234 PHYSIOLOGY OF NUTRITION 



oxidation is complete. The fermentation of a gram-molecule of glucose (forming 2 

 g. -molecules of ethyl alcohol and 2 g. -molecules of carbon dioxide) should give 57 

 kg.-cal. of heat, while the complete oxidation (to water and carbon dioxide) of the 

 alcohol thus produced (2 g.-molecules) should give 652 kg.-cal. in addition. The 

 complete oxidation of a g. -molecule of glucose should consequently give 57 + 

 652, or 709, kg.-cal. To set free a given amount of free energy, more than twelve 

 times as much glucose must be used in fermentation as would be required in aerobic 

 respiration. 



2. Alcoholic Fermentation by Yeast. — The Saccharomycetes absorb sugar from the 

 medium and derive free energy from the decomposition of this sugar by means of the 

 respiration enzyme zymase. The main products of this alcoholic fermentation are ethyl 

 alcohol and carbon dioxide, which diffuse from the cells into the medium. Some 

 nitrogenous material and mineral salts, as well as sugar, are necessary for the growth of 

 yeasts, just as for that of other plant forms. 



It appears that this fermentation process proceeds by two stages, employing an 

 inorganic phosphate (such as K 2 HP0 4 ) as a co-enzyme, along with zymase. In the 

 first stage carbon dioxide, ethyl alcohol, water, and a hexose phosphate are produced. 

 In the second stage the hexose phosphate is decomposed, giving more water and repro- 

 ducing the mineral phosphate and some of the original sugar. 



Yeast develops best in the presence of a plentiful supply of oxygen, although the 

 process of alcoholic fermentation is not directly influenced by the oxygen supply. It 

 appears that the reasons why ethyl alcohol is set free without being further oxidized in 

 this case (even in the presence of plenty of oxygen) are (1) that yeast is but poorly 

 supplied with oxidase and (2) that the alcohol diffuses out of the cells about as rapidly 

 as it is formed. As the concentration of alcohol increases in the medium, the yeast cells 

 ultimately become poisoned by the alcohol, and the fermentation process ceases 

 altogether when the alcohol concentration reaches a magnitude of about 16 per cent. 



Different kinds of yeast act somewhat differently. They may be separated and 

 identified by several methods, as by the time required for the production of ascospores, 

 by the forms of their giant colonies, etc. Many bacteria and moulds, as well as yeasts, 

 produce alcoholic fermentation. 



In the metabolism of yeasts and other alcoholic-fermentation organisms, the more 

 complex carbohydrates are generally first hydrolyzed (as when cane sugar is acted on 

 by the enzyme invertase) to form glucose sugar, and the latter is then fermented by 

 zymase. Other substances, simpler than glucose, some of which may occur as inter- 

 mediate products in the breaking down of the latter, may be fermented in a similar 

 manner. Thus, pyrotartaric acid (CH3COCOOH) forms carbon dioxide and acetic 

 aldehyde (CH 3 COH) under the influence of carboxylase, the aldehyde being subse- 

 quently reduced to ethyl alcohol (CH 3 CH 2 OH) by the action of reductase. The 

 reductase enzymes act by transmitting hydrogen from one substance to another. The 

 substance that supplies the hydrogen is a reducing agent, while the substance that 

 receives the hydrogen is an oxidizing agent. The latter is called the acceptor of hydro- 

 gen. Reductase action may be illustrated by the fermentation of lactic acid, which 

 may be pictured by means of the following equations, in which M represents a respira- 

 tion pigment acting as hydrogen acceptor. 



Lactic Acid Pyrotartaric Acid 



i. CH3-CHOH-COOH + M = CH3-CO-COOH + MH 2 



Pyrotartaric Acid Acetic Aldehyde 



2. CH3-CO-COOH = C0 2 + CH3-COH 



Acetic Aldehvde Ethyl Alcohol 



3. CH3-COH + MH 2 = CH 3 -CH 2 OH + M 



