Reactions of proteins with oxidizing lipids. 1. Analytical measurements of lipid oxidation and of amino acid losses in a whey protein-methyl linolenate model system.

The reactions between protein-bound amino acids and oxidizing lipid were investigated in a whey protein-methyl linolenate (C18:3)-water model system. The extent of fat oxidation was followed by measuring oxygen uptake, hydroperoxide formation and hydrocarbon (ethane and pentane) formation. Significant losses occurred with lysine (up to 71%), tryptophan (up to 31%) and histidine (up to 57%). Methionine was extensively oxidized to its sulphoxide but less than 2% was further oxidized to the sulphone. No other amino acids were affected. Increasing storage temperature (20 degrees, 37 degrees, 55 degrees) resulted in an enhancement of fat oxidation reactions and amino acid degradation. Increasing water activity (0.28, 0.65, 0.90) increased losses of lysine and tryptophan but had no influence on the oxidation of methionine, the level of remaining hydroperoxides or O2 uptake. Hydrocarbons were decreased. Limitation of O2 uptake to 1 mol/mol lipid instead of excess O2 (O2 uptake about 2.5 mol/mol lipid in 4 weeks) significantly reduced the degradation of lysine and tryptophan but had less influence on the oxidation of methionine. The level of remaining hydroperoxides was increased but hydrocarbons were unaffected.