GROWTH OF YEAST 183 



plained by the fact that before reproduction takes place, the 

 cells of the seed yeast absorb and fix oxygen which renders 

 possible this limited reproduction in anaerobic conditions. 

 This absorption of oxygen, which is a linear function of time, 

 takes place with great rapidity ; thus in one instance it was 

 found that -3 gram of pressed yeast per 100 c.c. of liquid com- 

 pletely absorbed the oxygen in two and a half hours. Horace 

 Brown concludes that the power of reproduction is impressed 

 in the cell at the very outset by the absorbed oxygen and that a 

 quantitative relation exists between this absorbed oxygen and 

 the number of units which the initial yeast cell can finally 

 gemmate. The action of the oxygen is one of induction and, 

 according to Horace Brown, all the known facts can be 

 explained on the assumption that the available oxygen is 

 equally divided between the initial cells and the consequent 

 variation in the oxygen charge which these cells must receive 

 when the ratio of the seed yeast to the available oxygen 

 varies. 



The amount of oxygen in aerated wort may be very small, 

 but its effect may be very great : thus 1 c.c. of oxygen in 

 aerated wort brings about a growth sixty times greater than 

 the same amount of oxygen in non-aerated wort.* Slator 

 is impressed by the importance of carbon dioxide as a con- 

 ditioning factor in the growth of yeast, and he considers that 

 the influence of this gas is much greater than is generally 

 supposed, and that some of the observed effects generally 

 ascribed to the direct influence of oxygen may be due to its 

 indirect action in lessening the supersaturation of the wort 

 with carbon dioxide. For measurements of the rate of growth 

 in wort and in wort saturated with carbon dioxide show much 

 retardation, possibly due to the carbon dioxide rather than 

 to the lack of oxygen. These measurements are confirmed 

 by controlled experiments in which the carbon dioxide was 

 the limiting factor. In fact, a correlation can be made out 

 between the crop of cells and the concentration of carbon 

 dioxide in the medium : — 



* Slator : '* Journ. Chem. Soc," 1921, 119. "5- 



