278 YEASTS AND YEAST-LIKE FtJNGI 



round shape, and "fairly good" growth of the former in media whose 

 only carbon source is alcohol and "scant to fairly good growth" of the 

 latter. All gradations from the species to the variety are found. 

 Nature does not seem to have divided the beer and the wine yeasts 

 into distinct species. 



There are two types of industrial yeasts that can be separated 

 fairly satisfactorily. These are the top yeasts and the bottom yeasts, 

 recognized as separate entities by Pasteur and by Hansen. The top 

 yeasts grow throughout the fermenting liquid evenly distributed, gas 

 is produced abundantly early in the fermentation, and the evolution 

 of gas tends to bring the yeasts to the surface. The bottom yeasts 

 tend to settle to the bottom of the liquids and gas production is 

 slower in the early stages of fermentation. Nearly all the top and 

 bottom yeasts of industrial importance may be separated into two 

 species with several varieties of each on the basis of melibiose fer- 

 mentation. S. carlsbergensis and varieties and a few rare species, 

 bottom yeasts, ferment melibiose, while S. cerevisiae and varieties 

 and a few rare species do not ferment this disaccharide. They fer- 

 ment only one third of the raffinose.-^ See page 63. These are top 

 yeasts. Baking yeasts, most distillery and wine yeasts, and those 

 used in the production of many English ales are top yeasts. Bottom 

 yeasts are of little use other than in the brewing industry. Most 

 American, Czech and German beers are produced with bottom yeasts. 

 The top yeasts occur much more frequently in nature. Mrak and 

 associates ^^' ^^ made a careful study of yeasts occurring naturally on 

 grapes and fermenting fruits in California and found very few strains 

 of Saccharomyces which fermented raffinose completely. In the past 

 three or four years, out of perhaps over two hundred isolations of 

 Saccharomyces from infusions of apples, pears, grapes, raisins, prunes, 

 raspberries, strawberries, and various spoiled jellies and jams, here 

 in Minnesota, we have not isolated a single melibiose-fermenting 

 strain. 



It must be noted that although bottom and top yeasts a«re melibiose- 

 fermenting and non-fermenting respectively, there are exceptions, and 

 top yeasts have been developed experimentally from bottom yeasts, 

 possibly by a segregation of characters incidental to sexual reproduc- 

 tion of heterozygous yeasts in cultures. It should be noted also that 

 *S. cerevisiae is the term usually given by the industry to most brew- 

 ery yeasts whether top or bottom. For instance, most brewers of 

 American beers insist that they use a strain of S. cerevisiae. Actually 

 most American beers are made with bottom yeasts, and indeed the 

 trade term for brews made with top yeasts is ale. According to 



