PHYSICAL AND CHEMICAL EXAMINATION OF MILK 189 



quired for casein estimation in order that the percentage may be 

 read directly from the buret. As milk has an average specific 

 gravity of 1.03, a volume of 10.5 c.c. will weigh 10.8 grams. The 

 volume of 10.5 c.c. is the basis for this method. 



The test is carried out as follows: "In a 200-c.c. Erlenmeyer 

 flask are placed 10.5 c.c. of the milk sample, 75 c.c. of distilled 

 water at room temperature, and 1 to 1.5 c.c. of a 10 per cent, solu- 

 tion of acetic acid. The flask is given a vigorous rotary motion. 

 Usually 1.5 c.c. of acetic acid gives a clear and fast filtering separa- 

 tion, but occasionally, with milks low in casein, a better separation 

 is effected if a little less acetic acid is used. The separated pre- 

 cipitate is now filtered through a 9-11 cm. filter. As the casein 

 accumulates on the filter there is a marked retardation of the filter- 

 ing process. This can be made rapid again by conducting a fine 

 stream of cold water against the upper point of contact of filter 

 paper and casein. This loosens the casein mass and accumulates 

 it at the apex of the filter. This is all essential to proper working 

 of the process." 



The acid must be removed by cold water for the success of 

 the test. When this is done, the casein is in a loose, easily soluble 

 form. The temperature should not be above 20° C. Any casein 

 particles that adhere to the glass of the flask need not be removed, 

 but the acid must be washed out of them. Washing of the pre- 

 cipitate should be continued until 250 to 300 c.c. of the filtrate 

 have accumulated and the filter has become perfectly clear. 



The precipitate, together with the filter paper, is now returned 

 to the flask in which precipitation was made; 75 to 80 c.c. of 

 neutral C02-free water added, and then a few drops of phenol- 

 phthalein and 10 c.c. of N. one-tenth potassium hydrate solution. 

 The flask is stoppered with a rubber stopper and vigorously shaken 

 either by hand or in a machine until solution is effected. Com- 

 plete solution is easily indicated, even in the presence of the filter 

 paper, by the disappearance of the white casein particles, which 

 otherwise would settle to the bottom. After solution the stopper 

 is rinsed off with neutral, C02-free water and immediately titrated 

 with N. one-tenth acid until the red color disappears. It is im- 

 perative that a blank should be run parallel with the entire de- 

 termination. The blank usually runs 0.2 to 0.3 acid. The cor- 

 rection for the blank is made by addition to the number of cubic 

 centimeters of acid used for titration. The difference between 

 this corrected acid reading and the 10 c.c. alkali used gives 

 directly the percentage of casein in the milk. 



"Example. — Suppose it took 6.7 c.c. of N. one-tenth H2SO4 in 

 the titration and the blank was 0.2 c.c, then the per cent, of casein 

 would become 10 — 6.9 = 3.1 per cent." 



