66 BACTERIA, YEASTS, AND MOLDS 



FOOD REQUIRED BY YEAST 



All common species of yeast require sugar for food, 

 and therefore will not grow rapidly unless sugar is pres- 

 ent in abundance. Bread dough ferments because it con- 

 tains some sugar. Flour itself contains a large amount 

 of starch, which is not fermentable ; but in the bread 

 dough some of the starch is changed to sugar by a chem- 

 ical process, so that fermentation is possible. Almost all 

 sugar solutions furnish a proper medium for yeast growth, 

 provided the solution is not too dense. Yeast cannot live 

 upon absolutely pure sugar, since it needs certain other 

 materials for food ; but all natural sugar solutions, such 

 as molasses, grape juice, etc., contain quite enough other 

 material for the yeasts to feed upon, and they ferment 

 readily enough. A high percentage of sugar is injurious 

 to the growth of yeasts, a fact that explains why almost 

 anything can be preserved if it is saturated with a large 

 amount of sugar. (See Preserves, p. 163.) 



Food is required for yeasts during the fermentation, 

 since they are growing and rapidly increasing in abun- 

 dance. The simple presence of yeasts produces no fer- 

 mentation. If anything prevents the growth of the yeast 

 plants, no fermentation occurs, and it is always found that 

 the yeast increases in bulk during the process. In the 

 large fermentative industries there is consequently pro- 

 duced a large quantity of yeast, which accumulates in 

 bulk at the close of the fermentations. 



This material has been mostly a waste product, although 

 a considerable amount of it has been utilized for bread 

 raising, as shown in the next chapter. Recently a new 



