SYSTEMATICALLY SELECTED YEASTS. 25 



branches of the fermentation industry, will be seen from the 

 review given in Chapter VII. 



5. THE PREPARATION OF PURE-CULTIVATED YEASTS 

 ON A LARGE SCALE. 



Preliminary Work. 



We naturally take our starting-point in the brewery itself. 

 If the yeast employed works satisfactorily as regards both 

 principal and secondary fermentations, and, what is of most 

 importance, gives a product having the desired properties, the 

 brewer naturally wishes to retain it. In many, and possibly 

 in most cases, the main bulk of such a yeast consists of 

 one culture species, with only an insignificant admixture of 

 other micro-organisms. Under these conditions the result 

 will also be essentially due to this species, and this will alone 

 give the desired product. 



Quite at the commencement of my practical studies on 

 low-fermentation yeasts and wild yeasts, I noticed that when 

 both were present in admixture in the pitching yeast, the 

 upper layers of the fermenting wort contained at the com- 

 mencement of the primary fermentation a much smaller 

 number of wild cells in proportion to culture cells than at the 

 end of the primary fermentation. Before pure yeast was 

 introduced, I had frequent opportunities of noticing this in 

 the fermenting vessels of both Old and New Carlsberg. The 

 same result was subsequently obtained from systematic 

 experiments made partly in small rounds in the fermenting 

 cellar, and partly in flasks in the laboratory with ordinary 

 wort, and with definite yeast mixtures of both Carlsberg 

 bottom yeast No. I and No. 2, and the three disease yeasts 

 Sacch. Pastorianus /., Sacch. Pastorianus ///., and Sacch. 

 ellipsoidens II. That the above frequently occurs in the 

 brewery is therefore proved, and there is much to show that 



