116 MICRO-ORGANISMS AND FERMENTATION. 



Saccharomyces cerevisice gave in the brewery products 

 having different characters. Starting from this, Hansen 

 elaborated his method, by means of which a pitching-yeast, 

 consisting of only one species, is employed. After some 

 resistance this system has been recognised and introduced 

 into practice in all countries where the brewing industry 

 is carried on. Velten of Marseilles, who formerly worked 

 with Pasteur, has, however, recently attacked this system, 

 the mistake of which he deems to be that Hansen' s 

 yeast consists only of one species. He considers it an advan- 

 tage in Pasteur's purified yeast that the latter consists of 

 several different kinds, and regards this combination of various 

 species as necessary in order that the beer may acquire the 

 desired taste and bouquet. Hansen's latest investigations 

 (see Chapter I., 7) show how completely this doctrine breaks 

 down. Hansen proved by experiment that when yeast is 

 treated with tartaric acid, according to Pasteur's method, the 

 conditions are so favourable for the development of the 

 yeasts which produce disease, that finally the culture-yeast 

 becomes completely suppressed. Pasteur consequently greeted 

 Hansen's method as an advance, in that he wrote, " Hansen 

 was tfye first to perceive that beer-yeast should be pure, and 

 not only as regards microbes and disease-ferments in the 

 narrower sense, but that it should also be free from the cells 

 of wild yeasts." 



As, however, Pasteur's work always retains its technical 

 importance, on account of the force with which the influence 

 of bacteria in the fermentation industries is asserted, so it also 

 possesses great theoretical interest, especially from the new 

 theory of fermentation enunciated therein, and which at the 

 time rightly attracted much attention. 



Contrary to Brefeld, who asserted that yeast could not 

 multiply without free oxygen, and Traube, who indeed granted 

 that yeast was able to develop without free oxygen, but main- 

 tained that it then required for its cell-formation the soluble 

 albuminoids in the liquid, Pasteur stated that the organisms 



