Broivn . — Some Studies on Yeast . 207 
to completely de-aerated wort, the intermediate members of the series 
being mixtures of the two in known proportions. 
In filling the flask with completely de-aerated liquid special precautions 
had to be taken in introducing the seed-yeast. This was spread on a thin 
cover-slip and dropped into the empty flask, a current of hydrogen being 
then passed through the flask for some time before it was filled with the 
oxygen-free wort. 
All the flasks were then allowed to ferment slowly at the same tem- 
perature for ten days or so with their outlet tubes dipping under mercury. 
Since the flasks were quite full it is manifest that as fermentation pro- 
Fig. 4. Curve showing influence on yeast-cell reproduction of varying amounts of dissolved 
oxygen in the culture-liquid (malt-wort). The cultures were made with definite mixtures of fully 
aerated worts and of wort fully deprived of oxygen. Seed-yeast = i cell per unit volume. The 
figures in small circles denote percentages of alcohol found at the termination of each experiment. 
ceeded there would be a slight loss of liquid from the outlet tubes and the 
upper part of the flasks in the first stages of fermentation. It was therefore 
more convenient to determine the cell-reproduction by the indirect method 
of nitrogen loss than by cell-counting. The nitrogen coefficient of the seed- 
yeast was therefore ascertained, and also the number of cells per unit 
volume given by mixing a definite weight of it with water and making up 
to 100 c.c. The estimation of the nitrogen in the wort before and after 
fermentation then completed all the necessary data. 
Fig. 4 shows the result of plotting the maximal cell-increases attained 
