278 TEXTBOOK OF PLANT PHYSIOLOGY 



The influence of external conditions on alcoholic fermentation 

 is, on the whole, the same as on respiration. As the temperature 

 rises, fermentation rapidly increases, reaching its maximum at 

 50° C, when yeast begins to die. Light somewhat retards fermen- 

 tation, as it curtails the vital activity of many lower organisms. 

 The greatest interest is attached to the study of the influence 

 exerted on fermentation by carbon dioxide and oxygen. Being 

 one of the products of fermentation and accumulating in large 

 quantities in the fermenting liquid, carbon dioxide has no evident 

 influence on fermentation. Yeast is as little sensitive to it as to 

 the accumulation of alcohol. 



The presence or absence of oxygen is of little importance in the 

 process of fermentation. Yeast ferments sugar in an anaerobic 

 medium, as well as in the air. With oxygen, however, yeast 

 undergoes oxygen respiration, producing as has been seen, a much 

 larger amount of useful energy. For this reason, yeast ferments a 

 somewhat smaller amount of sugar in the presence of air than 

 under anaerobic conditions. This economizing influence of oxygen 

 is different with the various yeast races. It is less marked in beer 

 yeast and more marked in wine yeast that lives in the wild state on 

 the surface of grapes, i.e., under the conditions of an abundant 



air supply. 



One must distinguish the direct influence on fermentation, as 

 on enzymatic process, from the influence of external factors on the 

 reproduction of yeast. In the industries, of course, the greatest 

 importance is placed on the reproduction of yeast in the fermenting 

 liquid, since, other conditions being equal, the rate of fermentation 

 of a liquid is dependent upon the number of cells producing it. 

 Here the oxygen plays a very important role, since in its presence 

 reproduction proceeds much more rapidly than without it. For 

 the same reasons it is also advisable to remove the carbon dioxide 

 which usually accumulates in the liquid. Hence, in the beer-brew- 

 ing industry fermentation proceeds in flat vats, or else air is blown 

 through the fermenting liquid. Moreover, the optimum tempera- 

 ture for the reproduction of yeast is 25 to 30° C, not 50° C. 



