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The Incidence of Oxidized Flavor in the Milk of Individual Cows within One Herd^*

W. J. Corbett and P. H. Tracy

Department of Dairy Husbandry, University of Illinois, Urbana, Illinois

ABSTRACT

The occurrence and degree of development of oxidized flavor in the milk of individual cows in the University herd was determined during six months of the year. A portion of the milk sample was kept relatively free from metal contamination and a portion was artificially contaminated with one part per million of copper. The milks were judged for flavor after 1-day and 3-day storage periods. Five hundred and thirty-eight samples were collected from 138 cows over a period of six months (March through July and September). Of the metal-free milk samples, 1.48 per cent and 10.01 per cent had an oxidized flavor after 1- and 3-day holding periods, respectively. Of the samples to which 1 ppm. copper was added 78.5 per cent and 91.7 per cent had an oxidized flavor after 1- and 3-day storage periods, respectively.

The data were analyzed to determine the correlation between oxidized flavor development and breed of cow, age of animal, period of lactation, milk yield and fat yield. For purposes of analysis the four flavor observations on each sample of milk were weighed and combined to give a single composite value or oxidized flavor rating.

There was little difference between the various breeds so far as occurrence and development of oxidized flavor is concerned, though the Ayrshire breed seemed somewhat less susceptible.

The stage of lactation appeared the most significant factor regarding the occurrence and extent of oxidized flavor in milk from individual cows. There seemed a greater tendency for an oxidized flavor to develop in metal-free milk from cows during the first month or two of lactation than during the later part of the lactation period. There was no marked correlation between degree of oxidation flavor in copper contaminated milk and stage of lactation.

Age of the cow also appeared correlated to the occurrence of oxidized flavors. A more intense off-flavor in case of copper contaminated milk and a greater frequency in case of the metal-free milk was observed in the milk from 2- and 3-year-old cows than for the older animals. Apparently there can be no adequate explanation for this until we learn why various milks behave differently to the development of oxidized flavors.

The milk from first calf heifers in the first month or two of lactation was very susceptible to development of oxidized flavor. The metal-free milk was especially susceptible. It is difficult to adequately explain why age and stage of lactation are correlated to development of oxidized flavor. Several possibilities might be mentioned. As an animal comes into milk production a great load is placed on the milk secreting glands as shown by inflammation of the udder. It is logical to assume that during the first few weeks of the lactation period that the conditions within the udder are not entirely normal. During this period there is a shift from the production of colostrum milk to normal milk. It might be possible for the milk fat to be partially oxidized when it leaves the udder. On the other hand if we assume that milks vary in their susceptibility to oxidation depending on the amount of protective substance they contain, it is possible to assume that less of this material is present in milk secreted during the first part of the lactation period than in the latter part. No proof is available for either of these suggested reasons.

No correlation was observed between milk yield and development of oxidized flavor. Neither was there any correlation between weekly fat yield and development of oxidized flavor.

There are several possible explanations which might be advanced to explain the variations in susceptibility of milk from individual cows to oxidized flavor. These are presented as follows:

1. It is known that when certain anti-oxidant substances (tyrosine, hydroquinone, ascorbic acid, cereal grains) are added to milk that the development of the oxidized flavor can be retarded or entirely prevented. It isalso known that milk can be processed in such a manner (high heat treatment, addition of hydrolytic enzymes) that antioxidant materials are liberated in the milk which retard fat oxidation. It has likewise been observed that milk from cows fed green feeds (high in carotene) is more resistant to oxidized flavor than cows on dry feed. It is possible, therefore, that milk as it is secreted from the cow might normally contain materials similar in nature to the ones mentioned above which have the power to retard the development of oxidized flavor. The susceptibility of the milk to oxidized flavor in turn would be related to the amount of protective material present.
   
2. It is commonly recognized that in order to prevent deterioration of animal fats that they must be purified as soon as possible after slaughtering. For example, lard rendered soon after butchering, and butterfat stored in the form of pure fat (ghee), have better keeping quality than fats allowed to remain in contact with the non-fatty material with which they are associated in nature. Since milk fat is rather easily oxidized it is possible that milk contains catalytic material which favors fat oxidation. It is reasonable to assume that if such a substance is secreted in the milk that the amount might vary, causing some milks to develop an oxidized flavor more readily than others.
   
3. Some milks are known to become oxidized within 24 hours after milking even though handled in such a manner as to prevent metal contamination or exposure to sunlight which might suggest that the milk fat is partly oxidized at the time it is secreted. No one has reported detecting an oxidized flavor in milk immediately after it was drawn from the cow's udder. However, this would not preclude the possibility that the milk fat has passed part way through its oxidation induction period at the time of milking and needed only a few additional hours of holding for the oxidized flavor to be evident.
   
4. Corbett and Tracy (2) have shown that homogenization and agitation retard development of oxidized flavor by enclosing the fat globule in a new protective casein type membrane. It seems possible that milks might vary in the type, amount and composition of the membrane which surrounds the fat globule. In such cases the protective action of this membrane might vary for the same reasons which would explain why milks vary in their resistance to development of oxidized flavor.
   

It should be pointed out that there is very little proof to support any of the above suggested theories. A more authentic explanation will have to be delayed until further facts can be secured regarding the physiological factors related to milk secretion, or until more complete information is available regarding the minor constituents of milk.

^* The data presented in this paper are from a thesis submitted to the Graduate School of the University of Illinois in partial fulfillment for Degree of Doctor of Philosophy, 1942.

Resigned January 1, 1942.