Catalysts of lipid peroxidation in meats

Abstract

AbstractHemoprotein and non‐heme iron components are active catalysts of lipid peroxidation. The behavior of these two catalysts under a number of conditions was compared as a basis for a study of their activities in meats. In model systems, MetMb accelerated linoleic acid peroxidation in a pH range from 5.6 to 7.8; it catalyzed especially rapidly at higher pH. A complex of ferrous ion [Fe(II)] and EDTA, a non‐heme iron model, in a 1:1 ratio accelerated peroxidation at lower pH; no catalysis took place above pH 6.4. Most chelating agents eliminated Fe(II)‐EDTA catalysis, but had no effect on MetMb catalysis. Reducing agents, on the other hand, accelerated Fe(II)‐EDTA catalysis but inhibited MetMb catalysis. In model systems in which fresh dilute (1.2%, w/v) meat homogenate was the catalyst, the effect of the heme predominated. An exception was ascorbic acid; it accelerated oxidation at pH 5.6. The pattern of linoleate peroxidation catalyzed by heme‐free (H2O2‐treated) beef homogenate and shrimp homogenate was similar to that in the Fe(II)‐EDTA model system. Again, ascorbic acid accelerated the catalysis and the acceleration could be eliminated by adding chelating agents. The presence of a non‐heme iron catalyst in meat is thus indicated. Evidence is presented for both types of catalytic activity in meats. In cooked meats, heme was the dominant catalyst, but significant lipid oxidation, apparently catalyzed by a non‐heme iron‐type catalyst, occurred in cooked meats in which the heme had been destroyed by H2O2. In raw meats, lipid oxidation was inhibited at high pH because of removal of oxygen by enzymatic reducing systems. Both heme and non‐heme iron were active at lower pH values. EDTA inhibited lipid oxidation during storage, presumably by its demonstrated effect on non‐heme iron catalysis. Ascorbic acid also inhibited lipid oxidation, probably indirectly by keeping the heme pigment in the catalytic inactive ferrous state.