144 WHAT IS THE PURE YEAST OF PASTEUR? 



the beer had the disagreeable bitter taste and unpleasant odour which 

 are produced by the disease yeast Sacch. Pastorianus I. Even from 

 this it might be concluded that it did not contain a pure culture of the 

 species of brewery yeast originally introduced. The problem now was 

 to more closely investigate what was the composition of the yeast ; for 

 this purpose a dilution and examination of the cells was carried out in 

 accordance with the method described in the first experiment. 



The result arrived at was that only a single species was 

 present, namely ', the disease producing Sacch. Pastorianus I. ; 

 this species alone had survived the treatment in the sugar 

 solution. 



Experiment VI. Whilst in the previous experiments equal quantities 

 of the different yeasts were taken, in this case the "disease" producing 

 yeasts were mixed with the culture yeasts in the proportion of one to 

 five. The brewery yeast present in each flask was thus in positive 

 excess at the very beginning. The experiment was conducted with the 

 following five flasks : 



In A, Sacch. cerevisice /., Sacch. Pastorianus I. 



In B, a brewery bottom yeast, Sacch. Pastorianus I. 



In C, Carlsberg bottom yeast No. 2., Sacch. Pastorianus I. 



In D, Sacch. cerevisice I., Sacch. ellipsoideus II. 



In E, a brewery bottom yeast, Sacch. ellipsoideus II. 



The sugar solution in this case contained only 3 8 per cent, of tartaric 

 acid. In other respects the conditions were the same as in the fifth 

 experiment. 



It was found that the yeasts contained in the flask D, namely, Sacch. 

 cerevisicz I. and Sacch. ellipsoideus 77., had perished after they had been 

 subjected for ten days to the described treatment in the sugar solution ; 

 on the other hand, after eight days' treatment, they were still alive. In 

 all the other flasks some, at least, of the species withstood the treatment, 

 not only for eight but for ten days. The yeasts were diluted, and in the 

 case of each flask a very considerable number (in some cases as many 

 as eighty) of cultures were made from the cells in accordance with the 

 methods described above ; the following results were obtained : 



A. Both species sown were alive, but whilst at the commencement 

 of the experiment the cells of Sacch. cerevisice I. were five times as 

 numerous as those of Sacch. Pastorianus /., the ratio was now completely 

 changed. The disease yeast predominated, whilst the brewery top-yeast 

 had become suppressed to such an extent that it could only be detected 

 by special cultivation in wort at 37-38 C., a temperature which, under 

 the given conditions, is still favourable to Sacch. ceremsics /., but not 

 to Sacch. Pastorianus I. 



B. Sacch. Pastorianus I. only was found, and not a trace of the 

 brewery bottom yeast was detected. 



