YEASTS. 31 



by cells before or after budding and may even change to asci and form 

 new spores. They are generally spherical or slightly ellipsoidal, rarely 

 kidney-shaped (S. marxianus) or furnished with a zonal ring (S. anomalus] 



(Fig. 13.) 



In nutrient solutions they swell, burst the mother cell, become free 

 and germinate by budding, usually producing vegetative cells directly, 

 though occasionally producing first a short promycelium (S. ludwigii). 



In Schizosaccharomyces octosporus the ascus is formed by the fusion 

 of two cells. Sometimes in other species, two or more spores in one cell 

 will fuse before germination. 



Staining with warm carbol-fuchsin and partial decolorization with 

 weak acetic acid leaves the spores red and the cell colorless. 



THE PRINCIPAL YEASTS OF IMPORTANCE TO FERMENTATION 



INDUSTRIES. 



TRUE YEASTS, SACCHAROMYCETES. The various yeasts used in brew- 

 ing and some of those used in producing distilling material are grouped 

 together as ,5. 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 pecularities such as the temperature and time limits of film and 

 spore formation, and the character of the fermented liquids. The various 

 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 somewhat 

 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. 



