64 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
will undergo with age. Most species of bacteria thrive 
best at about body temperature (37 to 38° C., 98.6 to 
100.4° F.), but at lower temperatures some multiply 
more rapidly than others. At temperatures below 15° C. 
(59° F.), and especially below 10° C. (50° F.), the or- 
ganisms of the peptonizing group develop more rapidly 
than any of the other common milk bacteria. At 15 to 
20° C. (59 to 68° F.), the true lactic acid bacteria in- 
crease faster than any of the others. At temperatures 
above 20° C. (68° F.), the acid-forming bacteria may 
continue to multiply more rapidly than the others, but 
the gas-formers are more likely to increase most rapidly. 
The higher temperatures are also favorable to the de- 
velopment of pathogenic organisms. 
3. The age of milk has considerable influence on the 
number of bacteria. The longer the period which has 
elapsed since the milk was drawn from the cow the more 
time afforded for the multiplication of the bacteria in- 
cluded in the original contamination. 
(The method of determining the number of bacteria 
is described on pages 260 to 272.) 
Proportion of Different Groups of Bacteria.—The 
kind or species of bacteria included in the original con- 
tamination, and the relative proportion in which the 
different groups are represented, will vary with the con- 
ditions under which the milk is produced. In milk pro- 
duced under good conditions and examined shortly after 
it was drawn from the cows, Conn found that the greater 
portion of the bacteria are udder cocci, including both 
peptonizers and the inert variety. Usually 1 or 2 per 
cent., sometimes more, are lactic acid bacteria. A few 
gas-producing bacteria and hay and potato bacilli are 
likely to be present, but they should never be numerous. 
