18 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
but these advantages are overbalanced by the expense 
of the process. 
When milk is shaken or agitated the fat globules 
gradually unite and form flakes or lumps—.e., butter. 
Small clumps of butter fat may form in milk during 
transportation when the container is not full. This 
change may affect the test for fat. If milk or cream 
is slightly acid and warm, the clumping of the fat glo- 
bules is facilitated. Hence cream is usually churned 
after it has attained an acidity 0.4 per cent., and at a 
temperature of 21° C. (70° F.). Cream from a cow 
near the end of lactation may not “butter” because of 
its alkalinity. 
The fat of milk differs in its chemical and physical 
properties from both the fat of the tissues and the fat 
of the food. It consists of a mixture of fats, princi- 
pally olein, stearin, and palmatin, together with some 
butyrin and other fats. Its composition varies, being 
influenced by the breed, feed, external conditions, etc. 
Cotton-seed meal, for example, increases the olein and 
raises the melting point. When milk fat decomposes, 
butyric acid is liberated and produces a rancid odor and 
taste. The color of the milk fat is more or less yellow. 
The fat globules, by reflecting the light, are partly re- 
sponsible for the opacity of milk. 
In regard to the origin of the milk fat, it appears 
most probable that it is derived in part from the splitting 
up of albuminous compounds in the udder or in another 
part of the body, or in both places. It may also be formed 
from the carbohydrates which are carried to the udder 
by the blood (Rievel). 
Lactose.—This substance, also known as “milk 
sugar,” is in solution in milk. It is a disaccharid and 
