175. The precipitation of the proteins in milk. I. Casein. II. Total proteins. III. Globulin. IV. Albumin and Proteose-peptone

Abstract

Improved methods have been evolved for the separation of the total protein, casein, albumin, globulin, and proteose-peptone substances of milk.I. The use of acetic acid-sodium acetate solutions for the precipitation of casein is elucidated, and it is shown that the maximum precipitation of casein is rapidly effected from milk samples of varying casein content by the addition to 10 ml. of milk of about 80 ml. of water at 40° C. and 1·0 ml. of 10% acetic acid solution, followed, after 10 min., by 1·0 ml. ofNsodium acetate solution.This maximum precipitation was found to be 1·0–1·4% greater than by Moir's method, and 2·4–3·8% greater than by the method of the Association of Oflicial Agricultural Chemists.A procedure is suggested for the determination of casein which avoids the tedious transfer and washing of the precipitate, and gives enhanced ease, accuracy and speed of working.II. The advantages of trichloroacetic acid for the precipitation of proteins in determinations of the total protein and the non-protein nitrogenous substances of milk are discussed.The trichloroacetic acid methods at present in use are shown not to give complete precipitation, and for this a rapid method employing, at room temperature, a final concentration of 12% acid in the milk-acid mixture is recommended.III. An accurate method for the precipitation of globulin uncontaminated with either albumin or casein is described.IV. Methods are given for the precipitation and separation of the albumin and proteose-peptone substances.