Other Modes of Nutrition; Conservation of Food Elements - 1 75 



in the surrounding medium. Starch cannot 

 always be used as a source of glucose, because 

 the digestive enzymes of the yeast do not in- 

 clude any of the amylases. However, ground 

 malt (germinated barley) is usually added to 

 a yeast brew, and the amylases from the bar- 

 ley cells serve to hydrolyze the starch to 

 maltose. 



Anaerobic Metabolism of Yeast. In the 

 presence of adequate oxygen, yeast cells, like 

 plants and animals generally, obtain energy 

 by oxidizing carbohydrates and other com- 

 pounds. But when free oxygen is scarce or 

 lacking, the yeast replaces this aerobic me- 

 tabolism by an anaerobic metabolism. 

 Under these conditions the yeast obtains en- 

 ergy by alcoholic fermentation (Fig. 8-5). 

 This decomposition of glucose into ethyl 

 alcohol and carbon dioxide does not require 

 the utilization of oxygen, as is shown in the 

 following over-all reaction: 



C 6 Hi 2 06 



glucose 



fermentation 



2C 2 H 5 OH + 2C0 2 + E (approx 1/5 Cal per 



g glucose) 



ethyl alcohol 



Because the nonoxidative metabolism of 

 glucose yields only about y 20 as much energy 

 as the oxidative metabolism (cf. Fig. 8-5), 

 yeast grows much more slowly and less effi- 

 ciently under anaerobic conditions. More- 

 over, the metabolic wastes, especially alcohol, 

 tend to accumulate in the medium after 

 escaping from the cells by diffusion; and the 

 alcohol finally exerts a toxic effect, despite 

 a unique tolerance that yeasts have devel- 

 oped to this compound. 



The importance of yeast in human econ- 

 omy lies in the fact that fermentation con- 

 stitutes the only practical source of alcoholic 

 beverages, and of ethyl alcohol, which is 

 widely used as a solvent and raw material in 

 chemical industry. In the brew mashes of 



beers and wines, most of the alcohol is 

 formed by the dense population of yeasts 

 near the bottom of the brew tank. Near the 

 top of the tank, which is more or less open 

 to the atmosphere, there is ample oxygen, 

 and the yeasts of this region produce little 

 or no alcohol. But the oxygen is soon used 

 up at the bottom of the tank, and new oxy- 

 gen entering from the atmosphere is con- 

 sumed by the upper layers of yeasts before 

 it can diffuse to the bottom layers. The pro- 

 portion of alcohol in a natural brew never 

 exceeds about 14 percent, since this is the 

 maximum tolerable to the yeasts themselves. 

 However, by distillation the proportion of 

 alcohol may be increased up to 95 percent. 

 In bread making, the alcohol produced in 

 dough is unimportant, since the relatively 

 small quantity is dissipated during baking. 

 But the carbon dioxide forms bubbles that 

 are trapped in the thick dough mixture, and 

 this "raises" the dough, giving a porous 

 texture to the bread. 



Nutrition in the Bread Mold. The spores 

 of the bread mold (Rhizopus nigricans) are 

 abundant in the atmosphere, especially in 

 populated regions. Consequently an exposed 

 piece of moist bread or other starchy food 

 almost invariably becomes covered by the 

 fuzzy white mycelium of the bread mold. 

 Later this growth turns blackish gray, indi- 

 cating that a new crop of spore capsules has 

 become ripe (Fig. 12-10). 



The mycelium of the bread mold (Fig. 12- 

 10) is composed entirely of a mass of much- 

 branched threads, or hyphae, which are of 

 three types: (1) short, clustered rhizoids, 

 which penetrate down into the substance of 

 the bread; (2) long, stout stolons, which form 

 an interlacing mass over the surface of the 

 bread; and (3) long, slender sporangiophores, 

 which extend vertically upward, each sup- 

 porting a single spore capsule at its upper 

 end (Fig. 12-10). 



The bread mold, which is a typical sapro- 

 phyte, uses starch as its main organic food. 

 Starch cannot be absorbed as such, but the 

 rhizoids of the mold secrete amylase (and 



