BIOLOGICAL RELATIONSHIPS OF YEAST. 279 



in regard to the maximum production of yeast which can be 

 developed from a given inoculation. 



Many fermentations take place in the industry at lower 

 temperatures ; indeed, in the case of bottom-fermentation 

 breweries, very considerably lower than the optimum for the 

 multiplication of the cells. In order that fermentation may 

 be completed at so low a temperature within a reasonable 

 time, and before other organisms have an opportunity of 

 infecting the liquid, relatively large amounts of yeast are 

 introduced, and propagation is assisted by aeration. At 

 times the pitching yeast is first placed in a smaller 

 quantity of the liquor at a higher temperature (about 

 20 C.), allowed to grow for a few hours, and the newly- 

 formed and vigorous cells are then introduced into the cold 

 liquor. There appears to be a tendency to forego the extremely 

 cold fermentations once customary in many places. In dis- 

 tilleries, where fermentation proceeds at a higher temperature, 

 it is often necessary to take special precautions to avoid 

 a considerable rise in temperature during the first stages of 

 the fermentation ; otherwise the propagation of cells ceases too 

 soon. Consequently the growth would be so enfeebled that it 

 would be impossible to carry the fermentation to completion. 



It has already been stated that the composition of the 

 nutritive fluid plays an important part in the propagation of 

 the yeast cells. Liquids containing a large percentage of sugar 

 have a weakening effect on the cells. According to Laurent, 

 growth ceases in a decoction of malt germs containing 

 60 grammes of sugar per 100 c.c. A few species of yeast, 

 nevertheless, appear to retain their activity even in the 

 presence of greater quantities of sugar. The aeration of yeast, 

 as carried out in practice, is of real importance for propaga- 

 tion. Exact conclusions regarding this were published by 

 Hansen in 1879. He used the cell-counting chamber, already 

 alluded to, and found that a beer yeast grown in wort at 

 12- 14 C. showed the formation of eleven cells from a 

 single cell in sixty hours without aeration, whereas with aera- 

 tion thirty-six cells were formed from each individual in the 

 same time. The importance of aeration depends not only 

 upon the fact that oxygen reacts more intensely on the indi- 



