Substances Adsorbed on the Fat Globules in Cream and their Relation to Churning. III. Analysis of the Adsorbed Protein

Abstract

Protein constitutes the major part of the material adsorbed on the sur- face of the fat globules in cow's milk (1). The protein has'been identified by various workers as a glycoprotein (2), casein (3), albumin (4), a globulin-like protein (5), and a new milk protein (6). Some of the observed difference in composition may have resulted from the method used in the isolation of the material. A method whereby the stabilized fat globules in milk are freed from milk plasma by allowing the globules to rise through a single comparatively short column of water does not prevent the occlusion of plasma protein by the clusters of fat. A much more drastic procedure was used in the preparation of the material analyzed in the present study. The detailed method has been described in two preceding papers (1, 7). The general procedure was as follows. Fresh heavy cream was repeatedly diluted with distilled water and passed through a centrifugal separator until the "skim milk" or wash water was free from milk plasma solids. The so-called washed cream was then churned to remove the major part of the fat. The butter was melted and the ma- terial which had remained adsorbed on the surface of the fat during the washing operation was recovered in the buttermilk and aqueous layer from the melted butter. These were concentrated and the protein fraction pre- cipitated from the concentrate at pH 3.9-4.0. After filtration the protein was exhaustively extracted with alcohol, chloroform, and ether. The results of analysis for total nitrogen, sulfur, and phosphorus have been reported (7). The following table also shows the nitrogen distribution analysis. The standard Van Slyke procedure described by Morrow (12) was followed. Corrections for the solubility of the basic phosphotungstates were not made inasmuch as they were filtere4 at a temperature of 40 ° F. For comparative purposes, the analyses of other milk proteins are included in the table. It will be noted from the table that the "membrane" protein from washed cream differs from other milk proteins in all respects. The most striking difference lies in the nitrogen fractions. In this regard, it agrees