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Deferment of an Oxidized Flavor in Frozen Milk by Ascorbic Acid Fortification and by Hydrogen Peroxide Oxidation of the Ascorbic Acid of the Fresh Milk¹

Division of Dairy Products Research Laboratories, Bureau of Dairy Industry, Agricultural Research, Administration, U. S. Department of Agriculture, Washington 25, D. C.

ABSTRACT

Discussion and Conclusions: It ought not to be implied from the work of Krukovsky and Guthrie (3, 4) on market milk stored at 0° to 5° C. that a rapid, complete oxidation of vitamin C in milk by hydrogen peroxide prevents the development of an oxidized flavor indefinitely. The results reported here indicate that, when such milk is preserved in frozen storage, this characteristic off flavor will be detectable eventually. From a chemical standpoint this is to be expected, if the flavor is due to a mild oxidative reaction. Whenever a physico-chemical system tends to have a higher oxidation-reduction potential, the more easily oxidizable constituents (for example, ascorbic acid in milk) will tend to oxidize. Free oxygen is not essential because oxidative changes can take place without its participation, merely by transfer of electrons from reductants to oxidants. So long as this can go on, as it can even in frozen milk, the system will be unstable and tend toward a higher potential. The addition of ascorbic acid to milk lowers the oxidation-reduction potential, the milk is more static and its tendency to develop the oxidized flavor is decreased.

Ascorbic acid, a strong reducing agent, is almost ideal for lowering the Eh of milk. It is a natural constituent of milk; it is one of two equally biologically active forms of vitamin C; it does not alter the flavor, nor does it appreciably increase the acid intensity of the system. Furthermore, it is inexpensive. However, its tendency to oxidize is relatively great. Added to market milk, ascorbic acid is effective in delaying, and usually in preventing, the onset of the oxidized flavor during the life of the product. In frozen milk, which has a longer commercial life than market milk, ascorbic acid defers or delays but does not prevent the defect. This is also true of hydrogen peroxide when it is added to milk that is to be preserved in a frozen state.

¹ This work was done with funds from the Agricultural Research and Marketing Act of 1946.