METABOLISM OF MICROORGANISMS 



371 



If the sugar content of the medium is raised to 5 per cent, the yield of 

 yeast is markedly reduced, and much alcohol is formed. In other words, 

 an anaerobic type of metabolism comes to the front, although an abun- 

 dance of air is present. The explanation for this is that ordinary yeast 



Courtesy of Dr. Charles N. Frcy, Fieiscbmauu Laboratories. 

 Fig. 14-2. Budding veast cells. 



has a weak aerobic enzyme system and a strong anaerobic system. If 

 more sugar is present than can be metabolized aerobically, the anaerobic 

 system begins to operate. 



By molds 



Molds cannot grow in the absence of air; carbon dioxide and water are 

 the usual products of metabolism. However, many species convert a 

 large percentage of the sugar in the medium into other carbon products. 

 Examples of products that make up more than 40 per cent by weight of 

 the sugar consumed and the molds producing them are given in Table 

 14—1. The highest yields of compounds in the table are for gluconic 

 acid. Actually over 100 per cent has been obtained, if allowance is 

 made for glucose going to mycelium. This yield is possible since one 

 oxygen is added per mole of glucose, which amounts to 196 g. of gluconic 

 acid from 180 g. of glucose, or 109 per cent. 



Citric Acid. This acid has been obtained many times in yields of 

 80-90 per cent, but the usual yields, without allowing for glucose going 

 to mycelium, are around 70 per cent. Very special conditions have to 

 be maintained to keep the mold growth low. Such conditions are low 

 concentrations of metals, particularly manganese, high concentrations 

 of sugar, and low pH in the medium. An explanation for the effect of 

 manganese is that this metal serves as a cofactor for some enzyme system 

 that functions in the breakdown and oxidation of citric acid. If there 

 is a deficiency of manganese, the enzyme cannot operate, and then citric 



