Phosphorus Compounds of Milk. V. The Phosphorus Partition in Milk, with Preliminary Observations on Milk Phosphatase

Abstract

1. Using the method described by Sanders for protein and fat precipitation from cow's milk, a fairly satisfactory method of determining the phosphorus partition in cow's milk has been devised.2. It is found that the gravimetric procedure of the American Association of Official Agricultural Chemists ((10), p. 3), which was not, in any case, specifically recommended for the analysis of milk, cannot be applied with safety to the determination of inorganic phosphate in filtrates prepared by Sanders' (1932) method, and that the conclusion that there is no appreciable amount of organically combined phosphorus in these filtrates is therefore invalidated.3. The ester P (organically combined, acid-soluble P) in milk obtained by the usual milking routine varies from 1/7 to 1/4 of the inorganic P of the milk (from 7 to 21 mg. P per 100 c.c. milk).4. An active phosphatase is present in cow's milky, and brings about changes in the ester P content of the milk on standing, possibly within and certainly without the mammary gland. Like phosphatases of all mammalian tissues, its optimumpH is in the neighbourhood of 9·0. In freshly drawn milk it is working, therefore, considerably on the acid side of its optimum.5. The phosphoric ester P of the milk appears to vary with the breed of cow, being low in the miiy of Canadian Holstein-Friesian cows and higher in that of Jersey cows.