82 PHYSIOLOGY OF YEASTS 



have access. The yeast grows at the bottom of the flask and finds 

 a bad supply of oxygen. In this case it uses little sugar for main- 

 taining itself and scarcely multiplies; the rest of the sugar is changed 

 into alcohol and carbon dioxide by the enzymes. 



In anaerobic life the yeasts are not able to secure their energy 

 by oxidation. Quite another chemical change is involved; this 

 is the enzymatic change of sugar into alcohol and carbon dioxide. 

 One easily conceives that much less energy is secured by this proc- 

 ess than by an oxidation. Much of the energy will be used for 

 building up a very small quantity of new protoplasm. The trans- 

 formation of sugar into alcohol will be considerable for a minimum 

 growth of the yeast. The memorable researches of Pasteur l have 

 enriched our information with regard to this change. This illus- 

 trious savant, by a series of experiments, demonstrated that the 

 scarcer the amount of oxygen, the greater was the amount of fermen- 

 tation. 



The best means of propagating a yeast under aerobic conditions is, 

 as we have stated above, growing it in a shallow dish with a few 

 centimeters of nutrient medium. Under such conditions, Pasteur, 

 at the end of 24 hours, has obtained 24 milligrams of yeast cells 

 with a consumption of 98 milligrams of sugar. No trace of alcohol 

 is found in the medium. At the end of 48 hours, Pasteur obtained 

 127 milligrams of yeast cells for 1.04 grams of sugar decomposed. 

 The yeast was almost exclusively an agent of oxidation and acted 

 like other plants. It consumed a large part of the sugar for its 

 maintenance and multiplication and its weight increased in a con- 

 siderable proportion. 



It does not act like this when put into a flask to which air docs 

 not have free access. For 10 grams of sugar decomposed Pasteur 

 secured only 0.44 gram of yeast cells. The yeast had scarcely mul- 

 tiplied; on the contrary, the proportion of sugar changed to alcohol 

 became greater. 



Pasteur has continued his experiments by putting a thin layer 

 of liquid into the same flask. This time, the fermentation was longer 

 and the weight of the yeast less perceptible; but the same propor- 

 tion of alcohol is found as in the alcohol fermentations so-called. 

 In this last experiment, the liquid in the flask was aerated, retaining 

 a small quantity of oxygen which the yeast utilized at the beginning 

 of its development. On the other hand, the yeast may come from 

 cultures which are in contact with air; the cells then have been able 



1 Pasteur. Memoire sur la fermentation alcoolique. Ann. de Chim. et de 

 phys. 1859: Influence de Toxygene sur le de"v. de la levure et de la ferm. ale. 

 Bull. Soc. Chim. 1861. 



