310 PLANT PHYSIOLOGY 



produce them. The bacteria of decay are also of great economic 

 importance to man, since they perform necessary steps in the or- 

 ganic cycles and thus enable the material of the proteins to be used 

 again by later organisms. Each link in the organic cycle is of as 

 great ultimate importance in the grand scheme of life as any other. 

 Acetic * 'Fermentation." — In the formation of acetic acid, ethyl 

 alcohol is oxidized by the acetic acid bacteria to acetic acid and 

 water, as shown by the equation : 



C 2 H 5 OH+0 2 = CH 3 COOH+H 2 0. 



This is an oxidative process with the addition of free oxygen and 

 is therefore not a fermentation process in the strict sense of the 

 word. Since it is popularly so called, it has been included here, 

 but it should be noted that the formation of acetic acid (aerobi- 

 cally) and the formation of lactic acid (anaerobically) belong in 

 two entirely different categories of reactions. 



Alcoholic Fermentation. — Of all the fermentations, this is the 

 one in which most people have been especially interested. From 

 the beginning of written records, alcoholic fermentation has in- 

 trigued man and taken much of his attention. Even the passage 

 of laws against alcoholic consumption has not affected the laws of 

 nature and of the fermentation process; it seems much easier to 

 legislate against the activities of human beings than against those 

 of the yeast plant. 



The general equation for the fermentation of dextrose by yeast is 



C 6 H 12 6 = 2C 2 H 5 OH+C0 2 (+energy) 



but it must be remembered that other organisms beside yeasts 

 bring about the formation of alcohol from sugars. Among these 

 are certain molds and bacteria, and even higher plants may pro- 

 duce alcohol under anaerobic conditions. Other sugars may be 

 fermented and other products developed including succinic acid, 

 amyl alcohol, glycerin, etc., but the chief commercial interest 

 centers in the formation of ethyl alcohol by members of the genus 

 Saccharomyces (yeasts) out of hexose sugars. 



When glucose is respired or oxidized under aerobic conditions 

 to carbon dioxide and water, one gram molecule (180 grams) pro- 

 duces 709 kg. calories of heat energy (Palladin). When fermented 

 or respired anaerobically, a part of these 709 kg. calories remains 

 in the alcohol. The alcohol derived from a gram molecule of sugar 



