YEASTS 37 



method o£ reproduction. In the bacteria, as we know, the spore is 

 formed by a shrinking of the cell contents without any division. 



56. Species of Yeasts. — The yeasts belong to the genus 

 Saccharomyces. There are many species and varieties, 

 which are not easily distinguished from one another. Com- 

 mercial yeast used in bread-making is ordinarily a mixture 

 of several species. The yeasts used in beer-making are of 

 two distinct sorts which are very different in form and size. 

 " Bottom " yeast, used in brewing most German and Ameri- 

 can beers, is found in the lower part of the malt solution, and 

 produces fermentation at low temperatures. " Top " yeast, 

 used for English ale, stout, and porter, lives at the top of the 

 liquid, and its fermentation goes on at relatively high tem- 

 peratures. The yeasts that ferment grape juice are of differ- 

 ent varieties, and the characteristic flavors of the many 

 different wines are due in large part to the different yeasts 

 which produce them and which cause, in addition to the ordi- 

 nary alcoholic fermentation, the formation of other sub- 

 stances that modify the flavor of the wine. 



Besides beers and wines, there are many other fermented 

 beverages due to the action of yeasts, such as kumiss and 

 pulque. Distilled liquors, like brandy and whisky, and 

 commercial alcohol itself, are also the result of a fermentation 

 by yeasts, followed by distillation. The beer yeasts and the 

 bread yeasts are known only in cultivation. The wine 

 yeasts live on the soil of vineyards and are carried to the skins 

 of the grapes in the dust that is blown about. They are 

 " wild " yeasts, and different varieties occur in different lo- 

 calities. In the same way, a wild yeast that lives on the soil 

 under apple trees causes the fermentation of cider; and 

 many other wild yeasts produce a variety of fermentations. 



57. Relationships. — The yeasts belong to the great group of fungi 

 (singular, fungus), which includes the relatively simple plants that are 

 without chlorophyl. In the broadest sense of the term, the fungi 

 include the bacteria also. The relation of yeasts to other fungi is not 



