DECOMPOSITION OF HEMICELLULOSES 81 



CH3COCOOH + H2O = CHs-COOH + H-COOH 



Pyruvic acid Acetic acid Formic acid 



CH3-CH20H + 3O2 = 2CO2 + 3H2O 



CHa-COOH + 2O2 = 2CO2 + 2H2O 



H-COOH + ^02 = CO2 + H2O 



The starch content of plants, with the exception of a number of 

 various seeds and other reserve food bodies, is very small. The 

 decomposition of the starches is very rapid, the process being 

 similar to the decomposition of the sugars. Some of the microbes, 

 especially the anaerobic bacteria, attack the starch molecule with 

 the formation of various organic acids, alcohols, aldehydes, acetone, 

 hydrogen, and carbon dioxide. Numerous soil microbes are able 

 to produce an enzyme, diastase, which hydrolyzes the starch to 

 maltose, and another enzynie, maltase which hydrolyzes the di- 

 saccharide maltose to two molecules of glucose. 



Decomposition of Hemicelltjloses. — Hemicelluloses are 

 present in considerable amounts in higher green plants and to some 

 extent in microscopic green and chlorophyll-free plants, where 

 they may occur as reserve food material. On hydrolysis, they are 

 changed to simple sugars, hexoses or pentoses : 



(C5H804)/i + nH20 = nCCsHioOo) 

 (CsHioOs)?! + nHsO = ^KCeHioOe) 



The most abundant group of hemicelluloses are the pentosans, 

 present in quantities ranging from 7 per cent in pine needles to 32 

 per cent in corn-cobs. 



Hemicelluloses, especially pentosans, are readily decomposed 

 by a large number of fungi, actinomyces, and bacteria. In the 

 degradation of fresh plant materials in the manure heap or in the 

 soil, the decomposition of the pentosans proceeds somewhat more 

 rapidly than that of cellulose. However, the organic matter in 

 certain peats and in mineral soils is apt to be free from cellulose, 

 while still containing considerable quantities of hemicelluloses. 

 This is due to the greater resistance of some of the hemicelluloses 

 to decomposition and to the formation of new hemicelluloses 

 through the synthesizing activities of the microorganisms. Such 

 substances formed by the organisms are commonly referred to as 



