334 BIOLOGICAL ACTIVITIES OF YEASTS 



aldehyde dehydrogenase which catalyzes the oxidation of diphos- 

 photriose to diphosphoglycerate. Enolase, the enzyme that catalyzes 

 the dehydration of 2-phosphoglyceric acid to phospho-enol-pyruvate, 

 is inhibited by sodium fluoride. The mechanism of this inhibition 

 was worked out by Warburg and Christian. ^^ They found that a 

 magnesium-fluor-phosphate complex is formed which inhibits the 

 action of the enzymes by combining with the protein. 



For further details on the enzymes of yeast, the reader is referred 

 to Chemistry and Methods of Enzymes by Sumner and Somers.^^ 



Symbiotic Fermentations. Although it is true that yeasts must 

 compete with bacteria (which generally grow more rapidly) in most 

 environments, except those in which the pH is fairly low, they are 

 sometimes found growing together in symbiotic relationships in cer- 

 tain situations. Thus the ferment of kefir is composed of a yeast 

 and a bacterium {Streptococcus kefir). Freudenreich, who studied 

 this ferment, claims that the yeast is unable to ferment lactose. The 

 milk sugar is first hydrolyzed by the streptococcus, the resulting 

 glucose and galactose being changed to alcohol and carbon dioxide by 

 the yeast. A similar relationship is present in the ginger beer "plant" 

 and in a curious ferment which has been used in rural America for 

 the manufacture of vinegar directly from molasses. In the latter 

 ferment the bacterial symbiont is apparently an acetic acid bac- 

 terium. The yeast first produces alcohol from the sugar; then the 

 bacterium oxidizes the alcohol to acetic acid. In all three of these 

 ferments the organisms grow together as curious irregular lumps of 

 cartilaginous consistency, part of the sugar being converted into a 

 gum which holds them together. These masses or grains are strained 

 out after the fermentation is complete, and will remain viable, after 

 drying, for considerable periods of time. 



INDUSTRIAL APPLICATIONS OF THE ALCOHOLIC 

 FERMENTATION 



Baking. The leavening agent used to raise the dough in miaking 

 bread is generally a strain of Saccharomyces cerevisiae. The amount 

 of gas produced is directly proportional to the amount of yeast added 

 to the dough, at least in the first few hours of the fermentation. In- 

 asmuch as the volume of gas produced varies with the type of yeast 

 used, the baker must have a source of yeast of uniform, definite, 

 gas-producing qualities. ' 



Starch in the dough is hydrolyzed to maltose by the wheat diastase 

 naturally occurring in the flour. Where low diastatic flour is en- 



