ACETIC ACID BACTERIA. 105 



frequently accompanied by mycoderma species. A careful 

 control of the manufacturing process in this respect should 

 never be omitted. 



While investigating the influence of acids, especially acetic 

 acid, on wine yeasts, Lafar found that each of the different 

 acids (malic, tartaric, lactic, acetic, etc.) exerts a peculiar 

 influence on the yeast, and not only on the proportionate 

 amounts of alcohol and carbon dioxide produced, but also 

 of glycerine ; the acetic acid samples contained the smallest 

 amount of glycerine and showed the weakest growth of yeast. 

 Contrary to the previously accepted view that even small 

 amounts of acetic acid prevent alcoholic fermentation, Lafar 

 found that the presence of 0-27 per cent, had practically no 

 influence on the rate of fermentation, the multiplying of 

 the cells, or the yield of alcohol and glycerine. In must, 

 before neutralisation, the yeast cells were not impaired by an 

 addition of 0-74 per cent. ; and in neutralised must, after 

 adding as much as 1 per cent, of acetic acid, 4-77 per cent, by 

 volume of alcohol was formed i.e., 60 per cent, of the maxi- 

 mum yield. Yeasts differ considerably, however, in their 

 sensitiveness to the action of acetic acid. Thus, a comparison 

 of fifteen different wine yeasts showed that all were able to 

 carry on fermentation in the presence of 0-8 per cent, of acetic 

 acid in a must that had previously been neutralised, whereas 

 with 1 per cent, of acid only three were active. With regard 

 to the propagation of cells, yeasts behave differently with the 

 same amount of acetic acid. Lafar also examined the influence 

 of these acids on the chemical activity of wine yeasts i.e., on 

 the proportion between the amount of alcohol produced and 

 the number of yeast cells formed. He found that in presence 

 of 0-88 per cent, of acid the amount of work done by one cell 

 was greater in the case of ten varieties, but smaller in two 

 varieties, than in the presence of 0-78 per cent. Those yeasts, 

 which are active in presence of 1 per cent, of acid, gave a 

 smaller yield than in presence of 0-88 per cent. 



According to W T . Seifert, the nitric acid present in wines 

 which have been diluted with water containing nitrates, is 

 completely decomposed by the action of certain acetic acid 

 bacteria. 



