318 MICRO-ORGANISMS AND FERMENTATION. 



According to Bau and Fischer, melibiase, the enzyme that 

 ferments melibiose, could only be detected in low-fermentation 

 yeasts, and not in those top yeasts that are unable to ferment 

 melibiose. 



According to Hansen's recent work (referred to in the 

 section on Variation), individuals may occur in old cultures of 

 bottom yeasts which exhibit top-fermentation phenomena, 

 and similarly, if in smaller numbers, individuals in top yeasts 

 which behave like bottom yeasts. 



After Hansen had introduced a pure culture of bottom 

 yeast into the Carlsberg brewery in Copenhagen it was possible 

 to discover how extensive and how deep-seated the differences 

 are which distinguish the various bottom-fermentation brewery 

 yeasts. With this object in view the writer undertook a long 

 series of comparative experiments with pure cultures of top 

 and bottom yeasts from every part of the world, noting in 

 particular the degree of fermentation, the clarifying power of 

 the liquid, the physical phenomena of fermentation, and the 

 stability of the fermented liquor. As early as 1886, in the 

 first edition of this book, he set forth a classification of typical 

 species or races, the correctness of which has been confirmed by 

 subsequent workers in this field. 



A. BOTTOM-FERMENTATION SPECIES. 



1. Species which clarify very quickly and give a feeble 



fermentation ; 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 some for lager beer. 



3. Species which clarify slowly and attenuate more strongly. 



The beer is very stable to yeast-turbidity. These 

 yeasts are suitable for lager beer, and especially for 

 export beers. 



