144 SCIENCE AND PRACTICE OF CHEESE-MAKING 



particles of casein coming together into large, vis- 

 ible aggregations. The cause of this change cannot 

 yet be fully explained. It has been usually ex- 

 plained by saying that acids unite with the calcium 

 of the calcium casein, and the casein, thus deprived 

 of its combined calcium, is changed from its con- 

 dition of finely divided, gelatinous particles into 

 larger masses and then appears as a solid, heavy 

 precipitate. This explanation is not entirely satis- 

 factory, since casein may be obtained in the form of 

 a precipitate when little or no acid is present. The 

 effect is probably to be ascribed rather to the forma- 

 tion of soluble calcium salts by the acid than entirely 

 to the direct effect of acid upon the calcium of milk- 

 casein. 



When milk sours in the ordinary way, the lactic 

 acid, thus formed, acts upon the calcium casein, 

 two definite changes taking place when sufficient acid 

 is present. First, the lactic acid combines with the 

 calcium of the calcium casein, forming calcium 

 lactate and calcium-free casein (casein set free from 

 its combination with -calcium). When more lactic 

 acid forms than is sufficient to combine with the 

 calcium, the second change takes place ; the free 

 casein or coagulum takes up the acid, forming a 

 mixture which is familiar as the curd of sour milk. 

 It was formerly believed that insoluble, precipitated 

 casein combines with a definite quantity of acid, 

 forming a definite compound; and that, under this 

 supposition, the curd of sour milk is a compound 

 known as casein lactate. But more extended, care- 

 ful, and accurate work has shown that the evidence 

 was misleading upon which was based the belief 



