MILK 15 



The same thing occurs when the lactose in the milk is 

 fermented by bacteria and a sufficient quantity of acid 

 is produced. The calcium caseinate is split up. The 

 acid combines with the calcium, while the free casein, 

 being insoluble, is precipitated in the form of a firm, 

 jelly-like white curd. Subsequently this curd contracts 

 and expresses a fluid called whey, which contains some 

 of the milk fat, the albumin and globulin, the milk sugar, 

 the mineral salts, and the calcium salt formed by the 

 combination of the acid with the calcium. Most of the 

 fat remains in the curd with the casein. This is the com- 

 mon sour curdling of milk. If lime water or a dilute 

 solution of an alkali is added, the casein will be redis- 

 solved and the acidity reduced. 



Rennet or chymosin also causes curdling of milk. In 

 this case the calcium caseinate is split up by the rennet 

 into calcium paracaseinate and a substance known as 

 whey-proteid. Calcium paracaseinate, being insoluble, 

 is precipitated and forms a curd, while the whey-proteid 

 is held in solution in the whey. Certain bacteria produce 

 a rennet-like ferment, which splits up the casein com- 

 pound of milk in the same manner. This is the sweet 

 curdling of milk, so called because the milk curdles with- 

 out souring. The curd produced in this way cannot 

 be redissolved by lime water or a dilute solution of an 

 alkali. Curdling of milk may occur from the joint action 

 of acids and the rennet-like ferment. The blood contains 

 a ferment, called anti-rennet, which inhibits the action 

 of rennet. This ferment is not present in normal milk, 

 but when inflammation occurs in the udder and there is 

 a transudation of serum from the blood-vessels into the 

 udder tissue the anti-rennet ferment is present in the 

 udder secretion, which is then not coagulated by rennet 



