A study of oxidation reactions in buffered emulsions of linoleic acid was undertaken as a means of investigating some of the important chemical reactions associated with the production of oxidised off-flavours in dairy products.
Previous investigations in this field had shown that metal, particularly copper, contamination was the most important single factor promoting lipid oxidation in milk and butter. A reducing agent, such as ascorbic acid, was found to greatly accelerate oxidation rates under some conditions. It was generally thought that the principal role of ascorbic acid was to reduce copper to the cuprous form and that cuprous copper was the active catalyst.
The method adopted in this study was to systematically observe the effects of variation in ascorbic acid and copper concentration on oxidation of the emulsions at various pH's. Oxygen consumption over a 90 hour period and thiobarbituric acid test values at the end of the period were used as indices of fatty acid oxidation.
Ascorbic acid at a concentration of 10-3M. was a more effective pro-oxidant in the presence of low copper concentrations (0.1 - 0.01 p.p.m.) than with high concentrations (10 p.p.m.), particularly in the initial stages of oxidation or in the lower pH range (5.5 - 4.7). This fact alone was sufficient to suggest a much more complex role for ascorbic acid than that of copper reducing agent. In fact, the ascorbic acid experiments and subsequent experiments with metal chelating agents gave strong indications that the initial oxidation state of the metal was a matter of minor importance. It was concluded that chelation of copper by ascorbic acid .and prevention of metallic termination of radical chain reactions was one important function of ascorbic acid. But this was not, in itself, sufficient to explain the pro-oxidant activity. Reaction of the ascorbic acid radical with oxygen and consequent formation of free HO• radicals was considered to be the most likely initiating reaction resulting from the oxidation of ascorbic acid to dehydro-ascorbic by copper.
Dehydro-ascorbic acid was also found to be a very effective pro-oxidant in the presence of copper. The evidence suggested that this was not due to regeneration of ascorbic acid, but there was insufficient data available to offer any substantial altemative explanation.
Metal chelating agents - 1,10 phenanthroline, E.D.T.A., and citric acid - on the pro-oxidant activity of copper were studied in further experiments. Citric acid had little effect. E.D.T.A. was found to inhibit the pro-oxidant effects of copper. 1,10 phenanthroline was found to be a particularly effective pro-oxidant when present in a concentration of more than three times the molar equivalent of copper concentration. Ascorbic acid was shown to have an inhibitory effect on oxidation of linoleic acid by the phenanthroline/copper complex, as well as an expected pro-oxidant effect. The predominance of one effect was dependant on the relative concentrations of the two chelating compounds.
The broad conclusion reached was that the role of copper as an oxidation catalyst was dependant on the nature and concentration of groups capable of co-ordinating with the metal. Lack of information in a number of areas prevented this conclusion being extended in more specific terms. Some suggestions were made on this basis as to possible areas for future research.