SYSTEMATICALLY SELECTED YEASTS. 27 



preparation of the pure culture. The method for this has 

 already been described (see p. 5).* 



In the foregoing, the case was mentioned in which a single 

 species of yeast preponderated in the mixture with which we 

 started, and effected, practically, the whole fermentation, and 

 determined the character of the product. Cases also occur, 

 however, in breweries where a single species does not pre- 

 dominate, but where there are several species which operate 

 together to produce the result. Under such conditions, a pure 

 culture of one of the species will, as a rule, give a product 

 having very different properties from that yielded by the 

 mixture. The different species contained in the mixture can 

 certainly be separated from each other, and it is, therefore, 

 possible at any time to make up the mixture, and with some 

 difficulty even to introduce the different species in the desired 

 proportions ; but as we are not able to regulate these for any 

 length of time, those species which are most vigorous will 

 gain the upper hand in the commencing struggle, and will 

 thus give rise to irregularities. Under these conditions a 

 constant product can only be attained with great difficulty, 

 and pure culture has, therefore, no practical value in this case. 

 If, however, we wished to attempt this, each portion of the 

 pitching yeast would have to be prepared separately, or the 

 fermentation with each species of yeast would have to be 

 carried out separately, and the resulting beers mixed at the 

 end of the primary fermentation. With the methods of 

 working now in vogue in breweries, I cannot regard the 

 adoption of such a complicated method as practical, and I 

 therefore advised against it even in the first German edition 

 (1888) of this book. 



Amongst the culture yeasts separated from such a mix- 

 ture, several species may often be found which are of use in 



* The flasks and apparatus which are used in my laboratory can be obtained 

 from F. C. Jacob, Hauserplads 14, Copenhagen, and from Dr. Rohrbeck. 

 Karlstrasse 24, Berlin. 



