METABOLISM OF MICROORGANISMS 



379 



duce more suitable polysaccharides than those obtained from L. mesen- 

 teroides. 



(d) Levans. Polysaccharides of this type are produced from sucrose 

 by several microorganisms, e.g., Bacillus subtilis. Yields of levan up 

 to 30 per cent, based on the sucrose used, have been obtained. Enzyme 

 preparations give approximately the same yields, and the reaction seems 

 to follow the same equation as for dextrans, but the polysaccharide and 

 free sugar are reversed. Thus: 



nlCi.HooOn) -^ (C6HioO,)„ + niC^Br^O^) 

 Sucrose Levan Glucose 



By yeast 



In the absence of air, ethyl alcohol and carbon dioxide account for about 

 90 per cent of the sugar fermented, as indicated by the following equation: 



CeHioOo -> 2C0H5OH + 2C0o 



Small amounts of acetic acid and glycerol are also produced. If the 

 medium is kept alkaline, pH about 8.5, large quantities of acetic acid 

 and glycerol are formed. The metabolism of the yeast is shifted so 

 that a minor product, glycerol, becomes a major product. The equation 

 for the fermentation may be represented as 



2C,;Hi,06 + H2O -^ 



2CO2 + CHoCOOH + CH,,CHoOH + 2CHoOH • CHOH • CHoOH 



This theoretical distribution of products is not realized, since more alcohol 

 and less glycerol are usually formed. 



Glycerol production may also be increased by adding sulfites to the 

 medium. This fixes the intermediate acetaldehyde as CH3CHOII • 0- 

 •S02Na and prevents its reduction to ethyl alcohol. A corresponding 

 amount of another intermediate, dihydroxyacetone phosphate, is con- 

 verted to glycerol. The sulfite process for production of glycerol was 

 used by the Germans in World War I. It is still under consideration, but 

 to date has not been operated successfully. The fermentation equation 

 may be written as 



CeHioOe -^ CH2OH • CHOH • CH2OH + CH3CHO + CO2 



Only about one-half of this yield of glycerol is obtained in practical 

 operations because some of the acetaldehyde escapes fixation and instead 

 goes to ethyl alcohol. 



Under anaerobic conditions the yield of yeast (dry weight) is around 

 5 per cent of the sugar fermented, about one-tenth as much as is pro- 

 duced under aerobic conditions. 



