16 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
at all, or only after several times the usual amount of 
rennet is added. A method based upon these facts has 
been proposed by Schern for detecting milk from cows 
affected with mastitis, but the test has not come into 
very general use, because it has not been found possible 
to obtain a standardized rennet solution which will not 
deteriorate. 
Fresh milk may be heated to boiling without coagula- 
tion occurring. A thin membrane, which consists prin- 
cipally of casein, forms on the surface, but real coagula- 
tion does not take place. After a certain degree of acid- 
ity has been reached a temperature of 75° C. (167° F.) 
is sufficient to coagulate the casein. 
Casein is formed by the secreting cells of the alveoli 
of the udder from the circulating albumin of the blood 
(Rievel). 
Lactalbumin.—The albumin of milk is in solution. 
It is similar to the albumin of the blood, but differs 
slightly in its chemical composition and polarization. 
It begins to coagulate at 65.6° C. (150° F.), and the 
coagulation increases with the temperature (Rupp). 
Whether the albumin originates from the breaking down 
of cells or is derived from the blood is not known. 
Lactoglobulin—The globulin of milk originates 
from the disintegration of cells, and is present in milk 
in solution. It coagulates at 75° C. (167° F.). 
Fat.——The fat is present in milk in an extremely 
finely divided condition—+.e., in an emulsion. Under 
the microscope it can be observed in the form of small 
globules. The specific gravity of the fat is lighter than 
that of any of the other milk constituents, including the 
water, being only 0.98. Consequently the fat globules 
in milk are buoyant, and when the milk is permitted 
