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Abstract
Both ascorbic acid and copper were strong prooxidants in the oxidation of linoleate in a buffered (pH 7.0) aqueous dispersion at 37 degrees C. Minimum concentrations at which catalytic activity was detected were 1.3 x 10(-7) m for copper and 1.8 x 10(-6) m for ascorbic acid. For concentrations up to 10(-3) m, the increase in rate of oxidation with increase in concentration of catalyst was greater for ascorbic acid than for copper. Ascorbic acid had maximum catalytic activity at 2.0 x 10(-3) m, but was still prooxidant at the highest concentration tested (5.0 x 10(-2) m). Dehydroascorbic acid was a weaker prooxidant than ascorbic acid. Further degradation products of ascorbic acid were not prooxidant. In early stages of the oxidation autocatalytic behavior was observed with copper, but not with ascorbic acid. Ascorbic acid functioned as a true catalyst, i.e., it accelerated the reaction but it was not oxidized simultaneously with the linoleate. It is proposed that the dehydroascorbic acid radical initiates the linoleate oxidation reaction.
Highlights
M for coppcr and 1.8 X 10-6 M for ascorbic acid
In early stages of the oxidation autocatalytic behavior was observed with copper, but not with ascorbic acid
Differences in the shapes of the curves indicate a different mode of catalysis for copper from that for ascorbic acid
Summary
M for coppcr and 1.8 X 10-6 M for ascorbic acid. For concentrations up to lop3M, the increase in rate of oxidation with increase in concentration of catalyst was greater for ascorbic acid than for copper. Further degradation products of ascorbic acid were not prooxidant. In early stages of the oxidation autocatalytic behavior was observed with copper, but not with ascorbic acid. Ascorbic acid functioned as a true catalyst, i.e., it accelerated the reaction but it was not oxidized simultaneouslywith the linoleate. E A R L , REPORTS [1, 2] noted that ascorbic acid promotes lipid oxidation in milk products, but Olson and Brown [3] reported the first definitive proof for its role. Ascorbic acid may inhibit lipid oxidation (7, 1012). Ascorbic acid should be an antioxidant because of its oxidation-reduction potential and the stability of its oxidation products [13]
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