76 MICRO-ORGANISMS AND FERMENTATION. 



3. BUTYRIC ACID BACTERIA. 



When milk which has stood for some time, in which lactic 

 acid bacteria have developed, is neutralised by the addition of 

 calcium carbonate (chalk), so that calcium lactate is formed, 

 it will, as a rule, undergo a butyric fermentation. PASTEUR 

 showed in 1861 that this fermentation is brought about by 

 particular micro-organisms which are able to live without 

 air (" vibrions butyriques "). This spontaneous butyric acid 

 fermentation takes place most vigorously at 35 to 40 C. 

 Starch, dextrin, cane-sugar, and dextrose are likewise decom- 

 posed by the butyric acid ferments, and these fermentations 

 are of frequent occurrence, as the bacteria belonging to this 

 group are very widely distributed in nature. Allied species 

 doubtless play a part in the ripening of cheese, discussed above. 

 In order to induce a butyric acid fermentation, FITZ recom- 

 mends the employment of a mixture of 2 litres of water, 

 100 grams of potato-starch or dextrin, 1 gram of ammonium 

 chloride, the ordinary nutrient salts, and 50 grams of chalk; 

 this mixture is to be maintained at 40 C. BOURQUELOT 

 recommends exposing slices of raw potatoes standing in water 

 for two or three days at a temperature of 25 to 30 C. At 

 35 C. this fermentation appears to be still more vigorous 



The most important products of the butyric acid ferment- 

 ation are butyric acid, carbon dioxide, and hydrogen. 



In the saccharine mashes of breweries, distilleries, and 

 pressed-yeast factories, species of butyric acid bacteria always 

 occur, and if the mashes are maintained for a lengthened 

 period at certain temperatures, these bacteria develop very 

 rapidly and exercise a retarding influence on the alcoholic 

 ferments. If butyric acid occurs to any extent in beer, it will 

 acquire a very unpleasant taste. 



According to PASTEUR'S experiments, the butyric acid 

 ferment can perform its functions without access to the free 

 oxygen of the air. More recent investigations have shown, 

 however, that many butyric acid bacteria exist which not 

 only yield different fermentation products, but also behave 

 differently with regard to free oxygen, in that some are not 

 capable of developing when the latter is present, so-called 



