66 MICRO-ORGANISMS AND FERMENTATION. 



same property and also produces a vigorous lactic acid 

 fermentation ; other slime-forming species have been described 

 by Adametz, Duclaux, Loffler, and Guillebeau ; Weigmann 

 isolated a species which imparts a bitter taste to milk and 

 secretes a ferment which dissolves caseine. Jensen likewise 

 found a species which causes marked abnormal changes in the 

 taste of milk and butter ; it has the form of thick, motile 

 rods, of varying length, partly resembling micrococci. Storch 

 proved that the disagreeable taste of tallow which butter 

 sometimes has, is caused by a certain species of bacterium, 

 which acidifies and coagulates the milk. 



3. BUTYRIC ACID BACTERIA. 



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

 lactic acid bacteria have developed, is neutralised by the 

 addition of lime ( chalk), so that calcium lactate is formed, it 

 will, as a rule, enter into a butyric fermentation, which is 

 caused by different species of butyric acid bacteria. This 

 spontaneous butyric acid fermentation takes place most 

 vigorously at 35 to 40 C. Starch, dextrin, cane-sugar, and 

 dextrose, are likewise fermentible by the butyric acid ferments, 

 and these fermentations set in very readily, as the different 

 bacteria belonging to this group are very widely distributed 

 in nature. It is doubtless also such species which take part 

 in the ripening of cheese, and which help to impart the 

 characteristic taste and aroma to the different kinds of 

 cheese. In order to induce a butyric acid fermentation, Fitz 

 recommends the employment of a mixture of 2 liters 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 water containing slices of 

 raw potatoes for two or three days at a temperature of 25 

 to 30 C. 



The most important products of the butyric acid fermen- 

 tation are butyric acid, carbonic acid, and hydrogen. 



