FERMENTS IN MILK. 47 
All bacteria do not have the same optimum growing tem- 
perature. Some species develop most rapidly at one tempera- 
ture, while other species prefer a different temperature for the 
greatest development. It is on this account that certain tem- 
peratures are employed in ripening of starters and cream. 
According to researches by Conn, Bacillus lactis aerogenes 
develops very rapidly in milk at 95° F. It produces much gas 
and an unpleasant flavor in the milk. This particular species 
sours milk very rapidly. As a rule, milk which has been held 
at this high temperature, contains a preponderance of this 
undesirable species of bacteria. At 77° F. results are more 
uncertain. The species of bacteria which will predominate in 
milk at this temperature depends in large measure upon the 
number of each kind present. According to Conn, Bacillus 
lactis acidi has the highest relative growth at about 70° F. 
This particular species produces no gas, and is desirable to have 
present in cream for butter-making. Milk kept at this tem- 
perature will, in most cases, providing it has previously been 
properly treated, develop a pleasant acid taste, will curdle into 
a smooth uniform coagulum, and will contain a preponderance 
of the species of germ mentioned above. 
At as low a temperature as 50° F. acid-producing types of 
bacteria do not develop very well. But Conn maintains that 
at this temperature miscellaneous species of bacteria develop 
that produce unfavorable results. While milk does not easily 
sour at this temperature, it should be remembered that un- 
desirable germs are constantly developing. 
As it is practically impossible to exclude: all of the bacteria 
from milk during milking and the handling of the milk, it 
is very essential that the multiplication of the germs present 
be checked, or at least retarded; and this can be done by 
controlling the temperature of the milk. As low temperature 
is effective in checking the multiplication of the bacteria, the 
sooner the milk can be cooled after it is drawn, the better itis 
for the keeping quality of the milk. 
Moisture.—Moisture is one of the essentials for bacterial 
