2 9 o SCIENCE PROGRESS. 



the propagation on a commercial scale of his pure yeast 

 cultures. 



Hansen's system consists in the employment of a single 

 and carefully selected species of yeast, the absolute purity 

 of which is ensured by the fact that it is grown from a 

 single cell. 



Before a pure yeast can be safely introduced on a large 

 scale into the brewery it is, however, necessary to make 

 preliminary trials on a small scale, for it is found that many 

 of the innumerable representatives of Saccharomyces cere- 

 visics differ widely in their properties and that a variety 

 which gives good results in one brewery may prove unsatis- 

 factory in another. This fact alone explains many of the 

 unsatisfactory results, which were at first obtained with 

 pure yeast, and in consequence also much of the opposition 

 which was brought to bear on the Hansen system. 



The Carlsberg Brewery, in which Hansen made his 

 experiments, is a low fermentation (lager beer) brewery, 

 and it is therefore natural that the first extension of his 

 system of pure yeast should have been in breweries of the 

 same kind. The number of lawr beer breweries in which 

 pure yeast of single race is employed has steadily increased 

 from year to year and is at the present time very consider- 

 able ; and the success of the application of such yeast to 

 low fermentation brewing is now very generally acknow- 

 ledged to be an accomplished fact. 



The main advantages accruing from the employment of 

 pure yeast of single race are (i) the possibility of effectually 

 guarding against all the troubles which arise from the pre- 

 sence of foreign organisms, (2) that the brewer can always 

 depend upon having a yeast constant in its properties and 

 upon obtaining a fresh supply of the same yeast as soon as 

 his store yeast shows signs of contamination. 



It is obvious that the above advantages should apply 

 with equal force to the high fermentation system of brewing 

 adopted in this country, and to a considerable extent also in 

 some continental countries, provided at least that a single 

 race of yeast is in this case also capable of exercising the 

 necessary functions of ordinary brewers' yeast, which is 



