44 PHYSIOLOGY OF BACTERIA 



Other a-amino acids also are changed to alcohols, e.g. tyrosin 

 OHC6H4-CH2CH(NH2)C02H gives OHC6H4CH2CH2OH, tyrosol, 

 a bitter tasting compound, probably the cause of the bitter taste of 

 certain beers (Ehrlich, 1911). Tryptophane is changed to tryptophol 

 by the same process, and these and similar compounds or their 

 esters may be one of the causes of the differences in the taste of dif- 

 ferent wines and beers. 



After this evidence that some of the products of yeast 

 fermentation are of protein origin, the products derived 

 from sugar can be traced with more certainty. 



(e) SirCCESSIVE STAGES IN THE FERMENTATIONS OF SUGARS 



The first great advance in the study of intermediate 

 products and successive steps in fermentation was 

 Neuberg's work on the fermentation of pyruvic acid by 

 yeast (Neuberg and Hildesheimer, 1911) and his further 

 developments from this starting point. The English 

 biochemists with Harden studied, in great detail, the 

 Colon fermentations as well as those by yeast. At the 

 same time, Warburg concentrated on the chemistry of 

 the respiration process, and Wieland developed his 

 theory of the activated hydrogen as the essential part 

 in all organic oxidations as well as reductions. Oxygen 

 is a '' hydrogen acceptor, '^ but in many cases it can be 

 substituted by other hydrogen acceptors, such as 

 nitrate or methylene blue. Warburg had found that 

 oxygen can oxidize only if ^' activated,'^ and the activator 

 in biological oxidations is iron. A combination of 

 Warburg's and Wieland's theories proved possible; 

 Kluyver and Donker (1926) have applied this combined 

 theory to fermentations and respiration in general, and 

 the discussions of fermentations offered here follows 

 closely their interpretation. 



As an illustration of Wieland's mode of presentation, 

 the Cannizzaro reaction between methyl glyoxal and 



