18 THE BOOK OF CHEESE 



(c) Acid peptonizing group. 



(d) Bacillus bulgaricus group. 



(e) Acid cocci or weak acid-producing group. 



23. Bacterium lactis-acidi group. There are many 

 strains or varieties in this group which are closely related 

 in their activities. They are universally present in milk 

 and are commonly the greatest causal agent in its souring. 

 They are widely distributed in nature. At a temperature 

 of 65 F. to 95 F., these bacteria grow and multiply very 

 rapidly; at 70 F. (approximately 20 C.) these forms 

 usually outgrow all others. The total amount of acid 

 produced in milk by these organisms varies from 0.6 of 

 one per cent to 1 per cent acid calculated as pure 

 lactic acid. These forms coagulate milk to a smooth 

 curd of uniform consistency. In addition to the lactic 

 acid, there are produced traces of acetic, succinic, formic 

 and proprionic acids, traces of certain alcohols, alde- 

 hydes and esters. Substances other than lactic acid 

 are not produced by organisms of this group to such 

 an extent as to impart undesirable flavors to the milk. 

 The action of this group on the milk proteins is very 

 slight. They produce no visible sign of peptoniza- 

 tion. The B. lactis-acidi group of organisms are essential 

 to the production of the initial acidity necessary in most 

 types of cheese. The practical culture and utilization 

 of them for this purpose under factory conditions are 

 discussed in Chapter IV, entitled " Lactic Starters." 



24. Colon-aerogenes group. This group takes its 

 name from a typical species, Bacterium coli communis, 

 which is a normal inhabitant of the intestines of man 

 and animals, and from Bacterium coli aerogenes, which 

 is similar in many respects to B. coli communis. The 



