24 Milk and Its Products. 
rennet or to certain vegetable substances, the casein 
is precipitated in a flocculent mass. Casein is not 
acted upon by heat. The albumin of the milk is 
in all respects similar to blood albumin. It is 
rendered insoluble by a heat of about 180° F., but 
it is not acted upon by weak acids or rennet, and 
in this way it is chiefly distinguished from the 
easein. The fibrin of milk, if present, is in ex- 
tremely minute quantities. It is supposed to be 
the same as blood fibrin, and coagulates upon ex- 
posure to the air, but is never present in sufficient 
quantity to form a elot, as in the ease of blood. 
Its coagulation is hindered by a reduction of tem- 
perature, and it has been supposed that when it does 
coagulate it forms a sort of network of threads 
through the mass of milk. 
The sugar.—Milk sugar, otherwise called lactose, 
exists in solution in the milk serum. It has the 
same chemical composition as cane sugar: that is, 
CyH2O0On + HeO. It erystallizes with considerable 
difficulty, and has very much less sweetening power 
than ordinary sugar. Under the influence of. vari- 
ous ferments it readily undergoes decomposition, 
each molecule of sugar breaking up into four mol- 
ecules of lactic acid. This change begins in the 
milk almost immediately after it is drawn, and con- 
tinues until about .8 of 1 per cent of lactie acid 
is formed. The presence of lactic acid in this 
amount acts as a check upon the growth of the 
ferments, and prevents the further formation of 
lactic acid, unless. ,thg ppd odsmeutralized with an 
