33» APPLIED BAGTEitlOJLOGy 



4. Fermentation by Eeduction. — It is to this class that the 

 various butyric acid ferments belong which are the cause 

 of ' rankness ' in butter ; they also take a great part in the 

 ' ripening,' and help to impart the characteristic taste and 

 aroma to the different varieties of cheese. When milk 

 which has undergone the lactic acid fermentation is 

 neutralised with carbonate of lime, so that calcium lactate 

 is formed, it will as a rule enter into a butyric acid fer- 

 mentation. This spontaneous butyric fermentation takes 

 place most vigorously at 35° to 40° C. Starch, dextrine, 

 cane-sugar, glucose, and cellulose, are among the large 

 number of substances which are fermentable by these 

 butyric acid ferments, which are very widely distributed in 

 Nature. The two chief butyric acid ferments are the 

 Bacillus butyricus and Bacillus amylohacter. 



Bacillus butyricus. — This forms short and long thin rods 

 with rounded ends, seldom forming threads. Large oval 

 spores are formed, which are very resistant to external in- 

 fluences. The bacilli are very motile ; they liquefy gelatine 

 very rapidly, giving rise to a strong butyric acid smell. In 

 milk it coagulates the albumen and decomposes it, forming 

 peptones and ammonium butyrate. 



Bacillus amylobacter, or Clostridium butyricum. — This, 

 which is always found in putrefying plant infusions, forms 

 large thick motile rods, which are often associated in the 

 form of chains. A large spore forms in one end of the 

 rod, thus causing the bacillus to become spindle or club 

 shaped — hence the name Clostridium. In solutions of 

 sugars, lactates, and in cellulose-containing plants, it gives 

 rise to decompositions in which butyric acid is formed. 

 The bacilli are strongly anaerobic, and have not yet been 

 satisfactorily cultivated. 



In the bacterial fermentations of this class many carbo- 

 hydrates and fatty acids undergo decomposition: a part 



