340 MICRO-ORGANISMS AND FERMENTATION. 



and that great progress .may be made in this field, and con- 

 siderable advantages may be derived by the use of a single 

 selected type. A further objection raised was that it is im- 

 possible by means of a single species to obtain a stable secon- 

 dary fermentation, a wrong assumption previously made 

 regarding a low-fermentation yeast. Van Laer strongly em- 

 phasised this view, and while he freely admitted that low yeast 

 types exist, capable of carrying through a normal secondary 

 fermentation, he believed that the author was wrong in ascrib- 

 ing the same properties to top yeasts. Notwithstanding the 

 practical results attained by exact experiments carried out with 

 selected types, even when due allowance was made for the 

 special English conditions referred to by van Laer, and in spite- 

 of the fact that no exact proof was forthcoming to warrant the 

 opposite view, the author's experience was ignored, and van Laer 

 prepared mixtures of top-yeast species which were distributed 

 for use in breweries. They were designed to satisfy practical 

 demands, the intention being that one species should carry 

 through the primary fermentation, the other the secondary 

 fermentation. It is true that the possibility is not excluded 

 that such a composite yeast could be prepared, but even when 

 van Laer's preparations gave good results in practice it could 

 not be proved that it was due to the activity of the composite 

 yeast as such. It must first be demonstrated that this new 

 yeast really reacted as a composite yeast i.e., that the different 

 constituents are really capable of acting together. In con- 

 junction with J. C. Holm, the author investigated many of 

 the preparations distributed in the industry, and it was shown 

 that even during the primary fermentation one of the species 

 very strongly predominated, whilst in the secondary fermenta- 

 tion the other species disappeared. Thus the problem of 

 preparing a truly composite yeast had not been solved. The 

 experience of subsequent years has always confirmed the cor- 

 rectness of the first results, even in fermentations carried out 

 on the English system. It is possible in both top and bottom 

 fermentations to carry out the whole primary and secondary 

 fermentation with a single selected species. 



Quite recently the question of applying two species in 

 English top-fermentation has been re-opened. It has been 



