FERMENTATION 39 



Besides nitrifying bacteria many de-nitrifying organ- 

 isms occur in the soil. They may (1) reduce nitrates to 

 nitrites ; (2) remove oxygen from nitrates and nitrites 

 and form ammonia ; (3) form nitrous and nitric oxides 

 or nitrogen from nitrates and nitrites. 



Fermentation. Another important group of changes 

 produced by micro-organisms is that comprised under 

 the comprehensive title of " fermentation," of which it is 

 difficult to give an accurate definition, for the distinction 

 between it and other chemical changes due to the activity 

 of micro-organisms is conventional rather than scientific. 

 The original conception of the term involved the occur- 

 rence of frothing of the fermenting liquid, owing to the 

 escape of gaseous products. Fermentation is brought 

 about by the action of ferments, two classes of which are 

 recognised, viz. the living or organised ferments, which, 

 in other words, are micro-organisms ; and the unorganised 

 or chemical ferments, bodies such as pepsin, which in 

 minute amount produce changes in a considerable quantity 

 of the substance acted upon, without themselves under- 

 going alteration. 



The term " fermentation " is now reserved for the 

 changes brought about by the organised ferments or 

 living prganisms, while the unorganised ferments are 

 known as enzymes, enzyme-action being termed " zymo- 

 lysis." As fermentations are investigated more critically, 

 the tendency is to find that they are brought about by 

 enzymes, extra-cellular or intra-cellular, produced by the 

 micro-organisms. 



The following are the chief varieties of fermentation : 



The alcoholic fermentation. This is mainly brought 

 about by the decomposition by yeasts of sugars of the 

 hexose group (C 6 H 12 6 ), principally dextrose and Isevulose, 

 into alcohol and carbonic acid, but some of the bacteria 

 and moulds also produce appreciable quantities of alcohol. 



