502 CHEMISTRY OF ALCOHOLIC FERMENTATION. 



certainty, that no reproduction has occurred. An unduly large 

 sowing of yeast also modifies the composition of the nutrient 

 medium, owing to the diffusion of soluble matters from the yeast, 

 more particularly from the dead cells. It is still an open question, 

 however, whether this change persists when air, carbon dioxide, 

 or hydrogen that is not perfectly pure, is blown through the 

 solution. 



Although the results of the majority of these experiments 

 tend to indicate that the amount of work performed per unit of 

 yeast is smaller in presence of oxygen, and that oxygen therefore 

 restricts fermentation, they cannot, however, be regarded as 

 decisive. For this to be the case it is an essential condition to 

 show that each cell has been continuously exposed to the influence 

 of the oxygen throughout the entire experiment. Now, in the 

 case of liquid cultures, the yeast cells frequently agglomerate to 

 small lumps, the interior of which cannot be reliably demonstrated 

 to be accessible to oxygen ; and the metabolism proceeding inside 

 these lumps differs from that in the outer cells that are exposed to 

 the ascending bubbles of air. This objection applies with still 

 greater force to all cultures on solid media, including streak 

 cultures on sugar-gelatin, the cells below the surface being 

 practically shut off from the oxygen in contact with those on the 

 surface. The only way to afford decisive proof is by experiments 

 in cell-less fermentation with zymase, and therefore the question 

 whether the fermentation set up by yeast is influenced in one way 

 or another by oxygen must be regarded as still unsettled. 



Practical experience in the alcohol industry is not opposed to 

 the foregoing particulars. The purpose of the aeration regarded 

 as necessary or useful in the case of fermenting mashes, especially 

 molasses, distillery wash and pressed yeast factories, is mainly to 

 increase the yeast crop and not to augment the fermentative power 

 of the individual cells. Any prolongation of the rousing process 

 beyond the attainment of this object is for the purpose of utilising 

 the favourable supplementary effects of this treatment as indicated 

 on p. 501, vol. ii. Due precaution must be observed in this, since 

 otherwise considerable loss may arise from another supplementary 

 effect, namely, the volatilisation of the alcohol. These circumstances 

 are merely referred to now, to prevent the impression that the fact 

 that aeration accelerates and increases the fermentation of a liquid 

 medium affords proof that oxygen stimulates fermentation. The 

 amount of alcohol carried away by the liberated carbon dioxide 

 was investigated by ECK (I.) as long ago as 1875, * n the case of 

 distillery washes; and the loss determined by Riss (I.) in experi- 

 ments made with a saccharine medium containing mineral salts, 

 amounted to 1.12 per cent., referred to the quantity of alcohol 

 present in the fermented liquid. 



Finally, brief consideration may be devoted to Pasteur's con- 

 ception of alcoholic fermentation as life without air (vol i. p. 20). 



