PHYSICAL AND CHEMICAL EXAMINATION OF MILK 191 



The milk sample must be neutralized exactly and excess of 

 alkali avoided. The temperature of the diluted and neutralized 

 milk should be 60° to 80° F. (15° to 25° C). Enough acid must 

 be added to promptly separate the casein into large flakes, leaving 



Fig. 68. — Apparatus and reagents required in the New York State station 

 volumetric casein test (Van Slyke). 



the supernatant fluid clear. The authors advise the use of a color 

 standard by which to judge the phenolphthalein end-point, as 

 this will insure consistent results. 



The Determination of Milk-sugar 



Milk-sugar may be determined by means of the polariscope 

 or by its reducing power on copper salts. The reduction of copper 

 salts is measured either gravimetrically or volumetrically. The 

 methods given by the Association of Official Agricultural Chem- 

 ists are as follows: 



Optical Method (Official). — "Preparation of Reagents. — 1. 

 Acid mercuric nitrate. Dissolve mercury in double its weight of 

 nitric acid, specific gravity 1.42, and dilute with an equal volume 

 of water. One c.c. of this reagent is sufficient for the quantities 

 of milk mentioned below. Larger quantities may be used without 

 affecting the results of polarization. 2. Mercuric iodid with acetic 

 acid. Mix 33.2 grams of potassium iodid, 13.5 grams mercuric 

 chlorid, 20 c.c. glacial acetic acid, and 640 c.c. of water. 



