CHAPTER XIII 



THE CIRCULATION OF CARBON DIOXIDE IN NATURE 



{continued) 



2. Bacterial Fermentation of Carbohydrates. 



The bacteria of lactic fermentation are very widely distributed in 

 nature, and play an important r61e, not only in dairy work, but also in 

 numerous other industrial processes — often as indispensable auxiliaries, 

 but sometimes as dangerous enemies and intruders. 



Grape sugar, cane sugar, and milk sugar are all accessible to lactic 

 fermentation. Other sugars (e. g. maltose) are not, nor are carbohydrates 

 like starch or cellulose. All these must be changed into a fermentable 

 form before they can be broken up by the lactic acid bacteria, which 

 themselves produce no such enzymes. 



The lactic fermentation is an aerobic process whose optimum lies 

 between 30 and 35° C. (for some species between 47 and 52° C). It 

 can go on for a long period only when basic substances, such as calcic 

 carbonate, are present to fix and neutralize the lactic acid as it arises, 

 a very small amount (0-15 per cent.) of free acid arresting the growth of 

 the bacteria. Eighty per cent, of the fermentable sugar is converted under 

 favourable circumstances into the so-called ' fermentation lactic acid,' 

 optically inactive ethylidene lactic acid, besides which are formed varying 

 amounts of acetic acid, optically active lactic acid, carbon dioxide, and other 

 by-products. 



The power of forming lactic acid from sugar is common to many 

 bacteria (e. g. the cholera and most other vibrios, B. prodigiosus, various 

 species from the intestines of infants, and many brewery Sarcinae). But 

 by lactic acid bacteria are generally understood those forms which are the 

 regular cause of the acidification of milk in dairies. These micro-organisms 

 were formerly looked upon as belonging to one species, Bacterium acidi 

 lactici (86), but more recent investigations have shown that not one species 

 but many are at work, sometimes one kind, sometimes another, prevailing. 



