SACCHAROMYCES 279 



Stelling-Dekker and most other authorities on yeast classification, 

 such strains should, in most cases, be termed S. carlsbergensis. Han- 

 sen's original S. carlsbergensis was a bottom yeast, isolated from the 

 inoculum used in Denmark. S. cerevisiae was isolated from English 

 ale and was a top yeast as described by Hansen. 



There are many varieties, races, and strains in both these species, 

 and these strains find their use in various industrial processes. For 

 certain purposes certain strains are desired. Those producing a high 

 alcoholic content in the mash are desired in distillery yeasts whether 

 for distilled alcoholic liquors or for industrial alcohol; baking quality, 

 high yield from inexpensive media, and high vitamin content are quali- 

 ties desired in compressed yeasts ; ease of clearing and flavor imparted 

 are qualities necessary in beverage yeasts. The flavor of wines and 

 beers is known to be partly due to the particular yeasts responsible 

 for the fermentation, and in many cases, especially in the brewing 

 industry, this is controlled. Mutations are said to occur, but more 

 likely it is a segregation of characters due to sexual reproduction, 

 and new races are being produced by artificial breeding. The older 

 races have been given various trade names. Brewing yeasts are 

 generally designated by names indicating the brewery from which 

 they had their origin, as Carlsberg Bottom Yeast No. 1. The Saaz, 

 Logos, and Frohberg yeasts are particularly well-known strains. 



Other species of Saccharomyces are more rarely encountered. Some 

 of these occur regularly in certain types of natural fermentations and 

 are desirable. Often they occur as "weeds" and may impart unde- 

 sired flavors, e.g., S. Pastorianus. This is a frequent contaminant 

 in brewing and may often be obtained from commercial yeast cakes. 

 It forms large sausage-shaped cells and a scum in old cultures. It 

 gives rise to disagreeable flavors in beer, and seems to be a common 

 organism in American "home brew." 



There are in use in various parts of the world, particularly south- 

 eastern Europe and adjacent Asia, fermented beverages made from 

 milk. Kefir and koumyss are the best known of these. The fer- 

 mentation is a complex one, as pure cultures are not used, and there 

 are various bacteria and usually several species of yeast present. 

 Not all of the latter are capable of fermenting lactose, but depend 

 upon bacteria for the preliminary conversion of the lactose to glucose 

 and galactose. These hexoses are then fermented to carbon dioxide 

 and alcohol by the yeasts. There are, however, some yeasts which 

 form lactase as well as zymase, and can carry on the alcoholic fer- 

 mentation by themselves. Of these the best known is S. fragilis. 

 The cells are oval to elliptical. From two to four spores are formed. 



