BEHAVIOUR OF YEAST CELLS. 241 



identical conditions as regards the composition of the nutrient 

 solution and the race of yeast, fermentation was arrested 



At 36 C. as soon as the alcohol content reached 3.8 per cent, by weight. 



27 7-5 



18 ,, 8.8 



9 9-5 



Practical brewers would seem, to have long recognised the existence 

 of some relation between temperature and the degree of sensitive- 

 ness of yeast toward alcohol, since they laid it down as an axiom 

 that, in order to obtain an active sedimental yeast as well as a 

 good beer, the temperature in the fermenting tun should be allowed 

 to rise at the start (when the amount of alcohol present is small) 

 and be afterwards gradually reduced. 



The composition of the nutrient solution also influences the 

 sensitiveness of the yeast toward alcohol, and therefore affects the 

 amount of alcohol that can be produced in the fermenting liquid. 

 Unfortunately, there is little reliable information available on 

 this point, though the researches of H. MULLER-THURGAU (XIV.) 

 have shown that, under otherwise identical conditions, the retard- 

 ing effect of alcohol increases with the amount of sugar in the 

 medium. This fact (the cause of which is still undetermined) is 

 utilised in certain fermentation industries, notably in molasses 

 distilleries. In countries where the excise duties are levied, 

 partly or entirely, on the dimensions of the mash tun or ferment- 

 ing vessel, the interests of the distiller lead him to work with 

 mash of the highest possible concentration. To overcome the 

 difficulty arising from the increasing sensitiveness of the yeast to 

 alcohol in these strong mashes, prolonged tentative experiment 

 has led to the mash being pitched at the highest gravity found to 

 be compatible with regular fermentation, and replenished with 

 concentrated mash in proportion as the sugar is consumed. In 

 distilleries where raw grain (maize, &c.) or potatoes are used this 

 method is impracticable, owing to reasons which need not be dis- 

 cussed here ; but in breweries it is not infrequently resorted to, 

 especially when the amount of pitching yeast is insufficient for a 

 brew and must be increased quickly. In the strong musts in- 

 tended for the production of choice wines, and fermented without 

 this artifice, fermentation comes to a standstill before the whole 

 of the sugar is consumed, and the resulting wines, though com- 

 pletely fermented, are sweet. 



Yeasts in general are capable of a certain degree of habituation 

 in respect of alcohol, so that they can be gradually accustomed to 

 work in a nutrient solution containing a larger proportion of 

 alcohol than was previously sufficient to arrest their activity. 

 However, as was shown by E. LAURENT (VI.), for a series of beer 

 and wine yeasts, this is possible only within comparatively narrow 

 limits. 



