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TEXTBOOK OF PLANT PHYSIOLOGY 



which influence in various ways the quality and the flavor of the 

 beverages obtained. Some species, as for instance, beer-yeast, 

 Saccharomyces cerevisiae, consist of many varieties or races, each of 

 which gives the beer a taste of its own. Almost every large 

 brewery, particularly in Germany, uses its own race of yeast 

 and has a special microbiological laboratory for maintaining its 

 purity. 



Only definite substances, carbohydrates with 3, 6, and 9 carbon 

 atoms (trioses, hexoses, and nonoses) are fermentable; di- and 

 polysaccharides may be fermented only after previous hydrolysis. 

 But since yeast possesses active invertase and maltase, but no 

 diastase, it fails to ferment starch directly, which must be pre- 



Cytoplasm 



Cell Wall 



Vacuole' 



Fig. 104. — Yeast. A, structure of cell; B, chain of budding yeast; C, D, E, 

 successive stages of budding; F, spores (after Smith et al.). 



viously changed into sugar. Hence, in the preparation of beer one 

 cannot start with barley before it has germinated, since it is very 

 poor in diastase ; but must use malt in which the quantity of dias- 

 tase has considerably increased during germination. Moreover, 

 before adding yeast, it is necessary to "cook" the beer mash, thus 

 changing the remaining starch into maltose at 60 to 70°, the op- 

 timal temperature for the action of diastase. 



Not all hexoses can be fermented by yeast. As in respiration, 

 so in fermentation only the rf-glucose, rf-fructose, d-mannose and 

 d-galactose are fermentable, the last two sugars being utilized 

 considerably slower than the others. Yeast can ferment rather 

 pure solutions of sugars, but like other saprophytic organisms, it 

 requires for its full development all the necessary mineral sub- 

 stances, and a ready source of nitrogen, preferably in the form of 

 peptones or amino acids. Ammonia is also suitable for the syn- 

 thesis of proteins by yeast. When amino acids are present in 

 abundance in the fermenting liquid, they are deaminized and often 



