64 Dairy Bacteriology. 



is about 0.4 per cent, but the curd becomes more solid 

 with increasing acidity. The action of the bacteria is con- 

 tinued until about 0.8 to 1.0 per cent acid is formed, although 

 the maximum amount fluctuates considerably with differ- 

 ent organisms. 1 Further formation then ceases, by reason 

 of the inability of the lactic acid organisms to continue 

 their development in such acid solutions. There is always 

 left in the milk a considerable amount of unfermented 

 milk-sugar which can be further acted upon by the con- 

 tinued growth of the bacteria, if a carbonate is added to 

 the milk to neutralize the developing acid that inhibits 

 their growth. 



Cream never develops as much acid as milk, because a 

 larger proportion of its volume is made up of butter-fat 

 which is not subject to this change. In the ripening of 

 cream in butter-making, it is necessary to take this fact 

 into consideration where the cream varies widely in per 

 cent of fat. 



The formation of lactic acid is a characteristic that is 

 possessed by a large number of bacteria, micrococci as well 

 as bacilli being numerously represented. Still the pre- 

 ponderance of evidence is in favor of the view that one 

 main type is responsible for most of this fermentation. 

 The most prominent organism associated with this change 

 is Bacillus acidi lactici, first described by Hiippe. 9 Giin- 

 therand Thierfelder 3 working on the spontaneous souring 

 of milk in the neighborhood of Berlin found what they 

 think is the same germ. Esten, 4 in this country, studied 

 milks from thirty different localities in New England and 



> Warrington, Jour. Chem. Soc., 1888, 53:727. 

 8 HUppe, Mitt. a. cL k. Gesundheitsamte, 1884, 2:309. 

 * Gtinther and Thierfelder, Arch. f. Hyg., 25:164. 

 Esten, 9 Kept. Storrs Expt Stat., 1896, p. 44. 



