THEORIES OF FERMENTATION. . 241 



that it may continue to develop as a living organism, it splits 

 up the sugar, and the residue of the oxygen, as well as the 

 carbon, constitute new compounds viz., the fermentation 

 products, alcohol, carbon dioxide, etc. At the same time, 

 Pasteur emphasised the idea that for each kind of fermentation 

 alcohol, acetic acid, butyric acid, etc. a specific kind of 

 organism occurs. 



It will be seen that Pasteur's theory consists both of a 

 biological and a chemical portion. The yeast cells fulfil 

 their normal existence with generous access of atmospheric 

 oxygen, and under these conditions develop, according to 

 his view, most vigorously, and prepare themselves in the best 

 possible way to continue their existence without air, and this 

 is the necessary condition for their existence as alcohol formers 

 i.e., decomposers of sugar. The first statement, which 

 clearly explained an important biological problem, still holds 

 good ; the second, which endeavoured to supply an answer 

 to the requirements of the chemical process, can no longer be 

 accepted. 



That Pasteur did not apply his definition in the narrowest 

 sense of the word is shown by the fact that he himself em- 

 phasised the fact that yeast can exercise fermentative power 

 in presence of a limited supply of air as well as in its absence. 

 This was established under certain conditions for low ferment- 

 ation beer-yeasts by Pedersen in 1878, and Hansen in 1879. 

 They arrived at the result that the quantity of dry substance 

 in beer-wort which a given quantity of yeast can convert into 

 alcohol, carbon dioxide, etc., is smaller when the liquid is 

 aerated during fermentation than when it is not. A similar 

 result was obtained by Eduard Buchner in 1885 in his experi- 

 ments on bacteria. 



Hansen so arranged his experiments that the cells during 

 aeration were in constant motion, carried hither and thither 

 by the vigorous blast of air. As they, nevertheless, continued 

 to give a distinct alcoholic fermentation, there can be no 

 doubt that this is not determined by life without air. 



Nageli, in 1879, in his Theory of Fermentation, proved that 

 access of oxygen is always favourable to alcoholic fermentation 

 in a sugar solution, if no nutritive material is present, and 



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