2 MILK 
tated by many substances, among which are 
acids, rennet, and magnesium sulfate, but not 
by heat. Acids precipitate it by breaking up 
the combination with phosphates. The action 
of rennet is complex and probably partly hy- 
hydrolytic, splitting the casein into several 
proteins, some of which are precipitated in 
the curd. Films of protein matter occur abun- 
dantly in milk, for which reason it is distinctly 
opaque, even when nearly all the fat has been 
removed by contrifugal action. 
The albumin of milk appears to be a distinct 
form, and is called lactalbumin. It is not 
precipitated by dilute acids, but is coagulated by 
heating to 70°—75°. The proportion in cow’s 
milk is usually from 0.35 to 0.50%, but col- 
ostrum may contain much larger proportions. 
Globulin is present only in minute amounts 
in normal milk, but colostrum may contain as 
much as 8%. It is coagulated on heating. 
Lactose.—This is a sugar peculiar to milk. 
Citric acid is a normal constituent of the milk 
of various animals. In human milk, the quantity 
is about 0.5 gram to rooo c.c.; in cow’s milk, 
from 1 to 1.5 grams. It is not dependent on 
the citric acid present in the food. 
Enzyms.—Several enzyms occur in milk but 
they are chiefly known by effects and not as 
isolated substances. Some are proteolytic, others 
