BEHAVIOUR OF YEAST CELLS. 239 



capable of hindering the appearance of other fungoid competitors 

 in saccharine nutrient media. However, when accumulated in 

 the medium during the progress of fermentation, it also restricts 

 the further development and action of its producer. In this case, 

 as with yeast poisons in general, the first result is the cessation of 

 cell reproduction, a larger quantity of alcohol being necessary 

 to arrest fermentation, and a still further quantity to kill the 

 cells. On this point again it is impossible to expect absolutely 

 applicable figures, because, as in other cases, the quantity of 

 alcohol requisite for producing a given effect, even with one 

 and the same species of yeast, is determined by external con- 

 ditions (e.g., composition of the nutrient medium) that cannot be 

 closely gauged. 



The restriction of cell productionis effected by a quantity of 

 alcohol that is smaller in proportion as its increase in the medium 

 is gradual, so that the cells have been subjected to its adverse 

 influence through many generations. Thus, according to M. 

 HAYDUCK (VII.) the reproduction of yeast cells in fermenting 

 distillery wash becomes sluggish when the alcohol content reaches 

 2 per cent, by volume, and ceases when 6 per cent, is attained. 

 This end point coincides approximately with the appearance of 

 ebullient fermentation, a phenomenon well known to the practical 

 man, and one that ordinarily appears about thirty hours after 

 pitching. From this point onward, reproduction proceeds very 

 slowly if at all, and therefore the maker of pressed yeast gathers 

 his crop during that preliminary stage, since the main point with 

 him is the amount of yeast produced, and the provision of suitable 

 conditions for reproduction his chief care, the amount of alcohol 

 formed being a minor consideration. On the other hand, the 

 conditions are reversed in distilleries, the object there being to 

 minimise cell reproduction, since this goes on at the cost of the 

 sugar in the wash and therefore of the production of alcohol. In 

 low-fermentation breweries also the end of the cell-reproduction 

 period is indicated by the characteristic appearance of a " head " 

 on the surface of the wort. During this initial period the alcohol 

 content of wort reaches 2-2.2 per cent., as was determined by 

 MOHR (II.) and confirmed by F. SCHONFELD (I.). 



When it is desired to prevent the development of active yeast 

 cells in a liquid that is as yet free from alcohol for example, to 

 preserve wine must in an unfermented condition the quantity of 

 alcohol to be added, in order to produce this effect, must be 

 larger than the 6 per cent, referred to above, and according to 

 HAYDUCK (III.) and E. LAURENT (VI.), at least 10 per cent, is 

 necessary. And even this limit must be raised considerably, for 

 H. MULLER-THURGAU (III.) not only confirmed the observation 

 of earlier workers that the various races of yeast differ in 

 their degree of sensitiveness toward alcohol, but also discovered 

 races that are capable of active reproduction in presence of 

 VOL. ii : PT. 2 Q 



