Relationship between Fat Globule Surface Area and Carotenoid and Vitamin A Content of Milk in Successive Portions of a Milking

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Abstract
Single-quarter milkings of first-lactation Holsteins were divided into successive portions, and skim milk and milk samples taken from each portion. Samples were analyzed for fat percentage, vit. A and carotenoid content, and globule size distr. Skim milk was significantly higher in carotenoids and yit. A/g. of fat and also in the globule surface-to-vol. ratio than was the whole milk from which it was separated. This indicated concn. of carotenoids and vit. A at the globule surface. Considering whole milk data only, both carotenoids and vit. A/g. of fat had significant positive linear trends with globule surface-to-vol. ratio. By considering the magnitude of the trends, it was possible to reject the possibility of continuous surface layers of carotenoids and vit. A and to advance, instead, a possible dilute soln. or loose chemical complex on the globule surface. Values were advanced for concn. of carotenoids and vit. A in various portions of the fat globule. Carotenoids per g. of fat showed a highly significant tendency to decline to a point near the milking midpoint and thereafter increase. Vit. A/g. fat showed no trend with stage-of-milking. Carotenoids and vit. A/g. of milk increased, with stage-of-milking, in upward-curving trends. Globule size, expressed either as surface-to-vol. ratio or mean diameter, showed no trend with stage-of-milking. Fat percentage increased significantly with stage-of-milking, more than doubling its value from the beginning to the end of the milking. There was no apparent tendency to curvature.