Retention of Ascorbic Acid, Changes in Oxidation–Reduction Potential, and the Prevention of an Oxidized Flavor During Freezing Preservation of Milk

Abstract

Outstanding among beverage milk problems is the prevention of the flavor described as oxidized, tallowy or cappy, that tends to develop in milk that is produced under sanitary conditions and has a low bacterial content. Preservation of milk by freezing is a means of studying the oxidized flavor problem which has been little utilized. In the frozen state, milk is more stable than in the unfrozen state, and its changing characteristics can be studied over a longer period. Babcock et al. (1), using samples packaged in paper at a large dairy, found the milk of acceptable beverage quality after storage at --32.8 ° for 115 days. At higher temperatures frozen milk is not as stable and, in the author's experience, always eventually becomes oxidized in flavor. The onset of this defect is a limiting factor in the preservation of milk by freezing and, therefore, is of economic importance. There are at least four forms of ascorbic acid--two that are levorotatory and are called ascorbic acid and dehydroascorbic acid and two that are dextrorotatory. The latter two are not biologically active. Of the former, each of which is equally biologically active, ascorbic acid is the only form of vitamin C present in milk in the mammary gland (6). The oxidized flavor ordinarily is detected in milk containing dehydroascorbic acid, the first oxidation product of ascorbic acid, as well as ascorbic acid. Krukovsky and Guthrie (7) concluded that ascorbic acid oxidation is an essential link in the chain of the reactions resulting in the development of the tallowy (oxidized) flavor in milk, and that apparently the oxidation of the lipid fraction of milk is coupled to that of ascorbic acid when a certain equilibrium between ascorbic acid and dehydroascorbic acid has been established. Greenbank (4) believes that the development of an oxidized flavor in milk is related to a change in the oxidation-reduction potential, and that variations in milks can be explained on the basis of differences in their poising action. DahIe and Palmer (3) have expressed the situation as follows: A condition in the milk which is favorable to the production of the oxidized flavor is apparently favorable to the destruction of vitamin C. They found that the ascorbic acid content gradually decreased during the holding period in all cases, regardless of the development of off-flavor. The reduction in vitamin C was nearly as great in the normal milk as in the milk that developed the off-flavor.