74 LOUIS PASTEUR 



tions. "The purity of a ferment," observes Pas- 

 teur, "its homogeneity, its free development with- 

 out any restraint with the aid of a nutriment appro- 

 priate to its peculiar nature form one of the essen- 

 tial conditions of good fermentation." One may 

 obtain mixed fermentations of a given material at 

 will by sowing in it diverse kinds of organisms. 



Lactic acid fermentation, then, is not a process 

 occurring independently of minute organisms, as 

 stated by Liebig. Pasteur had discovered that it 

 was due not to yeast plants, but to bacteria which 

 had not been noticed by previous observers. A 

 knowledge of the cause of this kind of fermentation 

 gave the key to its control. The memoir on lactic 

 acid fermentation showed the conditions under 

 which the small bacteria concerned could best be 

 grown and kept reasonably free from contamina- 

 tion. The mind of Pasteur was naturally led on 

 to the general conclusion that the different kinds of 

 fermentation in nature have each its own peculiar 

 organisms whose nutrient needs and metabolic 

 products determine the characteristic chemical sub- 

 stances arising from their activity. 



The illuminating investigation of lactic acid fer- 

 mentation was followed two years later by a 

 memoir on alcoholic fermentation. Pasteur showed 



