WHAT IS THE PURE YEAST OF PASTEUR? 139 



Experiment /.A 10 per cent, aqueous solution of cane sugar, to 

 which one-twentieth per cent, of tartaric acid had been added, was intro- 

 duced into two-necked Pasteur flasks and sterilised. After cooling, these 

 flasks were inoculated with absolutely pure cultures of the yeasts men- 

 tioned below.* The cultures employed were obtained from a ten days' 

 growth in wort cultivated at the ordinary room-temperature. A fairly 

 large quantity of yeast was introduced into each flask, and, as far as 

 possible, an equal amount of each species. 



In A, Sacch. cerevisia /., Sacch. Pastorianus /., Sacch. Pastorianus 

 III., Sacch. ellipsoideus II. 



In B, Carlsberg bottom yeast No. r., Sacch. Pastorianus /., Sacch. 

 Pastorianus III., Sacch. ellipsoideus II. 



In C, Carlsberg bottom yeast No. i, Carlsberg bottom yeast, No. 2, 

 Sacch. Pastorianus /., Sacch. Pastorianus ///., Sacch. ellipsoideus II. 



The flasks were allowed to stand for a month at the ordinary room- 

 temperature ; they were then shaken up and small average samples from 

 each were introduced into fresh flasks containing the same liquid ; a 

 month later a third series of flasks were similarly inoculated from the 

 second series, and these were again allowed to stand for a month. By 

 the inoculation of such average samples, the species which preponderate 

 are brought under still more favourable conditions, and thus the develop- 

 ment of a pure culture was favoured in this way. 



After this process of cultivation had been carried on during three 

 months, the problem was to bring about a new growth of any cells which 

 were still alive, and thus to determine which species were still living in each 

 flask. For this purpose average samples from each flask were introduced 

 into two other flasks, one of which contained beer wort, and the other a 

 10 per cent, solution of dextrose in yeast-water. These flasks were set 

 aside at a temperature of 25 C., and as soon as a growth had developed, 

 average samples were taken for fractional cultivation in gelatine to which, 

 in the one case wort and in the other the solution of dextrose in yeast- 

 water had been added. The flasks from which these samples had been 

 withdrawn were then left at the ordinary room-temperature until the 

 primary fermentation was finished, and average samples were then again 

 withdrawn for fractional cultivation as above. Similar cultivations were 

 likewise prepared from average samples taken direct from the growths 

 which had been cultivated for three months in the sugar solution in the 

 manner described. The experiments were thus made with the respective 

 yeast growths in three phases of development. The gelatine plates in 

 which the yeast cells had been sown were exposed to a temperature of 

 25 C. until numerous colonies had developed, and from these a large 



* These and the species mentioned later are described in different places in 

 my ' Recherches sur la physiologic et la morphologic des ferments alcooliques.' 

 They will also be found in Jorgensen's 'Micro-organisms and Fermentation,' 

 New edition, London, 1893 ; and in Zopf's Handbuch 'Die Pilze,' Breslau, 1890. 



