FERMENTATION 155 



By causing fermentation to take place in the presence of 

 sodium hydrogen sulphite or of dimethyl-dihydroresorcinol, 

 either of which combine with acetic aldehyde, Neuberg in- 

 creased the yield of glycerol produced in alcoholic fermenta- 

 tion. This, he suggests, is due to the hydrogen — which would 

 normally reduce the acetic aldehyde to ethyl alcohol — being 

 forced to find some other acceptor which he presumes is an 

 aldehydic precursor of glycerol. 



The main result of his experiments indicate that acetic 

 aldehyde is an intermediate product of alcoholic fermentation, 

 and he provided the means of establishing the presence of 

 small quantities of acetic aldehyde which has proved of great 

 value in determining indirectly whether, in any process of 

 carbohydrate degradation, a change of the nature of alcoholic 

 fermentation is operative — the significance of this will be seen 

 later (p. 160). 



Factors influencing alcoholic fermentation by yeast. — Some 

 space must be devoted to a consideration of the influence 

 of oxygen upon alcoholic fermentation with a view to tracing 

 the relation between the oxygen respiration of the yeast plant 

 with its activity in alcoholic fermentation, which, according 

 to the equation 



C 6 H 12 O - 2C 2 H B OH + 2C0 2 , 



does not involve atmospheric oxygen. 



Pasteur, to whom we owe much of our earlier knowledge 

 of alcoholic fermentation, held the opinion that alcoholic 

 fermentation was a mode of life of the yeast plant when unable 

 to obtain air, " la vie sans air" but he also knew that when 

 air is entirely withheld the plant sooner or later stops growing 

 and gradually dies, but the introduction of a little oxygen, 

 if not too late, restores the plant to activity. During fermen- 

 tation in anaerobic conditions, although the breakdown of 

 sugar continues, the reproduction of the cells stops. In at- 

 tempting to explain the role of oxygen in fermentation, sub- 

 sequent workers came to the erroneous conclusion that oxygen 

 stimulated fermentation since they obtained an increased 

 yield of alcohol when the fermenting liquors were aerated, 



