220 



MICRO-ORGANISMS AND FERMENTATION. 



gressive development of the yeast. The earlier statements, 

 that by continued cultivation of, e.g., bottom-yeast at a high 

 temperature, it could be converted into top-yeast, can only 

 be explained on the assumption that the bottom-yeast em- 

 ployed was impure and contained an admixture of top-yeast, 



FIG. 69. Carlsberg bottom-yeast, No. 1 (after HANSEN). 



which, at the high temperature, gradually developed at the 

 expense of the bottom-yeast, until it finally preponderated in 

 the mixture. 1 



As examples of two different bottom-fermentation species 



FIG. 70. Carlsberg bottom-yeast, No. 2. Some cells with spores (after HANSEN). 



of yeast, the Carlsberg bottom-yeasts "No. 1" (Fig. 69) and 

 "No. 2" (Fig. 70), employed in the Old Carlsberg brewery at 

 Copenhagen, may be more minutely described. 



2 LOISEATJ and BAU have established an essential chemical difference be- 

 tween the bottom- and fop-fermentation yeasts used in breweries. The former 

 ferments melibiose and melitriose completely ; the latter does not ferment 

 melibiose, and decomposes melitriose into fructuose and melibiose, the former 

 undergoing fermentation. According to E. FISCHER and these authors, meli- 

 biose is split up by an enzyme contained in bottom-fermentation yeast before 

 being fermented. No such enzyme decomposing melibiose could be detected 

 in the top-yeasts examined by FISCHER. 



