CONSUMPTION OF OXYGEN. 233 



without aeration, and 15. 8-fold with admission of air. In a 

 second experiment, at 13-! 5 C., the crop was 9 and 27.3 cells 

 respectively per original single cell. Other workers afterwards 

 obtained similar results. The assertion of N. VON CHUDIAKOW (I.) 

 that oxygen is indispensable for yeast reproduction only in the 

 case of imperfectly nutritive media, does not seem to rest on a 

 sufficient foundation. Other conditions being equal, the degree 

 of stimulation imparted to cell reproduction by aeration depends 

 on the race of the yeast a fact observed by G. KORFF (I.), M. 

 DELBRUCK (I.) and several other workers. Nevertheless, as 

 PRIOR (II.) has shown, the greater fecundity observed in aerated 

 cultures is attributable not merely to the excess of available free 

 oxygen, but also to the circumstance which should not be under- 

 estimated that the gas traversing the nutrient medium frees the 

 latter from various products (volatile acids) of yeast metabolism 

 that retard reproduction. 



Yeast has the power of absorbing free oxygen from the 

 environment, utilising it in completing internal chemical changes, 

 and then excreting the most part in the form of carbon dioxide. 

 The closer examination of this process of respiration is illumina- 

 tive. The quantity of oxygen taken up by the yeast cell has been 

 determined by P. SCHUTZENBERGER and E. QDINQUAUP (I.), in 

 an experiment wherein pressed yeast (containing 26 per cent, ot" dry 

 matter) was distributed in aerated water, the figures per i grm. 

 yeast in i hour being o.i c.c. at 9C., 0.4 c.c. at n C., i.2C.c. at 

 22 C., 2.1 c.c. at 33 C., 2.1 c.c. at 40 C., 2.4 c.c. at 50 C., and 

 o.o c.c. at 60 C. These workers assert that no further absorp- 

 tion of oxygen takes place in media which, like arterial blood, are 

 capable of cedeing 200230 c.c. of oxygen per litre, instead of the 

 6-7 c.c. present in the water used, but this is true only when the 

 experiment is conducted under the conditions employed by them. 

 By working under different conditions. A. HARDEN and S. ROW- 

 LAND (I.) found that i grm. of pressed yeast can take up an 

 average of 3.54 c.c. of oxygen per hour. Moreover, the consump- 

 tion of oxygen would probably reach a high figure if the cells 

 were abundantly supplied with respirable substances, instead of, 

 as in this case, being compelled to feed on one another. In fact, 

 it has been found by GILTAY and ABERSON (I.) that yeast grown 

 in a medium containing sugar consumed more and more of that 

 substance in proportion as the degree of aeration was increased 

 and the mixture of air and oxygen was richer in the latter con- 

 stituent, as much as 21 per cent, of the total sugar (utilised for 

 cell construction, respiration, and fermentation) being consumed 

 in this way. The higher final attenuation of wort that has 

 been strongly aerated during fermentation is partly due to the 

 increased respiration. 



Few of the investigations made in connection with the de- 

 pendence of yeast respiration on external conditions are reliable. 



