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 laciis 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 multipUcation of the bacteria, the 

 sooner the milk can be cooled after it is drawn, the better it is 

 for the keeping quahty of the milk. 



Moisture. — Moisture is one of the essentials for bacterial 



