QUALITATIVE EXAMINATION OP MILK 177 



mineral salts extracted with water, and the aqueous 

 solution filtered and evaporated down to a volume of 

 about 5 c.c. 



The solution is then examined qualitatively, accord- 

 ing to the method given on p. 9 for plant ash. 



(iii.) Casein. — About 75 c.c. milk are warmed to a 

 temperature of 50° to 55** C, and allowed to run 

 slowly into 200-300 c.c. dilute HgSO^ with constant 

 stirring. The casein separates out in the form of a 

 curd, which holds mechanically a proportion of the 

 fat of the milk. To separate this, the liquid is filtered 

 and the filtrate retained for examination for milk sugar 

 (below). The curd is stirred thoroughly with absolute 

 alcohol to remove water, and is then twice shaken up 

 with small quantities of ether. The ether is then filtered 

 off and examined for fat as below. The casein is then 

 examined by the usual tests for proteins (p. 16 et seq.). 



(iv.) Fat. — The ethereal solution of butter-fat 

 obtained by treatment of the casein with ether is 

 evaporated on the water-bath. An oily residue of fat is 

 obtained which will leave a permanent greasy stain on 

 paper. 



(v.) Milk Sugar. — The acid filtrate from the casein 

 precipitation above is boiled and filtered. A small pro- 

 portion is then concentrated by boiling, and Fehling's 

 test applied to the liquid. The production of the red 

 precipitate of CugO shows the presence of reducing 

 sugars, glucose, and galactose that have been formed 

 by hydrolysis of lactose or milk sugar. Lactose itself 

 will reduce Fehling's solution. 



The Specific Gravity of Milk. 



The specific gravity of pure fresh milk is about 1-032, but 

 undergoes a slight increase of about '001 during the five hours 

 following milking, this phenomenon being possibly due to some 

 change in the state or condition of the casein. 



M 



