YEASTS 73 



tion, is replaced by a compensating phenomenon which intervenes at 

 the germination and consists in the fusion of the ascospores two by two 

 (Fig. 55). The ascospores anastomose at their extremities by a chan- 

 nel of copulation which, as soon as the nuclear fusion is accomplished, 

 becomes the seat of a budding. 



THE PRINCIPAL YEASTS OF IMPORTANCE TO FERMENTATION 



INDUSTRIES* 



TRUE YEASTS, SACCHAROMYCETES. The various yeasts used in 

 brewing and some of those used in producing distilling material are 

 grouped together as S. cerevisia. They are large and round or slightly 

 oval. 



They are divided into three main groups the bottom yeasts which 

 are used in the manufacture of German beer, and which, usually, are 

 capable of producing only a moderate amount of alcohol; the top yeasts, 

 used in English beers and compressed yeast, capable of producing more 

 alcohol, and the distillery yeasts, which have great fermentative power 

 and produce large amounts of alcohol. 



Many forms of these yeasts have been described in great detail by 

 Hansen and others but the distinctions are based principally on physio- 

 logical peculiarities such as the temperature and time limits of film and 

 spore formation, and the character of the fermented liquids. The vari- 

 ous forms seem to be fixed, and to retain their characteristics unchanged 

 under almost all forms of treatment. 



The wine yeasts, S. ellipsoideus, seem to be even more diverse than 

 the beer yeasts, but have been less thoroughly studied. They are some- 

 what smaller than the latter and usually slightly more elongated. They 

 form spores much more abundantly and easily than the beer yeasts 

 and the cells in film formation are often much elongated. 



Their fermentative power is considerable, some of them being capa- 

 ble of producing over 16 per cent by volume of alcohol. W. V. Cruess 

 has obtained 21 per cent from a Burgundy wine yeast. They differ in 

 the flavors and aromas which they produce in the fermented liquid, and 

 especially in the rapidity with which they settle. Some yeasts, such 

 as those of Champagne and Burgundy, form a compact sediment which 

 settles quickly and leaves the liquid clear. Others remain suspended 

 for a long time and settle with difficulty. 



* Prepared by F. T. Bioletti. 



