CHAPTER III 1 



PHYSIOLOGY OF YEASTS. NUTRITION, RESPIRATION, 

 AND ALCOHOLIC FERMENTATION 



YEASTS are able to undergo two very different kinds of life. 

 Sometimes they live in contact with air and respire under 

 aerobic conditions; at other times, in the absence of air. In 

 this latter case, they take the energy which is necessary from another 

 process. They transform the greater portion of sugar at their dis- 

 posal into alcohol and carbonic acid. They thus induce an alcoholic 

 fermentation, which is then anaerobic. One must distinguish be- 

 tween the yeast plant which acts like other ordinary plants and the 

 yeast ferment which is the agent of alcoholic fermentation. 



We shall take up in this chapter the general nutritive processes 

 of the yeast, that is, its nutrition, respiration, and alcoholic fermenta- 

 tion, reserving for a following chapter the study of the relation of 

 yeasts to their external environment, of the conditions which determine 

 their multiplication, sporulation, and parasitism. 



We shall begin by investigating the chemical make-up of the 

 yeasts and by studying the various enzymes which prepare foods for 

 absorption. 



GENERAL PHENOMENA OF NUTRITION OF 

 THE YEASTS 



Chemical Composition of the Yeasts 



The different analyses of yeasts undertaken by various authors 

 have given variable proportions of C, H, N, 0, and S. (Dumas, 

 Schlossberger, Mitscherlich.) Analysis of the ash of yeasts has given 

 equally inconstant results. Phosphoric acid, silicic acid, carbonic 

 acid, sulfuric acid, hydrochloric acid, potassium, sodium, sulfur, 

 magnesium, calcium and ferric oxide and manganic oxide have all 

 been found to exist in different proportions. 



Thus, as we have seen in the preceding chapter, the yeast cell is 

 composed essentially of a membrane which seems to be made up of 



1 In the preparation of this chapter, the obliging collaboration of M. A. 

 Polecard, D.Sc., Chief of the Department of Physiology of the College of Medi- 

 cine of the University of Lyon, is acknowledged. 



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