164 NUTRITION AND METABOLISM 



The intermediate product can be accumulated by precipitating it with 

 lime which neutralizes the acidity. This principle is used in the com- 

 mercial manufacture of citric acid by Citromyces^ a mold closely 

 related to the genus Penicillium. This mold oxidizes sugar to citric 

 acid according to the following equation: 



C6H12O6 + 3O = CeHsOr + 2H2O 



Citric acid 



This fermentation is much more complicated than this equation indi- 

 cates, on account of the entirely different chemical structures of citric 

 acid and dextrose. The practical yield in the factory is only about one- 

 half of the theoretical, since complete oxidation cannot be avoided 

 altogether. 



The oxidation processes, just recited, can take place only in the 

 presence of oxygen; the other four types of carbohydrate decompo^si- 

 tion require no, oxygen, and take place as well in the absence of oxygen; 

 the butyric fermentation is brought about only in the absence of oxygen. 



Alcoholic fermentation is caused only by yeasts and a few molds; 

 no bacterium produces alcohol according to the well-known equation 

 mentioned above. ' Alcohol is formed by several bacteria but only in 

 small quantities and always together with several acids; this is a 

 distinctly different type of decomposition. 



In the above groups and the \f oUowing groups of microorganisms, 

 there appears to be a close agreement between the morphological 

 characters of the organisms involved and the specific type of fermenta- 

 tion. Practically all the alcoholic organisms are yeasts, and the lactic 

 acid-producing organisms are streptococci or closely related bacteria. 



The lactic bacteria as they are briefly named, such as are responsible 

 for lactic fermentation, are readily recognized by their scanty growth 

 on agar, and their excellent growth in milk, bringing about a solid 

 curdling in one to three day&. They change sugar to lactic acid only. 



C6H]206 = 2C3H6O3 '■ 



No gas and no volatile acids are formed by these bacteria. The best- 

 known representative of this group is the organism which causes the 

 normal souring of milk. It was originally called Bacterium lactis acidi, 

 but on account of its very close relation to the streptococci, it is more 

 commonly now named Streptococcus lacticus. Many streptococci will 

 produce the true lactic fermentation. 



