204 PHYSIOLOGY OF NUTRITION 



2.0 



equal to — > or 10. It thus appears that but very little oxygen is absorbed, 



even with an abundance of this gas, while much carbon dioxide is produced; 

 oxygen respiration is here very weak but the decomposition of sugar into 

 alcohol and carbon dioxide is very pronounced. Another series of experiments 

 by Ivanovskii gave concordant results. Equal amounts of nutrient solution 

 were placed in two vessels, the space above the liquid being filled with air in 

 one case and with nitrogen in the other, and equal quantities of yeast were 

 added to the vessels. At the end of the experiment the rate of sugar fermen- 

 tation, per gram of dry yeast, per day, was determined. In one test, for ex- 

 ample, where the yeast introduced into each vessel had a dry weight of 

 0.16 g., this weight increased to 0.516 g. in the presence of air and to 0.497 g- 

 in its absence. With air, 6.009 S- °f sugar was decomposed in twenty-four 

 hours and without air 5.804 g. Thus, the amount of sugar decomposed in 

 twenty-four hours per gram of dry yeast was 8.9 g. in both cases. A marked 

 difference between the two cultures is to be noted, however, in regard to their 

 reproduction; with access of air the yeast multiplied considerably faster than 

 in the absence of oxygen. With a long exposure to oxygen-free air, growth ceases 

 entirely, but the cells still remain alive and capable of decomposing sugar. 

 Reproduction continues indefinitely when the supply of oxygen is not cut off. 



The researches of Gromow and Grigoriew ^how that zymin (acetone-treated 

 yeast, see page 174) produces carbon dioxide at the same rate in a stream of 

 air as in a stream of hydrogen, and these results were substantiated by 

 Buchner and Antoni. 2 



Palladin 3 showed that oxidation enzymes are present in yeast in but slight 

 amount, and this explains the fact, which seems remarkable at first, that yeast 

 produces alcoholic fermentation even with an abundant supply of oxygen. It 

 is on account of the absence of these enzymes that yeast is unable to oxidize 

 alcohol in the presence of air, but this organism usually develops in the absence 

 of oxygen, where oxidation enzymes are not needed. Moreover, alcohol 

 readily diffuses out of the cells and thus becomes inaccessible to the action of 

 intracellular enzymes. 



In the industries, it is well to carry out the fermentation process under condi- 

 tions of good aeration, since the multiplication of the yeast is hastened by the 

 presence of oxygen and the process is thus accelerated. Although each individ- 

 ual cell produces the same amount of alcohol in the absence as in the presence 

 of air, the number of active cells is larger when oxygen is supplied. Oxygen 

 thus exerts, indirectly, an accelerating influence upon fermentation. 



The concentration of alcohol in the solution influences the rate of fermenta- 

 tion; with increasing alcoholic concentration an anesthesia of the yeast cells 

 finally sets in, and the rate of sugar decomposition is diminished. If the alco- 

 holic concentration reaches 16 per cent, fermentation ceases altogether. 



1 Gromow and Grigoriew, 1904. [See note 7, p. 174.] 



- Buchner, Eduard, and Antoni, Wilhelm, Weitere Versuche iiber die Zellfreie Garung. Zeitsch. physiol. 

 Chem. 44: 206-228. 1905. 



3 Palladin, W., Ueber das Wesen der Pflanzenatmung. Biochem. Zeitsch. 18: 151—206. 1909. 



