212 PHYSIOLOGY OF NUTRITION 



§3. Other Kinds of Fermentation. — Lactic acid fermentation (the souring 

 of milk) is caused by Bacillus lactici acidi, which has the form of small paired 

 rods from 1.0 to 1.7 micra long and from 0.3 to 0.4 micron broad. Many other 

 bacteria are able to produce lactic acid fermentation; such as Bacterium lactis 

 acidi, Bacillus lactis acidi, Bacterium limbatum lactis acidi, Micrococcus lactis 

 acidi, S phcerococcus lactis acidi, Streptococcus acidi lactici and Bacillus acidificans 

 longissitnus. 



The process of lactic acid fermentation is represented by the following 

 equation: 



Lactose Water Lactic acid 



C^H^On-j- HoO = 4 C3H6O3. 



This fermentation occurs when milk is simply exposed to a temperature of 

 from 35 to 42°C. for a short time. The process stops when a certain amount of 

 acid has accumulated, but if the acidity thus produced is neutralized with 

 calcium carbonate fermentation begins again. Some acetic acid and other 

 volatile acids usually occur, as well as lactic acid, the amount of these being 

 dependent both upon the kind of bacterium and upon the composition of the 

 nutrient medium. Besides lactose, other kinds of sugars, such as cane sugar, 

 fructose and maltose, can be fermented to lactic acid if the proper kind of 

 bacteria is used. 



Lactic acid may also be obtained if a mixture of 100 g. of cane sugar and 10 g. 

 of casein or old cheese, in a liter of water saturated with calcium carbonate, is al- 

 lowed to stand in an open vessel at a temperature of from 35° to 40°C, with 

 occasional shaking. After fermentation has ceased the liquid is evaporated and 

 calcium lactate is deposited, from which free lactic acid is obtained by decompos- 

 ing the lactate with sulphuric acid. The optically inactive variety of lactic 

 acid is obtained in this process, but in some cases the optically active isomers 

 arise. When Micrococcus acidi paralactici acts in a medium containing sugar, 

 appreciable amounts of the dextro-rotatory paralactic acid are formed ; Bacillus 

 acidi levolactici forms the levo-rotatory acid. The different powers possessed 

 by different bacteria to form optically active isomers of lactic acid may be used 

 in identifying related species of these organisms; thus, Bacterium coli commune 

 decomposes grape sugar, giving dextro-lactic acid, but Bacillus typhi abdominalis 

 produces levo-lactic acid under the same conditions. Lactic acid bacteria have 

 been widely applied in the industries; for example, Berlin white beer is obtained 

 by the action of these forms. 



Butyric acid fermentation is produced by the bacterium, Clostridium butyri- 

 cum, which has recently been shown to consist of a mixture of at least three 

 different species. There are also many other bacteria that produce butyric 

 acid. Butyric acid fermentation occurs in the complete absence of oxygen, 

 and both hydrogen and carbon dioxide always arise as gaseous products of the 

 process. The reaction is represented by the following equation: 



Glucose Hydro- Carbon Butyric acid 



gen dioxide 



C 6 H 12 6 = 2 H 2 + 2 C0 2 + C 4 H 8 2 . 



