PURE CULTURES IN PRACTICE. 415 



liquid for yeast activity. Hansen experimentally proved that 

 some of the most dangerous diseases of beer are caused by 

 wild yeasts.* 



When the absolutely pure culture developed in flasks in 

 the laboratory has attained certain dimensions e.g., 1 to 

 2 litres of fluid yeast it is ready for practical application. 

 In the vast majority of cases the pure culture is further de- 

 veloped in a couple of small vats in which successive quantities 

 of the nutritive medium are added in each case as soon as a 

 vigorous development has taken place e.g., from 5-50-100- 

 300 litres or from 5-50-500 litres, etc., depending on whether 

 the yeast is a low- or a high-fermentation species. As a rule, 

 the chief point to observe is that the mass of cells should be 

 brought as quickly as possible to development, until the 

 requisite quantity is secured for carrying out the normal 

 fermentation on a large scale. In the preliminary stages the 

 same nutritive liquid must be used as in the large fermenta- 

 tions. It will be obvious that during such a rapid development 

 the specific character of the race of yeast will not be brought 

 out. If it is desired to observe this during the small fer- 

 mentations, they must be carried through to completion. 

 If a regular supply of absolutely pure yeast must be kept in 

 stock, it is necessary to use the pure propagating apparatus 

 designed by Hansen and by A. Kiihle. 



The apparatus (Fig. 98) consists of three chief parts and 

 the necessary connecting tubes. First, the air apparatus, 

 with air pump (A) and air holder (B), secondly, the fermenting- 

 cylinder (C), and, thirdly, the wort-cylinder (D). The air, 

 which has previously been partially purified, is pumped into 



* The " natural " selection of yeasts proposed by Delbriick must not be 

 confounded with the preparation of a single pure race. His process consists in 

 subjecting the whole impure yeast-mass to a treatment which may consist of 

 the application of a higher fermentation temperature, or pumping into a new 

 vat after the appearance of foam on the surface, or pitching with wort from 

 the first stages of fermentation, etc. A summary process of this kind will 

 always yield an uncertain result, because the impure mass contains elements 

 of very different character, and, even in the most favourable case, if by good 

 luck the disease germs are restricted, it is evidently impossible to depend on 

 securing the best-selected type of culture yeast. It is essential to isolate the 

 yeast species and then to select those which best fulfil the stated requirements. 



