186 MICRO-ORGANISMS AND FERMENTATION. 



microscopic appearance as species which give a low attenua- 

 tion. It will only be possible to form an opinion in this 

 direction when our knowledge of the structure of the plasma 

 is much more advanced. Statements to the contrary which 

 have hitherto appeared in the literature of the subject are 

 simply erroneous assertions. 



A preliminary grouping for practical purposes of the 

 different species or races of bottom- and top-fermentation 

 beer-yeasts which have been prepared in pure culture by 

 Hanserfs method in my laboratory, is as follows : 



A. BOTTOM-FERMENTATION SPECIES. 



1. Species which clarify very quickly and give a feeble 



fermentation in the fermenting-vessel ; the beer holds 

 a strong head. The beer, if kept long, is liable to 

 yeast -turbidity. Such yeasts are only suitable for 

 draught-beer. 



2. Species which clarify fairly quickly and do not give a 



vigorous fermentation ; the beer holds a strong head ; 

 high foam ; the yeast settles to a firm layer in the 

 fermenting vessel. The beer is not particularly stable 

 as regards yeast-turbidity. These yeasts are suitable 

 for draught-beer and partly for lager beer. 



3. Species which clarify slowly and attenuate more 



strongly ; the beer has a good taste and odour ; 

 the yeast deposit is less firm in the fermenting 

 vessel. The beer is very stable against yeast- 

 turbidity. These yeasts are suitable for lager beer, 

 and especially for export beers which are not pas- 

 teurised or treated with antiseptics. 



B. TOP-FERMENTATION SPECIES. 



1. Species which attenuate slightly and clarify quickly. 



The beer has a sweet taste. 



2. Species which attenuate strongly and clarify quickly. 



Taste of beer more pronounced. 



