ACETIC FERMENTATION. 619 



was formerly looked on as the exciting agent of the 

 acetic fermentation. According to Nageli, however, 

 this form of yeast only prepares the soil for the acetic 

 fungus hy using up the fruit acids when they are pre- 

 sent in large amount in the fermentescible material, and 

 the presence of which would hinder the development of 

 the bacteria ; in this way it neutralises the medium. 

 But this explanation of the peculiar concurrence of 

 these two species of fungi, which is so often observed, 

 is not completely satisfactory, because the acetic 

 bacterium can bear the presence of acetic acid in the 

 nutrient fluid in much larger amount than other 

 bacteria. 



The development of the acetic fungus only occurs conditions of 

 when the ordinary nutrient materials, nitrogenous sub- fermentation, 

 stances, and salts, are present. The fermentescible 

 material (the alcohol) must not be present in too great 

 concentration (at most 10 per cent.). The develop- 

 ment of the acetic fungus occurs best when a certain 

 quantity of acetic acid (1 to 2 per cent.) is already pre- 

 sent. The formation of acetic acid ceases below 10 C. 

 and above 35 C. ; the optimum lies between 20 and 

 30 C. ; by heating the fermenting fluid to 60 C. for 

 twenty minutes, the fermentative action is permanently 

 stopped unless new bacteria obtain access to it. As to 

 the other products which appear in the fermenting mix- 

 ture nothing is known ; according to Nageli, the acetic 

 fungus forms extremely minute quantities of carbonic 

 acid and water. 



According to the few certain observations which have been Doubtful rule 

 made as to the acetic fermentation, this process seems to be ? g f ^ ^the* 1 " 

 very different from that of the other fermentations. Marked acetic fer- 

 decomposition of the alcohol molecule and marked formation mentation, 

 of carbonic acid does not occur ; the formation of acetic acid 

 runs its course much less violently than the other fermenta- 

 tive processes ; the same effect is obtained by spongy 

 platinum and by charcoal. All this has led Pasteur to the 

 view that the formation of acetic acid is not really a physio- 

 logical action of the fungus, but that the organisms only act 

 in a similar manner as spongy platinum, as carriers of oxygen. 

 Mayer has, however, pointed out, as against this view, that 

 the optimum of the action of the acetic fungus is quite 



