Ascorbic acid and copper in linoleate oxidation. III. Catalysts in combination

In promoting oxidation of 0.02 m potassium linoleate in a buffered (pH 7.0) aqueous dispersion at 37 degrees C, ascorbic acid at low concentrations (1.8 x 10(-6) and 1.8 x 10(-5) m) in combination with copper (1.3 x 10(-7) to 1.3 x 10(-3) m) had greater catalytic activity than the additive activity of the two catalysts individually. Possible explanations for the enhanced catalysis include reduction of copper by ascorbic acid to the cuprous form, increased concentration of semidehydroascorbic acid radical, and formation of a metal-ascorbic acid-oxygen complex. Some combinations of ascorbic acid (1.8 x 10(-4) and 1.8 x 10(-3) m) and copper (1.3 x 10(-6) and 1.3 x 10(-3) m) inhibited the formation of conjugated dienes but not the oxidation of ascorbic acid, and caused rapid loss of part of the conjugated dienes that were already present. It is suggested that free-radical inhibitors formed by the combination of catalysts inhibit initiation of lipid oxidation but not copper-catalyzed oxidation of ascorbic acid. Effects of the inhibitory combinations on changes in UV absorption by conjugated dienes, and absorbance in the TBA test, indicate the presence of at least two conjugated dienes that differ in stability.