| Milk and milk fats |
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Milk contains several potential catalysts of lipid oxidation in the form of native Cu2+ and metalloenzymes, like xanthase oxidase and lacto peroxidase. Yet, it remains remarkably stable in spite of the fact that the concentration of ascorbic acid and tocopherols steadily decreases during storage. Therefore, it is assumed that milk itself is protected by means other than conventional antioxidants. It appears that major proteins, especially casein, are quite effective at binding metals, such as Cu2+ and Fe3+. This can physically prevent contact between the metal ion and its triglyceride substrate. In addition, raw milk – and to a lesser extent pasteurized milk – has its triglyceride protected by the fat-globule membrane. Processing milk does not remove the factors that render the fat liable to oxidation, even though pasteurization deactivates the enzymes lacto peroxidase and xanthine oxidase. On the contrary, separating, churning, drying and additives can often increase lipid oxidation by interrupting the fat-globule membrane, intermixing catalysts with the lipid, and introducing contamination from the plant. For example, migration of Cu2+ into cream on churning can cause rapid flavor impairment. Synthetic antioxidants (BHA, BHT, propyl gallate) and metal chelators (citric acid, phosphoric acid, salts of EDTA) have proven effective in inhibiting oxidation, but use of these compounds in milk fat is not permitted in the USA, EU and most other countries. So, milk fat is not treated with any antioxidant, but rather is refrigerated to prolong shelf life. However, there are a few examples of milk fat products that must be stored at room temperature and therefore need protection against oxidation: Milk powder for vending machines is one example. Tocopherols and ascorbyl palmitate can be used unrestricted in most countries and some very effective antioxidant blends have been developed to successfully replace the synthetic products in many applications. |
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