134 MICRO-ORGANISMS AND FERMENTATION. 



alcoholic fermentation in the true sense of the term." The 

 researches of LECHARTIER and BELLAMY, which were sub- 

 sequently extended by PASTEUR, have shown that when grapes, 

 oranges, and other fruits on which no yeast-cells were present, 

 were preserved in vessels filled with carbon dioxide, a develop- 

 ment of alcohol and carbon dioxide took place. " The 

 fermentative character is consequently not a condition of the 

 existence of yeast ; the fermentative power is not peculiar to 

 cells of a special nature, is no fixed structural characteristic, 

 but is a property which is dependent upon external conditions 

 and upon the mode of nutrition of the organism " (" Etudes sur 

 labiere," page 258). 



" In short, fermentation is a very general phenomenon. It 

 is life without air, life without free oxygen ; or more generally 

 still, it is the necessary result of chemical work carried out on 

 a fermentable substance, which by its decomposition is capable 

 of evolving heat ; the heat necessary to effect this work being 

 borrowed from a part of that which is liberated by the decom- 

 position of the fermentable substance. The class of fermenta- 

 tions properly so-called is limited by the small number of 

 substances which are capable of evolving heat on decomposi- 

 tion, and which will serve as nourishment for the lower 

 organisms when the admission of air is excluded " (" Etudes 

 sur la biere," page 261). This is, briefly, PASTEUR'S famous 

 theory of fermentation. 



Fermentations dependent upon oxidation such as the 

 acetic acid fermentation, which, as PASTEUR himself had 

 observed, requires an abundant supply of air were con- 

 sequently not regarded by him as true fermentations. It is 

 seen, moreover, that he does not strictly adhere to his defini- 

 tion, in that he emphasises the fact that yeast also possesses 

 fermentative properties when air is present, although to a less 

 degree than when oxygen is excluded. The correctness of this 

 under certain conditions has been confirmed in the case of 

 bottom yeast by PEDERSEN (1878), and by HANSEN (1879), 

 who came to the conclusion that the amount of substance in 

 a wort which a definite quantity of yeast can convert into 

 alcohol and carbonic acid is smaller when the liquid is aerated 

 during fermentation than when no aeration takes place. ED. 



