296 SCIENCE PROGRESS. 



where the secondary fermentation is dependent solely upon 

 the diastase of the hops, the result is really very far from 

 satisfactory, and that neither the diastase, of the hops nor 

 the wild yeasts which they also contain can be depended 

 upon to give more than a very feeble and unsatisfactory 

 after-fermentation. 



It has been shown by Morris and Wells 1 that when a 

 wort is fermented by a single race of yeast " a certain definite 

 limit-attenuation " is attainable and when this is reached the 

 fermentation will cease. The limit-attenuation varies with 

 different races of yeast. When a mixture of yeasts is em- 

 ployed a limit-attenuation is also attained and in this case 

 "corresponds with the limit-attenuation of the lowest type" 

 present in the mixture (Van Laer, loc. cit., p. 78). 



It is generally acknowledged that there are many yeasts 

 which are unable to bring about any secondary fermenta- 

 tion. Such yeasts ferment the whole of the carbohydrate 

 matter which they are able to decompose during the 

 primary fermentation ; the limit-attenuation is thus attained, 

 and in the absence of other yeasts capable of carrying the 

 fermentation further, or of diastase, which gives rise to the 

 formation of maltose and thus renders it possible for the 

 original yeast to recommence fermentation, the beer will 

 remain flat. On the other hand it is also acknowledged 

 that races exist which readily ferment the free maltose of 

 a wort but only slozvly decompose certain other constituents 

 (Van Laer, loc. cit.). This latter property is necessarily of 

 importance in the selection of a suitable race of yeast which 

 has to effect both primary and secondary fermentation, and 

 although, as shown by Van Laer, it may not be sufficient to 

 ensure a satisfactory result in other respects, it affords an 

 explanation of the fact that it is possible to obtain a secon- 

 dary or cask fermentation with pure yeast of single race. 

 In the case of such yeasts, the limit-attenuation can, at any 

 rate in many cases, be rapidly reached if a high tempera- 

 ture {e.g., 30°C.) is maintained during fermentation, but under 

 the normal conditions obtaining in a brewery the limit is, at 



1 Tra?is. Inst. Brewing, vol. v., p. 133 ; vol. vii., p. 86. 



