298 science progress. 



If the so-called secondary yeasts were present in large 

 proportion, e.g., twenty-five per cent, of the total yeast, no 

 secondary fermentation takes place (Van Laer, loc. at.). 

 In this case they are able to exert their full activity during 

 the primary fermentation, and when this is finished the 

 limit-attenuation of the mixed yeast is attained and conse- 

 quently no after-fermentation is possible. 



The above and other similar results obtained by Van 

 Laer conclusively proved that in his experiments the 

 secondary fermentation was brought about by the so-called 

 secondary yeasts. The fact that the latter types are able 

 to carry on the fermentation of cashed beer and thus take 

 part in the secondary fermentation has, however, long been 

 known to brewers, and the fact that in the above experi- 

 ments the secondary fermentation is brought about solely 

 by these lower types merely proves that under the condi- 

 tions adopted by Van Laer, his principal yeast (E 2 ) had 

 reached its limit-attenuation with the completion of the 

 primary fermentation. 



Interesting as these laboratory experiments are they 

 cannot be regarded as proving that the same holds good 

 in practice. Ordinary brewers' yeast certainly contains 

 wild or secondary types and these are seen to develop 

 during the secondary fermentation, but that they are the 

 sole cause of the latter and that certain of the principal 

 yeasts do not also play a prominent part is not proved by 

 Van Laer's experiment. 



The chief objection which has been raised to the employ- 

 ment of composite yeasts is that it is practically impossible 

 to maintain that constancy of composition which is neces- 

 sary in order to ensure uniform results in the brewery. 

 In a mixed yeast one variety is liable to develop with 

 greater vigour than another, so that after a few fermenta- 

 tions the yeast will have a different composition and conse- 

 quently different properties from those which it originally 

 possessed. According to Van Laer this is by no means 

 necessarily the case, and constancy of composition may be 

 maintained, owing to the higher fermentative energy of one 

 species over another being counterbalanced by a more 



