48 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



upon the type of fermentation. Some yeasts settle rela- 

 tively rapidly to the bottom of the fermenting liquid and 

 do not form a heavy scum or pellicle over the surface. 

 They are termed bottom yeasts. Certain of the wine and 

 cider yeasts are of this type. In other species growth is 

 usually characterized by the development of a foamy scum 

 on the surface. Such are termed top yeasts. Interme- 

 diate forms, however, are known. 



Many of the great brewing districts in Europe have 

 developed special strains of yeasts which are used for manu- 

 facture of particular types of beverages. Such yeasts have 

 been named after the town, locality, or establishment in 

 which they originated. For example, one may read of 

 Saaz yeasts, of Carlsberg bottom yeasts, etc. 



Among the earlier investigators an attempt was made 

 to separate the various species of yeasts on the basis of 

 shape. The ordinary brewers' or bread yeast, known as 

 Saccharomyces cerevisiae, was described as oval or nearly 

 spherical ; wine yeasts were found usually to be ellipsoid 

 or elongate, and were termed Saccharomyces ellipsoideus. 

 The name Saccharomyces pastorianus was given to those 

 which produced long or cylindric cells. The conditions 

 under which yeasts are grown, however, influence to such 

 a marked degree the shape and size of the cells that this 

 method of differentiation is frequently difficult to use. 



The most satisfactory methods for differentiation of 

 yeast species are based upon such morphological differences 

 as may be observed microscopically, the development of 

 spores, the ability of yeasts to ferment particular sugars, 

 to grow in the presence or absence of air, and to produce 

 definite quantities of alcohol. It is customary to recognize 

 six groups of yeasts, the groupings being determined by 

 the carbohydrates which may be fermented by the forma- 

 tion of alcohol and carbon dioxide. The following table 

 gives the characteristics of these groups : 



