88 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



From the above figures it may be seen that, both 

 in osmotic pressure and electrical conductivity, pressed 

 yeast gives values which are much higher than those of 

 wort. Baker's yeast, however, gives a low osmotic pres- 

 sure, but a high conductivity even after washing. 



The figures afforded by the sap of yeast and by the 

 surrounding nutritive fluid may be seen in Table XXV. 



On comparing the results given by beer with those of 

 wort, it is at once apparent that, while the electrical con- 

 ductivity remains much the same, the osmotic pressure 

 becomes approximately three times as great during fer- 

 mentation, when interrupted at the usual stage in the 

 commercial process. Very complete fermentation, how- 

 ever, judging from the single experiment performed, 

 occasions a fall in osmotic pressure after the initial rise, 

 and is accompanied by a marked increase in the conduc- 

 tivity (see No. 597). It is, however, possible that the 

 conditions of this fermentation were abnormal, and there 

 was probably considerable loss of liquid by evaporation. 

 The above-mentioned experiment is substantiated by 

 No. 609, which is the beer of No. 606 allowed to stand at 

 air temperature in a closed vessel with a little yeast. It 

 will be noted that there is a fall in pressure, but a slight 

 rise in conductivity. 



Turning now to the yeast in Nos. 595, 598, 612, the 

 osmotic pressure of the juice is much higher than that of 

 the beer, corresponding in two cases to a difference in 

 freezing-point of about 0-5. In these cases the yeast was 

 separated from the beer by centrifuging to remove ad- 

 herent liquid as completely as possible, and was then 

 frozen. This process occupies some time. In No. 612, 

 where it was effected as rapidly as possible, less than one 

 hour elapsed between the separation and the freezing in 

 liquid air. In this case the divergence between the os^ 



