FERMENTATION. 123 



gradually become more languid, and if they do multiply it is in strange and 

 monstrous forms. A little older still, they remain absolutely inert in a 

 medium deprived of free oxygen. This is not because they are dead ; for in 

 general they may be revived in a marvellous manner in the same liquid if it 

 has first been aerated before they are sown. ... At this point we must 

 observe for it is a matter of great importance that in the operations of 

 the brewer there is always a time when the yeasts are in this state of 

 vigorous youth of which we have been speaking, acquired under the in- 

 fluence of free oxygen, since all the worts and all the yeasts of commerce 

 are necessarily manipulated in contact with air, and so impregnated more or 

 less with oxygen. The yeast immediately seizes upon this gas and acquires 

 a state of freshness and activity which permits it to live afterwards out of 

 contact with air, and to act as a ferment. Thus, in ordinary brewery prac- 

 tice, we find the yeast already formed in abundance even before the earliest 

 signs of external fermentation have made their appearance. In this first 

 phase of its existence yeast lives chiefly like an ordinary fungus." 



But as soon as the process of fermentation ends, and some- 

 times even before the whole of the sugar has been converted, 

 the yeast if originally not sufficiently rejuvenated gradually 

 loses its power of living by deriving its oxygen from its 

 nutrient medium, and the cells revert to their original con- 

 dition of senescence. They become dormant, and until again 

 supplied with oxygen can bring about no further fermenta- 

 tion. It is for this reason that to obtain the best fermenting 

 yeasts, cultivations must always be made in the presence of 

 free oxygen, and although this was done before Pasteur 

 explained the reasons for its necessity, it is now carried on in 

 a much more systematic manner. Brewers knew perfectly 

 well that they had to clear out their vats from time to time, 

 not only to get rid of foreign organisms, but also that they 

 might obtain oxidation or more perfect aeration in the early 

 stages of the process of fermentation. 



It is very interesting to note that Pasteur, although laying 

 such stress on the connection between vital processes in the 

 cell and the process of fermentation, at the same time holds a 

 very definite opinion that the vegetative activities of the yeast- 

 cells are independent of their characters as ferments ; and 

 he maintains that the presence of oxygen, although increas- 

 ing the activity of the cells as regards their subsequent 

 power of setting up a rapid fermentation, may, nevertheless 

 during its presence, give rise to weakening of their 

 fermenting action. He says : 



" Free oxygen imparts to the yeast an increasing vital activity, but at the 

 same time qua oxygen, impairs rapidly its power as yeast inasmuch as under 



