140 MICRO-ORGANISMS AND FERMENTATION. 



PRIOR is of opinion that the different behaviour of yeast 

 species in this respect may be partly accounted for by the 

 supposition that the cell-walls are not equally thick in different 

 species, and that the permeability varies ; he assumes that the 

 thickness of the outer slime layer of the cell-wall also plays a 

 part. From comparative experiments with the same type of 

 yeast, which he made to act upon different sugar species, he 

 further concludes that the individual sugar species fiossess 

 different rates of diffusion through the cell-wall, the diffusive 

 power of cane-sugar being greatest, that of maltose least. 



We may aptly conclude this survey with a brief mention 

 of the preliminary communications of ED. BUCHNER on the 

 separation of the active ferment from the living yeast-cell. 

 Common brewers' yeast, mixed with quartz sand and kiesel- 

 guhr, was finely ground, mixed with water, and then subjected 

 to a pressure of 4-500 atmospheres. The expressed juice, 

 containing substances extracted from the contents of the yeast- 

 cells, is capable of quickly fermenting highly concentrated 

 sugar solutions of various kinds, even if the juice is filtered 

 through kieselguhr (as in a Berkefeld filter), and the mixture of 

 expressed juice and sugar solution saturated with chloroform. 

 BUCHNER infers from his experiments that the fermentative 

 power of the expressed juice is embodied in a soluble enzyme- 

 like substance isolated from the living cell-plasma, undoubtedly 

 an albuminoid, which he terms zymase. If allowed to stand, 

 this expressed juice soon loses its power, whereas if mixed 

 with a 75 per cent, saccharose solution, it retains its activity 

 for a long time. On evaporating and drying the expressed 

 juice, a brittle, yellowish mass is obtained, which is capable of 

 exhibiting fermentative activity for a considerable time. 



Thus, BUCHNER'S results harmonise with the chemical 

 theories of TRAUBE and FISCHER, alluded to above. 



EAYMAN and KRUIS have added to our knowledge of the 

 biology of yeast-fungi by their experiments on beers which, for 

 several years, had undergone fermentation with absolutely pure 

 cultures, prepared by HANSEN'S method. These investigators 

 found that the fermentation product obtained by means of 

 pure cultures of Saccharomycetes all the normal conditions 

 of temperature, etc., obtaining in the brewery being maintained 



