1 62 AIDS TO BACTERIOLOGY 



liquid, they form mature cells, which multiply, as usual, 

 by gemmation. Dextrose, laevulose, mannose, and (some- 

 times) galactose, are the only hexoses fermentable by 

 the yeast cell. Maltose, cane-sugar, malto-dextrin, and 

 lactose are fermented, but different species vary in 

 fermentation ability. 



Saccharomyces Cerevisise. Of the typical brewery 

 yeast there are two main varieties, which are intercon- 

 vertible: the typical English brewery yeast a ' top ' 

 fermentation variety and a yeast of the Continental 

 lager beer brewer a ' bottom ' fermentation. The ' top ' 

 and ' bottom ' varieties are known also as ' high ' and 

 ' low ' yeasts respectively, as they ferment at high and 

 low temperatures respectively. The rounded or slightly 

 ellipsoidal cells are from 8 ^ to 9 //, in diameter, and occur 

 both singly and in short chains. Spores occur three or 

 four together in a mother cell, each being 4 /n to 5 /,<, in 

 diameter. 



Fermentation with ' low ' yeasts, in the manufacture of 

 lager beer, takes place at 5 to 10 C., at which tem- 

 perature other forms of yeast are inert. The process 

 requires about fourteen days. 



' High '-fermentation yeast consists of cells which are 

 rather larger and more globular, and have a greater 

 tendency to form branched chains than the ' low ' yeasts. 

 The temperature best suited for this fermentation is 

 between 15 and 18 C. The reaction in the fermenting 

 vats is much more violent than is the case with the 

 ' low ' yeasts. The rapid emission of carbonic acid 

 brings the cells to the surface. 



A soluble ferment (invertase) is present, which can be 

 extracted by precipitating with chloroform, and which 

 converts sucrose into invert sugar. Another enzyme 

 occurs within the cells (maltase), which is capable of 

 converting maltose into glucose. 



Buchner, by grinding up yeast with kieselguhr and 

 filtering under pressure, obtained a liquid which caused 

 the evolution of carbonic acid and formation of alcohol 

 in solutions of glucose. This he termed zymase, and 

 regarded it as a solution of an intracellular ferment. 

 Von Lebedeff prepares zymase by macerating 1 part 

 of dried yeast with 2-5 3 parts of water, and filtering 

 through paper after being allowed to remain overnight. 



