278 MICRO-ORGANISMS AND FERMENTATION. 



develop at a higher temperature than bottom-fermentation 

 yeasts has no general application. 



Kayser's more recent work along the same lines has con- 

 firmed these results. He also proved that the species with- 

 stand considerably greater heat in a dry than in a moist con- 

 dition. Thus a yeast species isolated from pale ale was killed 

 in a moist condition by heating for five minutes at 60-65 

 C., whilst in a dry condition it withstood a temperature of 

 95-105 C., and in the case of a wine yeast (St. Emilion), the 

 corresponding temperatures were 55-60 C. and 105-110 C. 

 The spores withstand temperatures 10 and even 20 higher 

 than the vegetative cells. 



Vegetative cells which are derived from the heated spores- 

 show a somewhat greater power of resistance than normal 

 vegetative cells. This increased power of resistance is not 

 transmissible ; by cultivation in beer - wort it disappeared 

 entirely in the second generation. 



The temperature limits within which budding of cells can 

 take place in wort were investigated by Hansen. The upper 

 limit for S. Past. I. is 34 C., for S. membrancefaciens 35- 

 36 C., for S. anomalus and 8. Ludwigii 37-38 C., for 8. 

 Past. II., III., and S. ell. I., II., and for S. cerev. I. about 

 40 C., and for S. marxianus 46-47 C. The lower limit 

 for each of these species is 0-5 C., with the exception of 8. 

 cerev. I. and 8. Lvdwigii with a limit of l-3 C. Miiller- 

 Thurgau found that the wine yeasts that he examined are 

 incapable of propagation at temperatures above 40 C. 



It is, of course, impossible to establish any one temperature 

 that shall serve as the optimum for the growth of yeast cells, 

 because the composition of the nutritive liquid has a greater 

 effect than it has on other determinations. The formation of 

 new cells in the same liquid goes on at a diminishing rate when 

 the development proceeds at a constant temperature, because 

 the increasing quantity of the products of metabolism and 

 the simultaneous impoverishment of the nutritive fluid acts 

 restrictively upon the growth, especially at higher temperatures. 

 An approximate temperature of 28- 30 C. is found to be 

 favourable for the development of many species. Without 

 doubt the species behave differently in this respect as well as 



