254 MICRO-ORGANISMS AND FERMENTATION. 



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. 



3. With HANSEN'S investigations on the alcoholic ferments 

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

 fermentation industries. His earliest publications on this 

 subject date back to 1879. In the year 1883 he demon- 

 strated that the universally dreaded yeast-turbidity and 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, etc., as was then commonly believed, but that 

 these diseases must be attributed to the yeast itself; for in such 

 cases the pitching-yeast contains, in addition to the cultivated 

 species, other Saccharomycetes, which act as disease germs (Sacch. 

 Pastorianus I. and III., Sacch. ellipsoideus II.). A basis was 

 thus acquired for the new system. 



He subsequently showed that the name Saccharomyces 

 cerevisice embraces many different races or species (both bottom- 

 and top-fermentation), which communicate very different char- 

 acters to beer. 



As the rational result of these scientific investigations he 

 completed the cycle of his new system by his method for 

 the pure cultivation 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 Saccharomyces 

 cerevisice, we are still dealing with a mixture which is just as 

 uncertain as before the purification, and, moreover, the composi- 

 tion of such a yeast-mass is always liable to change during 

 fermentation. In fact, HANSEN has shown in recent investiga- 

 tions that cases occur in which two yeasts, each of which by 



