392 MOULDS AND YEASTS 



Most of the races of yeast used in the manufacture 

 of beer in this country carry on rapid fermentation at 

 temperatures between I 2 to 24 C., and are lifted to the 

 surface of the liquid as a frothy scum by the bubbles of 

 carbon dioxide set free : such are known as top-fermenta- 

 tion yeasts. Others, termed bottom-fermentation yeasts, and 

 utilized for the production of lager beer at 5 to 10 C. 

 produce alcohol more slowly, and accumulate as a muddy 

 layer on the bottom of the vessel in which the fermenta- 

 tion takes place. 



A great number of species and varieties of yeasts 

 have been described : a few only need mention here. 



(i) Saccharomyces cerevisiae, Meyer. Beer yeast. 

 The typical form of this species, of which many varieties 

 exist, is roundish oval : usually I to 4 ascospores from 

 3 to 6 (L in diameter, are produced in each cell the 

 optimum temperature for their formation when grown on 

 gypsum blocks is about 30 C. It ferments glucose, 

 saccharose and maltose, but not lactose. 



(ii) S. pastorianus, Reess., possesses oval, round and 



long sausage - shaped cells 

 (Fig. 58). The ascospores 

 are generally I to 4 in num- 

 her, and from 2 to 4 ^ in 

 diameter. They are formed 

 most rapidly at 27 to 28 C. 

 It is a wild yeast which gives 

 a bitter taste and unpleasant 



ferment lactose. 



(iii) S. ellipsoideus, Reess., is one of the species 

 or forms of yeast concerned in the manufacture of wines 

 from grape juice : it occurs on the exterior of grapes, 



