TYPES OF HACTERIA FOfM) IX MILK 



43 



BUTYRIC ACID FERMENTATION^ 



The presence of butyric acid, accompanied by a rancid odor, 

 is not uncommon in milk products. It is well known in samples 

 of butter and, once in a while, is found in old samples of cheese. 

 In milk, however, butyric acid fermentation rarely occurs. It 



FIG. 22 BACTERIA OF BUTYRIC ACID (SHATTENFROH AND GRASSBERGER) 



is sometimes produced by certain types of bacteria (Fig. 22), 

 which are practically always present in milk, although they 

 rarely have an opportunity to develop. 1 These organisms are 

 commonly anaerobic, and will not develop in milk kept under 

 ordinary conditions. Moreover, the rapid growth of lactic 

 bacteria in milk, in almost all cases, prevents the butyric acid 

 germs from developing at all. For these reasons, this fermenta- 

 tion does not occur in ordinary milk, and in the common bac- 

 teriological study of milk, butyric bacteria do not appear. They 

 can, however, usually be found by a simple device, since the 

 most common types of these organisms are spore producers and 

 are, therefore, not killed by ordinary boiling temperatures. If 

 milk be boiled for a few moments, and then plates be made from 



1 Kedroski. Zeit. f. Hyg. xvi., p. 445, 1894. 



Schattenfroh and Grassberger. Arch. f. Hyg. xxxvii., p. 54, 1900; xlii., 

 p. 219, 1902; lx., p. 40, 1907. 



