684. Factors influencing the vitamin content of milk fat: VII. The possible existence of protein-bound carotenoids and vitamin A alcohol in mammary secretion

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Abstract
1. The distribution of carotenoids and vitamin A in milk-fat globules has been further investigated. The results obtained support previous observations that the carotenoid content of the fat obtained by solvent extraction from the smaller globules in milk is greater than in the fat obtained from the larger globules. It has also been shown that the vitamin A alcohol content of the extracted fat varies in the same way as the carotenoid content, but that the reverse appears to be the case for vitamin A ester.2. It is suggested that these differences in concentration are not due to a surface layer of carotenoids and vitamin A alcohol on the fat globule but to the presence in mammary secretions, as in blood, of protein-bound forms of these substances. In support of this it has been shown that when fat is obtained by centrifuging rather than by solvent extraction, its carotenoid and vitamin A content appears to be almost independent of globule size. Furthermore, some vitamin A and carotenoids remained in the virtually fat-free serum obtained from normal milk, colostrum and pre-partum secretion and was precipitated with the various serum proteins, particularly the globulin fraction. The increased carotenoid and vitamin A alcohol content of mastitis milk may also be explained by the presence of abnormally large amounts of these protein-bound forms.The author is grateful to the Department of Scientific and Industrial Research for a grant towards the cost of the investigation; also to the farm staff of The Dairy Research Institute (N.Z.) for assistance with the collection of milk and colostrum samples.