vi] THE BACTERIA OF MILK 93 



evolution of carbonic acid. Other lactic ferments 

 produce small quantities of secondary by-products, 

 especially alcohol. The most important practical 

 application of lactic fermentation is in the souring 

 of cream in the manufacture of butter ; and as it is 

 known that the various different lactic bacilli are not 

 all equally adapted to act as ferments in effecting this 

 change, attempts have been made to isolate and 

 cultivate, in pure cultures, such as will produce the 

 best butter with the finest aroma. A point of much 

 interest in lactic fermentation is that it only continues 

 for a certain time, this being the case with many 

 other kinds of fermentation. When it has proceeded 

 to a certain extent, the acid generated becomes fatal 

 to the bacterial life causing it, and it ceases ; con- 

 sequently when much over one per cent of lactic 

 acid is generated fermentation ceases. Curdling, it 

 may be added, only takes place after the lactic acid 

 has reached a certain amount. 



A number of the bacteria causing lactic ferment- 

 ation have been isolated and studied by different 

 observers ; among others by Hueppe, Grotenfelt, 

 Marpmann, Krueger, and Weigmann. Thus Hueppe 

 has discovered a bacillus about 2 5 \ Q O th of an inch 

 long and 6 2 \ Q Q th of an inch broad. It develops at 

 temperatures between 10 to 45 C. (50 Fahr. and 

 113 Fahr.), and most rapidly at 35 C. (95 Fahr.). 

 Other bacilli, micrococci, sphaerococci, streptococci, 



