132 ANALYSIS OF MILK. 



heated, can be made as follows : — Twenty-five grammes of milk 

 are evaporated to dryness, ignited, and the phosphoric acid 

 estimated in the ash, as directed on p. 82. Twenty-five c.c. of 

 the filtrate, produced by adding 3 c.c. of acid mercuric nitrate to 

 100 c.c. of milk, are taken, evaporated down, and ignited ; the 

 phosphoric acid is estimated in the ash. 



The casein is calculated from the following formula : — 

 Let Pi = P205 obtained from 25 grammes of milk, 

 P2 = P205 obtained from 2.5 o.c, of filtrate, 

 F = percentage of fat, 

 and S = the specific gravity. 



Then casein!= (P, - Pa x M^LIM) x 203 '2. 



The coagulated albumin is deduced by subtracting this figure 

 from the apparent percentage of casein estimated by one of the 

 methods previously described. 



The Volumetric Determination of Casein in Miik. — Van 



Slyke and Bosworth's method is to place 20 c.c. of milk in 



a 200 c.c. flask, add 100 c.c. of water, 1 c.c. of phenolphthalein 



N 

 solution, and titrate with r-r sodium hydroxide solution till a 



faint pink colour appears ; raise the temperature to between 



N 

 18° and 24° C, and add =-r acetic acid in 5 c.c. portions, shaking 



vigorously after each addition, till the casein separates in the 

 form of white flakes ; usually 25 c.c. is sufficient, but the addition 

 of acid should be continued, 1 c.c. at a time, after 25 c.c. have 

 been added, till on standing a short time the liquid appears 

 quite clear. Water is now added to make up 200 c.c, the con- 

 tents of the flask vigorously shaken, and poured upon a dry 

 filter, and the whole of the filtrate collected. Of the filtrate, 



N 

 100 c.c. are titrated with :— - sodium hydroxide solution till a 



faint pink colour appears, care being taken that the shade of 

 pink is the same as that obtained before (a colour standard, 

 see p. 133, may usefully be employed). A second titration may 

 be made with 50 c.c. of the filtrate, the results being doubled. 

 The percentage of casein is deducted by subtracting the number 

 of c.c. of sodium hydroxide solution used per 100 c.c. of the 

 filtrate from half the volume of the acetic acid added and multi- 

 plying by 1-0964. 



The method works well with fresh milk, but milk that is suffi- 

 ciently sour to coagulate on boiling does not give satisfactory 

 results. 



Estimation of Curd. — Lindet proposed to deduce the amount 

 of curd from the difference of specific gravity of milk and the 



