234 NUTRITION AND METABOLISM 



Complete oxidation of carbohydrates is observed most commonly 

 among molds and mycodermas, and also in a few bacteria, e.g., in A zoto- 

 bacter. It is possible only where there is a ready oxygen supply, as, 

 e.g., in soils of an open texture, in trickling filters, and on the surface 

 of decaying fruits. 



The incomplete oxidation is, as a rule, more common in nature. 

 Frequently microorganisms produce first an incomplete oxidation, but 

 later oxidize the intermediate products completely. The molds are 

 typical examples. Aspergillus niger is noted for its formation of oxalic 

 acid. If it is grown in a sugar solution, it will bring about at first a 

 rapid increase in acidity, but after a while, it decreases again, when the 

 acid is oxidizing completely. The following processes may be noted: 



C 6 H 12 6 + 90 = 3(C0 2 H) 2 + 3 H 2 



Oxalic acid 



(C0 2 H) 2 + O = 2 C0 2 + H 2 O 



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: 



C 6 Hi 2 6 + 30 = C 6 H 8 7 + 2H 2 



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 decomposi- 

 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 following groups of microorganisms, 

 there appears to be a close agreement between the morphological 



