielding by protein resulting in a reduced tendency of the lipid to autoxidize.