Inhibition of methyl linoleate peroxidation by maize zein in powder model system at high water activity

When stored at high relative water activity (A_w0.9), zein has a prominent antioxidant activity against the peroxidation of methyl linoleate, while its papain hydrolysate does not. Using a lipid-soluble radical initiator, AMVN [2, 2′-Azobis(2, 4-dimethyl-valeronitrile)], to alter the induction period of peroxidation of methyl linoleate, we found a turning point just at 24 hr where the rate of oxygen absorption declined. The formation of a solid, tightly aggregated outer shield was observed by scanning electron microscopy. A rather amount of embedded substrate oil (methyl linoleate-methyl stearate, 7:3) was obtained when aggregations formed, and there was little change in the level of embedded oil for at least 16 days. On the other hand, a hydrolysate of zein had scarcely any embedding effect. The ratio of methyl linoleate to methyl stearate showed that embedded oil (hexane unextractable) was more protective to oxidation than hexane-extractable oil. These findings indicate that the substrate oil must have been embedded in the shield formed by zein protein at A_w0.9 to avoid outer oxygen attack.