RESULTS OF SCIENTIFIC RESEARCH IN PRACTICE. 207 



is just as uncertain as before the purification, and, in addi- 

 tion, the composition of such a yeast-mass is always liable 

 to change during fermentation. In fact, Hansen has shown 

 in recent investigations that cases occur in which two yeasts, 

 each of which by itself gives a faultless product, will when 

 mixed give rise to disease in the beer. He made these 

 experiments with the two species of Carlsberg bottom-yeast 

 No. 1 and No. 2 (see page 184) ; in one set of experiments 

 the pitching-yeast consisted chiefly of No. 1 with a small 

 admixture of No. 2, and in the other set, the reverse was 

 the case. It was found that in all cases the small quantity 

 of. the admixed yeast, whether No. 1 or No. 2, made the 

 beer less stable as regards yeast turbidity, than when the 

 chief constituent of the pitching-yeast was employed alone. 

 Thus the two cultivated yeasts under these conditions behaved 

 in such a manner as to produce effects similar to those brought 

 about by the wild yeasts (Sacch. Pastorianus III. and Sacch. 

 ellipsoideus II.). We can therefore only obtain true uni- 

 formity in working when a suitable species has been 

 obtained from the yeast-mass by systematic selection, and 

 cultivated by itself (compare pages 26-32). 



The systematic studies which Hansen has carried on for 

 many years on the constancy of the characters of different 

 species of yeast, have proved that, under the conditions of 

 the brewery, they only undergo slight changes, which are 

 of no importance in practice, and this result has been con- 

 firmed by various investigators. 



On the other hand, he found that, when the conditions 

 of the life of the yeast are disturbed by a systematic and 

 more vigorous treatment, it was possible to produce 

 varieties (see page 151), which remained more or less 

 constant in their properties, and even to produce new 

 species. As a result of these investigations Hansen obtained 

 useful varieties of some brewery yeasts. 



The biological and physiological characteristics of the 

 species discovered by Hansen led him also to a method 



