324 MICRO-ORGANISMS AND FERMENTATION. 



As an admirable example of the application of Hansen's- 

 biological methods to the differentiation of yeast species, we 

 give the comparison of four low-fermentation brewery yeasts 

 carried out by Will. This, again, emphasises the fact that, 

 within this group, just as clearly distinguished species occur 

 as in those groups of Saccharomycetes which have not yet 

 found an industrial application. Will began his characterisa- 

 tion by adopting the classification of brewery-yeast types 

 published by the author in 1886. He classed Races 93 and 2 

 as high-fermenting, Race 7 as a low-fermenting type, and 

 Race 6 as a yeast of intermediate fermentation. 



The four yeasts can be distinguished as follows : 



Race 2 has large roundish or oval cells ; the colonies on> 

 gelatine are spherical or lenticular ; the spores are formed 

 easily and freely ; spore-formation takes place between 31 

 and 11 ; the optimum is 25- 26 ; film-formation occurs 

 between 31 and 7 ; very slow. 



Race 6 ; oval cells predominate, but the species has a 

 great tendency to form sausage-shaped cells ; colonies on 

 gelatine are spherical or lenticular ; the spores form easily 

 and freely; spore-formation occurs between 31 and 11 C. ; 

 the optimum is 28 ; film-formation between 31 and 7,. 

 occurring later than with Race 7. 



The cells of Race 93 are typically oval with a tendency to- 

 assume a roundish shape. . The colonies in gelatine are spherical 

 or lenticular ; spores are freely and easily formed ; spore- 

 formation occurs between 30 and 10 C. ; optimum 28 ; 

 film-formation between 31 and 4 ; very feeble and slow ;. 

 resting cells occur freely in the film. 



Race 7 has oval cells which closely approach the spherical 

 shape ; giant cells regularly occur, and at the end of the 

 fermentation large budding colonies with small oval cells 

 frequently occur ; the young colonies on gelatine are irregular 

 with a marked wavy and fringed outline ; the species develops 

 spores with great difficulty ; spore-formation occurs between 

 30 and 13 ; optimum 25 to 26 ; film-formation between 

 28 and 4 ; appears earlier than with the other species ;. 

 resting cells are to be found only in small numbers in 

 film. 



