206 MICRO-ORGANISMS AND FERMENTATION. 



The experiments on aeration described in " Etudes sur 

 la biere " gave rise to extensive series of investigations which 

 yielded valuable information on the fermentative and repro- 

 ductive power of yeast in the presence of varying amounts 

 of air. This relation plays an important part in the 

 distillery and in pressed-yeast works. No results, however, 

 have as yet been obtained, which can be directly applied in 

 practice. 



The reason why the method proposed by Pasteur for the 

 purification of yeast has acquired no real importance for 

 practical purposes, has been already stated (page 24). 



3. With Hansen's investigations on the alcoholic ferments 

 there began, as Aubry says, a new era in the history of the 

 fermentation industries. In the year 1883, Hansen demon- 

 strated that the universally dreaded yeast-turbidity, and like- 

 wise the disagreeable changes in taste and odour, in fact some 

 of the commonest and worst diseases of beer, are not caused 

 by bacteria, by the water, by the malt, by the particular 

 method of brewing, &c., as was then commonly believed, 

 but that these diseases must be attributed to the yeast itself ; 

 for in such cases the pit ching-y east contains, in addition to 

 the cultivated species, other Saccharomycetes, which act as 

 disease germs (Sacch. Pastorianus L and III., Sacch. 

 ellipsoideus //.). A basis was thus formed for the neiv 

 system. 



He subsequently showed that the name Saccharomyces 

 cerevisice embraces many different both bottom- and top- 

 fermentation races or species, which communicate very 

 different characters to beer. 



As the rational result of these scientific investigations he 

 completed the third link of his new system his method for 

 the pure cidtivation of yeast. If it were possible to free the 

 impure yeast mass from wild yeasts as well as from bacteria 

 and mould-fungi, we should still not attain all we desire ; for 

 if the purified yeast contains several species of Saccharo- 

 myces cerevisice, we are still dealing with a mixture which 



