Fermentations in Milk. 69 



the lactic acid bacteria are destroyed, as in boiled, sterilized 

 or pasteurized milk, these rennet-producing, digesting 

 species develop. 



Butyric acid fermentations. The formation of butyric 

 acid in milk which may be recognized by the "rancid but- 

 ter" odor is not infrequently seen in old, sour milk, and 

 for a long time was thought to be a continuation of the 

 lactic fermentation, but it is now believed that these or- 

 ganisms find more favorable conditions for growth, not so 

 much on account of the lactic acid formed as in the ab- 

 sence of dissolved oxygen in the milk which is consumed 

 by the sour-milk organisms. 



Most of the butyric class of bacteria are spore-bearing, 

 and hence they are frequently present in boiled or steril- 

 ized milk. The by-products formed in this series of changes 

 are quite numerous. In most cases, butyric acid is promi- 

 nent, but in addition to this, other organic acids, as lactic, 

 succinic, and acetic, are produced, likewise different alco- 

 hols. Concerning the chemical origin of butyric acid 

 there is yet some doubt. Duclaux l affirms that the fat, 

 sugar and casein are all decomposed by various forms. 

 In some cases, the reaction of the milk is alkaline, with 

 other species it may be neutral or acid. This type of fer- 

 mentation has not received th'e study it deserves. 



In milk these organisms are not of great importance, as 

 this fermentation does not readily gain the ascendency 

 over the lactic bacteria. 



Ropy or slimy milk. The viscosity of milk is often 

 markedly increased over that which it normally possesses. 

 The intensity of this abnormal condition may vary much; 

 in some cases the milk becoming viscous or slimy; in others 



1 Duclaux, Principes de Laiterie, p. 67. 



