Influence of milk proteins on lipid oxidation in aqueous emulsion: II. Lactoperoxidase, lactoferrin, superoxide dismutase and xanthine oxidase

SUMMARY The milk-related model system described by Allen & Wrieden (1982) was used to assess the effects of a number of purified milk metalloproteins, at concentrations relevant to those in milk and dairy products, on triglyceride oxidation in emulsions. Oxidations were monitored by the rate of O 2 utilization (ROU) and by the thiobarbituricacid (TBA) assay. The proteins lactoferrin, lactoperoxidase, superoxide dismutase and xanthine oxidase were used either on their own or in conjunction with 10 μM-Cu 2+ or Fe 2+ . Lactoferrin alone had little effect on oxidation, although both the iron-free and iron-saturated protein were able to protect the lipid to some extent from Fe 2+ -catalysed oxidation. Cu 2+ -catalysed oxidation was slightly promoted. Lactoperoxidase was pro-oxidative in the absence and presence of added Cu 2+ or Fe 2+ . Heat treatment at 72°C/20 s had little effect, but oxidation was greatly reduced by treatment at 80°C/20 s. Superoxide dismutase strongly reduced the rate of lipid oxidation in the absence of added metals, but had no effect in the presence of 10 μM-Cu 2+ . xanthine oxidase had little effect on lipid oxidation in the absence of added metals, but was strongly pro-oxidative in the presence of 10 μ-Cu 2+ . Similar effects were observed down to 1 μM-Cu 2+ . The enzymic activity of xanthine oxidase was rapidly eliminated by 10 μM-Cu 2+ , but not by 10 μM-Fe 2+ . A moderate pro-oxidative effect was observed in the presence of 10 μM-Fe 2+ . Heat treatment of xanthine oxidase at 80°C/20 s reduced its lipid pro-oxidative effect in the presence of Cu 2+ more effectively than treatment at 72°C/20 s.