A Review of Oxidation in Milk and Milk Products as Related to Flavor

Summary Of all the factors concerned in bringing about oxidative changes in dairy products metallic contamination, particularly by copper, is at present the most important. The control of copper contamination in processing becomes increasingly more important as the sanitary quality of the products is improved. However, not all milk must have copper contamination to develop an oxidized flavor. Fortunately, the percentage of milk which develops oxidized flavor spontaneously is relatively small and when this milk is mixed with normal milk it remains normal in flavor. For this reason it is not of as great commercial importance as is milk susceptible to metal-induced oxidized flavor. Oxidized flavor in milk has been shown to be associated with milk of low bacterial count. The growth of bacteria in the milk, either by using up the oxygen or by reduction of the potential, render milk non-susceptible even in the presence of copper. Oxidized flavor in milk was originally believed to be the result of oxidation of the fat catalyzed by copper and brought about through the action of an enzyme. The recent trend in literature points to the phospholipid fraction as the source of the flavor and recent work on cooked flavor questions seriously the action of an enzyme in bringing about the defect. Grass feeding has been shown to reduce the susceptibility of milk to oxidized flavor even though the milk fat is made more susceptible to ordinary chemical oxidation. This is explained on the basis of reducing substances in the feed or in the milk, or in both. Various investigations have shown that ascorbic acid and carotene in the feed tend to reduce the susceptibility of the milk. Also a number of antioxidants have been demonstrated to have protective qualities. The mechanism whereby an oxidized flavor develops has not been completely demonstrated nor has the mechanism whereby the various factors exert their effects. Tallowy flavor in butter appears to be the result of metal-induced oxidation of the fat, the point of attack probably being the double bonds in the oleic and linoleic acid radicles. Light and oxygen have been demonstrated to be important factors favoring oxidative changes. Oxidation will take place in the absence of light but the rate of reaction is slow. The source of oxygen may be either the free or the combined form. Both strong acid and alkaline reactions have been shown to favor oxidative changes. Over-neutralization is known to favor oxidative changes while butter made from high acid cream in the presence of copper contamination and salt is prone to become fishy upon storage. Both salt and moisture appear to play a role in the development of oxidative changes but they are of minor importance when compared to metallic contamination. Temperature is important only as a regulator of the rate of oxidative change. As the temperature increases the rate of oxidative change increases, all other factors being constant. Low temperature storage favors a slow rate of oxidative change. Development of tallowy or oxidized flavor in ice cream is undoubtedly an oxidative change. In this product, however, we have the possibility that the oxidative changes may affect either the phospholipid fraction or the butter-fat or both. The present work points toward fat as the substance oxidized. If the present trend of research on oxidized flavor of milk continues, a reex-amination of tallowy flavor in ice cream would seem desirable. The elimination of copper contamination is one of the major problems confronting the dairy industry today as the flavor problems resulting from chemical reactions have a copper history in the vast majority of cases. If these problems are to be eliminated by removal of the cause, it will be necessary for copper to be eliminated from all surfaces with which milk comes in contact because of the extremely small amount of copper required to develop the flavor in many cases. The authors wish to express their appreciation to Dr. R. B. Dustman, Department of Agricultural Chemistry, West Virginia Agricultural Experiment Station, for advice and criticism in the preparation of this paper.