CONSTITUENTS OF MILK 147 



tained in rennet. This property makes possible 

 the manufacture of cheddar and many other kinds 

 of cheese from milk. The curd formed by the action 

 of rennet is called paracasein or, more properly, cal- 

 cium paracasein. The coagulation of milk-casein pro- 

 duced by rennet is quite different from that produced 

 by acids. Calcium paracasein behaves, in general, 

 much like casein toward acids and alkalis. The details 

 of rennet action on milk-casein will be considered more 

 fully in Chapter XXII (p. 299). 



Other changes caused in milk-casein. Under the 

 action of chemical reagents, of enzyms and of various 

 micro-organisms, calcium casein and paracasein 

 may be changed into a large number of other sub- 

 stances. Among the compounds and classes of com- 

 pounds thus formed are caseoses (albumoses), 

 peptones, amino acids (crystallizable bodies) and 

 ammonia. These products are never found in nor- 

 mal milk as it leaves the cow's udder, but may be 

 present in milk that has stood exposed to air for some 

 time. 



Brine-soluble substance formed from casein. 

 When milk is treated with rennet and the curd 

 is handled in the usual manner followed in cheese- 

 making, a most interesting change begins to take 

 place, which becomes especially prominent in the 

 cheddaring operation (p. 32). The curd changes 

 into a form which is soluble in a warm solution of 5 

 per cent brine (common salt) ; at the same time, the 

 curd forms long strings on a hot iron and acquires the 

 peculiar texture of the cooked meat of a chicken's 

 breast, with a characteristic velvety mellowness of feel- 

 ing and glistening, silky appearance. These changes 



