46 MICRO-ORGANISMS AND FERMENTATION. 



to which a small amount of tartaric acid has been added. The 

 object of the method is to free the yeast from any disease 

 germs with which it may be infected. Hansen's investiga- 

 tions have, however, proved that, even if the bacteria are 

 suppressed or checked by this treatment, simultaneously the 

 wild yeast, and among them those productive of diseases in 

 beer, will develop abundantly, and in many cases the culture 

 yeast, which it was intended to purify, is entirely suppressed. 

 Even if there is primarily only a trace of the wild yeasts 

 or " disease " yeasts, these are apt to develop to such an 

 extent by Pasteur's treatment that they may eventually form 

 the predominant part of the yeast-mass. 



The use of hydrofluoric acid or its compounds, such as 

 ammonium fluoride, for the purpose of purifying an impure 

 yeast brewers' or distillers' yeast as proposed by Eflront, 

 is liable to lead to the same dangers as the use of tartaric 

 acid. Methodical experiments made by Holm and the author 

 have shown that by treating impure yeast according to Eff rent's 

 process, the growth of wild yeast and Mycoderma species is 

 forced more than that of the culture yeast ; they have also 

 shown that such a dangerous species as Bacterium aceti is in 

 many cases not suppressed at all by the treatment in question, 

 but, on the contrary, multiplies more rapidly in presence of 

 hydrofluoric acid or fluorides. 



If, now, we ask, whether it is advisable to employ any 

 of the various methods mentioned above for the purification 

 of an unknown and impure yeast-mass, the answer must be 

 in the negative ; and this will be the case whether the culture 

 is intended for purely scientific or for industrial purposes, 

 for the danger will never be excluded that in prolonged cultiva- 

 tion other species than the one desired will gain the supremacy. 

 The starting point being uncertain, it necessarily follows that 

 the result must be so too. In fact, all such methods must 

 now be regarded as antiquated, and as complete failures. 

 Nevertheless, they may possibly be used in isolated cases 

 before proceeding to the preparation of a pure culture. In 

 this way it is possible by suitable treatment of the impure 

 material to secure a preponderance of the group of the 

 desired species in the mixture, so that a pure cultivation 



