BACTERIA. 67 



To those in practice, it is interesting to note that the best 

 known of these aerobic bacteria exert no influence on the colour 

 or brightness of the beer, whilst most other bacteria cause 

 turbidity. 



Likewise in distilleries, and more especially in air-yeast 

 factories, acetic bacteria may occur in large quantities, as 

 shown by numerous experiments made by the author; they 

 are most frequently accompanied by mycoderma species. A 

 careful control of the manufacturing process from this point of 

 view 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 on 

 that of glycerine ; the acetic acid samples contained the 

 smallest amount of glycerine and showed the weakest propaga- 

 tion of yeast. Contrary to the previously accepted view that 

 even small amounts of acetic acid prevent alcoholic fermenta- 

 tion, LAFAR found that the presence of 0*27 per cent, of this 

 acid 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.; but 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 maximum 

 yield. Yeasts differ, however, considerably in their sensitive- 

 ness to the action of acetic acid. Thus, a comparison of fif- 

 teen 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 pro- 

 pagation of cells, yeasts behave very differently with the same 

 amount of acetic acid. LAFAR also examined the influence of 

 this acid 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 



