266 TEXTBOOK OF PLANT PHYSIOLOGY 



of the sugar molecule into 2 molecules of lactic acid, according 

 to the formula 



C 6 Hi 2 O c = 2CH 3 CHOHCOOH. 



Bacteria producing lactic acid cause the normal souring of 

 milk. Once the sour reaction is initiated, the development of 

 other bacteria ceases, and the coagulation of casein is produced, 

 resulting in curdling of the soured milk. As coagulation proceeds 

 further, the clots of casein contract and whey is separated out. 

 Further accumulation of lactic acid stops the development of the 

 very organisms that cause this fermentation; hence, there never 

 accumulates more than 1 per cent of acid, and part of the sugar does 

 not decompose. But if fermentation is conducted in the presence 

 of calcium carbonate, CaCOa, which neutralizes the acid as soon as 

 it is formed, fermentation of the sugar proceeds to the end. The 

 same hexoses which undergo alcoholic fermentation, as well as 

 maltose and lactose, can be subjected to lactic-acid fermentation. 

 Cane sugar, on the contrary, is not fermented by most lactic-acid 

 bacteria, since they do not produce invertase. 



Impure lactic fermentation is a rather complex process. Be- 

 sides lactic acid, there are formed great quantities of acetic acid, 

 a little ethyl alcohol, carbon dioxide, and hydrogen. Fermenta- 

 tion of this type proceeds best in the presence of air, while the bac- 

 teria causing pure lactic fermentation do not need it. Impure 

 fermentation only spoils milk. In order to obtain faultless curd- 

 ling, therefore, boiled or at least pasteurized milk is used with pure 

 cultures of lactic-acid bacteria. For the preparation of certain 

 food products, such as sauerkraut, pickles, etc., impure lactic fer- 

 mentation is used. This process is accompanied by rapid elimina- 

 tion of gases, while the accumulating acetic and lactic acids pre- 

 vent the development of bacteria which cause decay and help to 

 keep the products in good condition. Care must be exercised, 

 however, not to let the air have access to the fermented products, 

 since they are a favorable substrate for molds preferring a sour 

 medium. 



Butyric-acid fermentation differs from the lactic-acid type in 

 being anaerobic. Bacteria causing it are not only able to live 

 without atmospheric oxygen, but are even unable to develop in 

 the presence of it. According to their response to oxygen, all 

 microorganisms may be divided into three fundamental types: 



