258 APPLIED BACTEEIOLOGY 



There are two kinds of fermentations employed in 

 breweries, the ' low ' and ' high.' Whether the Saccharomyces 

 cerevisicB producing these two kinds of fermentation are 

 identical species or not is still a disputed point, but they un- 

 doubtedly retain their distinctive mode of action through 

 many generations, although Eees states, in opposition to 

 Pasteur, that the one kind may be transmuted into the other. 

 ' Low ' Yeasts. — These are employed in making the 

 German and Austrian lager-beers, which differ from English 

 beers, prepared by the ' high ' fermentation process, by not 

 containing so much alcohol or extractive ; but they are of 

 more delicate flavour, and they seem likely in time to 

 entirely replace the heavier beers prepared from the 'high' 

 yeasts. The ' low ' yeasts consist of round or oval cells 

 8 to 9 /i in diameter. In sporulation there are three to four 

 spores of 4 to 5 /A in each mother-cell. This fermentation 

 takes place at a very low temperature, not higher than from 

 5 to 10° C, a temperature at which other forms of yeast 

 are inert. This low temperature is maintained by passing 

 currents of purified cooled air over the surface of the 

 fermenting vessels, or by floating metal cones containing 

 ice in the beer; the number used is regulated by the 

 temperature of the external air. As would be expected, 

 this fermentation proceeds more slowly than the ' high ' 

 process, taking on the average about fourteen days, the 

 cells during the fermentation falling to the bottom of the 

 fermenting vat. 



'High' Yeasts. — These are especially used in the 

 manufacture of English beers, whose bouquet and richness 

 in alcohol render them more acceptable to English tastes 

 than the somewhat milder German beers, prepared by the 

 ' low ' fermentation process. The ' high ' fermentation 

 yeast consists of cells which are rather larger and more 

 globular, and have a greater tendency to form branched 



