28 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
PHYSICAL PROPERTIES 
The physical properties of milk which are of great- 
est importance in milk hygiene are the color, odor and 
taste, specific gravity, and refraction. Other physical 
properties which have been extensively studied are the 
viscosity, surface tension, freezing point, and electrical 
conductivity. 
Color.—Milk is a white, opaque fluid, sometimes with 
a yellowish or bluish tinge. The white color is due to 
the calcium caseinate, while the opacity is due in part to 
the same substance and in part to the fat. Not only the 
quantity of fat, but also the size of the fat globules, 
affects the opacity, the opacity being less when the glo- 
bules are large than when they are small. Since opacity 
does not depend entirely on the quantity of fat, tests for 
fat based upon transparency, such as the lactoscope test 
(p. 239), are not accurate. The yellowish tinge of milk 
is due to a pigment in the fat (carotin) ; it is more pro- 
nounced in milk from cows of certain breeds, as the 
Guernseys. A bluish tinge indicates that the milk has 
a low fat per cent., and is sometimes associated with skim- 
ming and watering, but it must be remembered that any 
milk in a thin layer has a bluish tinge. 
Odor and Taste—Normal milk has a slight odor, 
resembling the exhalations from the cow’s skin, and a 
slightly sweetish taste. During the colostral period and 
near the end of lactation, individual cow’s milk may have 
a salty, bitter, or a rancid, animal-like taste. A large 
proportion of “strippers” in a herd may give the mixed 
milk a similar taste. The milk of the individual cow may 
also be salty or bitter in advanced pregnancy, after abor- 
tion, in mastitis, and when digestion is disturbed. Milk 
with a certain degree of acidity will acquire a bitter, 
