FERMENTATION. 127 



added to the exhausted yeast, the conditions of nutrition 

 are rendered so favourable that the yeast cells again acquire 

 in a most marked degree their original and characteristic 

 power of absorbing oxygen, of vegetative proliferation, and 

 of setting free alcohol and carbonic acid gas. 



We may briefly give in his own words the position that Schiitzenberger 

 holds, so that it may be compared with Pasteur's position, previously stated. 

 "The cell ferment is not developed in the absence of free oxygen, even in 

 its most favourable medium, the must of grapes. The ferment, in process of 

 development, continues to increase in suitable media, even in the absence 

 of all trace of oxygen, as M. Pasteur had already shown. The contrary 

 assertions of Brefeld are erroneous. M. Pasteur's theory, according to 

 which yeast, in the absence of air, takes from the sugar the oxygen neces- 

 sary for its development, is not well founded ; in fact, this development 

 stops long before the greater part of the sugar is decomposed. Is it from 

 the albuminoid matter that the ferment takes its oxygen in the absence of 

 air ? Yeast sets up alcoholic fermentation in a solution of pure sugar in 

 the absence of all trace of oxygen, but without developing. This is 

 contrary to the affirmation of M. Pasteur that fermentation is bound up 

 with the organization of the yeast, or is a phenomenon correlative to the 

 vital activity of the cells." 



We find that in nature there is, in protoplasm, not only an 

 extreme adaptability to surrounding circumstances, but an 

 attempt to utilize, as far as possible, the whole of the 

 energy set free from cells. We find that in this matter 

 the protoplasm of yeast cells differs in no essential respect 

 from other kinds of protoplasm, and we have already seen 

 that in the process of fermentation of saccharose there is a 

 preliminary change brought about by the soluble products 

 of the yeast-cells (invertin), by which we obtain dextrose and 

 levulose, both of which materials may, under certain con- 

 ditions, be split up into alcohol and carbonic acid gas. 



It may now, further, be noticed that if the fermentation 

 be stopped at a comparatively early stage, there is found in 

 the fermenting solution a larger quantity of levulose than 

 of dextrose ; *>., the dextrose is more readily converted into 

 the characteristic products of fermentation than is the levu- 

 lose. It has also been proved that if the yeast-cells be heated 

 to a temperature of about 60 C., they are destroyed, as is 

 evidenced by the fact that they are no longer able to 

 multiply, and they never give further evidence of vitality. 

 But there is left in the fluid a substance known as enzyme 

 (ivZvfirj^ leaven yeast), which, added to the mixture of dextrose 

 and levulose, does not affect the levulose in the slightest. 



