30 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
Bacilli of the coli-aerogenous group may produce an 
unclean, even nauseating, taste with a stable or manure- 
like odor, while the lactic acid bacteria give to milk a 
sour odor and taste. Specific organisms have been iden- 
tified which produce bitter, soapy, oily, and burnt tastes 
and a stable-like odor and taste. Other bacteria produce 
a rancid odor and taste, and some produce an unclean 
odor and taste. While some of the peptonizing bacteria 
(udder cocci) are normal inhabitants of the udder, the 
other bacteria usually enter the milk after it is drawn 
from the cow. Sometimes, however, bacteria which are 
the cause of abnormal odors and tastes become estab- 
lished in the udder. Odors and tastes of bacterial origin 
are often not apparent until a certain period after the 
milk has been taken from the udder and usually become 
more pronounced as the milk increases in age. Milk 
acquires a cooked taste when heated above 68 to 71° C. 
(155 to 160° F.) Heating in open vessels has a more 
pronounced effect on the taste than heating in closed 
vessels or bottles. 
The senses of smell and taste tire very quickly and 
cannot be depended upon to judge many samples of 
milk. Odors and tastes are more apparent when the 
milk is warm. 
Specific Gravity—As would naturally be expected 
from the statements made in regard to the variation in 
the composition of milk, the specific gravity or density 
of different samples of milk varies considerable. The 
range of variation is greater for individual milk than 
for market milk. The specific gravity of the milk of 
individual cows will range from 1.027 to 1.040, while 
that of market milk will fluctuate between 1.028 and 
1.034, with an average of 1.032, at the standard tempera- 
