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 
