CHAPTER XVI 



LACTIC ACID FERMENTATION— THE GENEEA LACTOBACIL- 

 LUS, STEEPTOCOCCDS AND BACTBEIUM 



Lactic acid is produced in nature commonly as the result 

 of the action of various microorganisms upon carbo- 

 hydrates; it is also found in small quantities in tissues of 

 the body as a result of muscular activity. Lactic acid fer- 

 mentation is the most common change observed in milk, and 

 fermentation of this type is most important in the develop- 

 ment of the acid so necessary in the preservation of certain 

 foodstuffs such as silage, sauerkraut, pickles and other fer- 

 mented foods. 



The exact chemistry of the formation of lactic acid has 

 not been adequately explained. It is generally assumed to 

 be due to the activity of an intracellular enzyme, the so- 

 called lactacidase. The transformation does not require the 

 presence of free oxygen, that is, it is essentially anaerobic. 

 The change is one brought about by the organism as a 

 means of securing energy. It is quite possible that the 

 chemical transformation of sugar into lactic acid occurs in 

 several steps. Disregarding the intermediate products, 

 however, the reaction may be written : 



C.Hi^O, = 2CH3 • CHOH • COOH 

 Glucose Lactic acid 



It will be noted that the lactic acid molecule contains 

 one asymmetric carbon atom. There are, therefore, two 

 lactic acid forms, a levo acid and a dextro acid, the former 

 turning the plane of polarized light to the left, the latter to 

 the right. The kind of lactic acid developed in a particular 



192 



