ANAEROBIC RESPIRATION AND FERMENTATION 309 



fermentation has hence returned to its original meaning as the 

 decomposition of organic materials under anaerobic conditions. 

 There is therefore no fundamental distinction between fermenta- 

 tion and anaerobic respiration, but the tendency still remains to 

 restrict the term to the anaerobic respiration of microorganisms. 

 The various kinds of fermentations are named after the substances 

 produced (alcoholic, lactic, butyric, etc.), and we shall now consider 

 some of the more important of these respirations or fermentations. 

 Lactic Acid.— The production of lactic acid is seen especially 

 in the souring of milk. Milk sugar (lactose) is used up or respired 

 by various forms of lactic acid bacteria to form lactic acid : 



C 12 H 22 O n +H 2 = 4C 3 H 6 3 . 



At least a half dozen different species are able to bring about this 

 type of fermentation. As commonly happens, the products of 

 the fermentation injure the bacteria which produce them, and 

 when about 8% lactic acid is formed, the action is brought to a 

 standstill. If the acid is neutralized with calcium carbonate 

 (CaC0 3 ), fermentation will begin again and, in this way, large 

 amounts of calcium lactate may be formed, from which the free 

 lactic acid may be obtained later. The optimum temperature 

 for this type of respiration is about 38° C. Other sugars, includ- 

 ing cane sugar, may be fermented to lactic acid, which is produced 

 along with hydrogen, methane, and nitrogen, depending upon 

 the kind of bacterium involved and the general environmental 

 conditions. According to some investigators, lactic acid may also 

 be produced by roots during anaerobic respiration. 



Butyric Acid.— This acid with a very disagreeable odor is 

 developed in rancid butter by various forms of bacteria, chiefly 

 Clostridium butyricum, which, in the complete absence of oxygen, 

 breaks lactose or lactic acid down to hydrogen, carbon dioxide, 

 and butyric acid: 



2C 3 H 6 3 = 2H 2 +2C0 2 +C 4 H 8 2 . 



Putrefactions.— In the anaerobic respiration of proteins (putre- 

 factions) many disagreeable end products are formed, such as 

 hydrogen sulphide, skatol, indol, etc. The toxins produced by 

 forms like the diphtheria bacterium may also be a kind of fermenta- 

 tion product. Little is known about the details of these processes, 

 but they are undoubtedly advantageous to the bacteria which 



