SYSTEMATICALLY SELECTED YEASTS. 23 



mentation. We are in a position to prepare such mixtures of 

 different species of brewery yeasts. In some few breweries 

 two species of culture yeast are employed separately, and the 

 resulting beers are then mixed at the end of the primary 

 fermentation. / can, however, by no means recommend such 

 mixtures, not even if they are guaranteed by trustworthy 

 analyses. The ideal in every manufacture is to conduct it 

 in as simple and certain a manner as possible, and it is a 

 recognised fact that it is easier to regulate one than several 

 factors, even when they are all well known. In several cases 

 the mixture will even give rise to a less satisfactory fermenta- 

 tion. Finally, it must be remembered that the ratio originally 

 existing between the species in the mixture will be changed 

 after a short time. Such a yeast will therefore give varying 

 results, wiiilst this will not be the case when a pure yeast of 

 single race is employed. 



The method given in the first German edition of this 

 book, which appeared in 1888, is still followed in the different 

 countries. The pure yeast propagating apparatus, devised by 

 Captain Kiihle and myself, has, however, acquired no small 

 practical importance in recent years. By its means we are in 

 fact enabled to introduce at short intervals large quantities of 

 pure cultivated yeast into the brewery. A detailed descrip- 

 tion of this apparatus will be given in the next section. An 

 improvement which is also closely related to the employment 

 of my pure cultivated yeasts was introduced into Old Carls- 

 berg in 1885, by the late J. C. Jacobsen. This consisted of 

 an apparatus in which the boiling hot wort from the copper 

 could be cooled and aerated without coming into contact with 

 the micro-organisms present in atmospheric dust. The idea 

 of employing closed sterilised vessels in the place of the open 

 coolers originated, as already stated, with Pasteur ; sub- 

 sequently such an apparatus was constructed by Velten ; 

 hitherto, however, it acquired no really practical importance, 

 because what was most essential was wanting. Thus, of what 



