372 Bacteria in Relation to Country Life 



of their ability to form lactic acid which rapidly accumu- 

 lates in the milk and thereby creates conditions unfavor- 

 able for the development of the other organisms. The 

 acid thus accumulated causes the precipitation of the 

 casein; in other words, the curdling of the milk. The 

 same may be accomplished by the addition of vinegar 

 (acetic acid), or of other mineral or organic acids. 



It was formerly thought that the lactic-acid bacteria 

 did not produce rennet-like or pepsin-like enzymes, 

 and that the curdling of milk caused by them was ac- 

 complished merely by the presence of the lactic acid. 

 Boekhout and de Vries have shown, however, that lactic- 

 acid organisms exist which can secrete the two enzymes. 

 They described an interesting organism isolated by them 

 from a sample of cheddar cheese made in New York 

 state, and demonstrated that while producing lactic 

 acid it secretes a milk-coagulating enzyme as well as a 

 casein-digesting enzyme. This fact is of particular 

 interest in its possible bearing on the role of lactic-acid 

 bacteria in the ripening of cheese. 



The normal changes in milk are dependent upon the 

 rapid development of the acid-forming species which 

 continue to increasev until their predominance is clearly 

 established. Considerable variations exist even among 

 different strains of the same species of lactic-acid bac- 

 teria in so far as their power of souring milk is concerned. 

 For instance, B. lactis acidi from different sources has 

 been found to show very considerable variations, 

 moreover, the power of producing acid in milk could be 

 increased or decreased by cultivation in different media 

 and under different conditions. Aside from the vigor 





