CULTIVATED YEASTS 2-2 



TOP AND BOTTOM YEASTS 



Cultivated yeasts include (1) bottom and top fermenting beer 

 yeasts, (2) distillery yeasts, (3) baking yeasts, (4) wine yeasts, 

 (5) food yeasts including vitamin-, protein-, or fat-producing forms. 

 Some yeasts belongii^ to the genera Torulopsis, Rhodotorula, and 

 Mycoderma, are nonfermenters, but most yeasts are fermenters. 

 In top fermenting beer yeasts, distillery yeasts, or baking yeasts 

 the yeast is suspended in the culture broth and finds its way into the 

 foam. Bottom fermenting wine and beer yeasts produce a heavy sed- 

 iment of yeast at the bottom of the container and a clear supernatant 

 fluid. Anomolous yeasts (pickle barrel yeasts) form a membrane 

 over the nutrient solution after 24 hours, being neither top nor bot- 

 tom fermenters, in fact, they are generally nonfermenters as their 

 growth in the direct contact with the air would suggest. Yeasts not 

 belonging to the anomolous group may form a membrane after sev- 

 eral weeks. Typical top fermenting yeasts ferment with a yeasty 

 foam, sporulate abundantly, and ferment only one -third of the raffi- 

 nose molecule. The transformation of a bottom fermenting beer 

 yeast into a typical top fermenting bakers' yeast (and the reverse) 

 has been reported frequently. Top fermenting yeasts produce 

 "stormy" fermentations with rapid growth at a relatively high opti- 

 mum temperature (30O) as compared with a typical bottom fermen- 

 ter. Most top fermenters characteristically ferment only one third 

 of the raffinose molecule; they are unable to ferment melibiose. 

 When shaken with water they produce a milky fluid while an agglu- 

 tinated bottom fermenter cannot be dispersed. They grow more 

 rapidly and are much more vigorous and generally more viable than 

 bottom fermenters. 



A typical bottom fermenter may have an optimum temperature 

 of around 25°C. to 26°C., but may ferment in beer vats at a much 

 lower temperature (5°C. to 8^C.). The cells are characteristically 

 clumped and the supernatant fluid is clear and when shaken up the 

 clumped cells do not form an even suspension but the clumps settle 

 out quickly. Small amounts of alcohol are produced by the ineffi- 

 cient fermentation and a pleasant aroma is characteristic. Bottom 

 fermenters usually produce a membrane covering the nutrient broth 

 sooner than top fermenters, possibly because of their general habit 

 of coherent growth. The original bottom fermenting yeasts were 

 capable of fermenting melibiose, i.e., of fermenting raffinose com- 

 pletely, but it seems unlikely that this is a characteristic of all bot- 

 tom fermenters. Bottom fermenting beer yeasts are relatively weak 

 fermenters, producing small amounts of alcohol. 



