238 MILK ! ITS ORIGIN AND COMPOSITION 



(3) Rennet changes caseinogen into a soluble body, 

 " soluble casein " (sometimes termed paracasein), which 

 remains in solution if no soluble calcium salts are 

 present 



Soluble calcium salts precipitate it as casein (or 

 paracasein). 



Lact-albumin is another protein of milk, but, unlike 

 caseinogen, it coagulates on heating and can be obtained 

 as a slimy substance by boiling the residual " whey " 

 after the curd thrown down on the addition of an acid 

 has been removed by filtration. The skim formed on 

 the surface of fresh milk when it is boiled consists of 

 coagulated lact-albumin with a certain amount of dried 

 caseinogen. 



(jii) Fats. These consist of mixtures of the common 

 solid animal fats, palmitin and stearin (compounds of 

 glycerine with palmitic and stearic acids), with variable 

 proportions of olein and other liquid fats, compounds of 

 glycerine with oleic, butyric, caproic and other acids 

 (see p. 78). The fats are present in the form of small 

 spherical globules of variable sizes, the diameter of some 

 being as high as .01 mm. while in others it is as low as 

 .0016 mm. Like the caseinogen they do not appear 

 to be merely collected from the blood-stream as such 

 and transferred to the milk, but are formed by the 

 chemical activity of the secreting cells from proteins and 

 possibly from fats supplied to the gland. 



(iii) Milk Sugar. Milk sugar or lactose is present to 

 the amount of 4 to 5 per cent, in solution in the milk and 

 can be obtained by evaporating whey. It is a crystalline 

 material with a moderately sweet taste, and is easily 

 changed into lactic acid by many species of micro- 

 organisms. Like the other constituents of milk it is 



