158 Milk and Its Products. 
casein, coagulation may be brought about by several 
reagents, but the one universally employed in cheese 
making is a soluble ferment found in the stomachs 
of young mammalia and in certain other animals, 
known as rennet. By the addition of rennet to 
milk, the casein takes on the form of a homogene- 
ous gelatinous solid, and in changing its form en- 
closes in its mass the globules of fat. In bringing 
about the change in the casein in this way, the ren- 
net acts by contact; that is, its own constitution is 
in no way disturbed, and a minute amount of rennet 
is capable of causing the coagulation of a _ large 
amount of milk. 
Quality of milk for cheese making.—While it is 
searcely necessary to demonstrate that milk is val- 
uable for butter making in proportion to the amount 
of fat it contains, the proposition that its percent- 
age of fat is also a measure of the value of nearly 
all milk for cheese making has not heen so readily 
accepted. Indeed, until within a very short time, the 
prevailing opinion among dairymen and cheese-makers 
has been that a milk poor in fat was likely to be 
rich in casein, and hence more valuable for cheese 
making purposes. But both fat and casein are con- 
stituents of cheese, and both are of nearly equal im- 
portance ; henee, the richer a milk is in fat, the 
more cheese it will make, and recent research has 
shown that for milks containing a normal amount of 
fat the yield of cheese will be nearly proportional to 
the percentage of fat in the milk. 
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