PREPARATION OF FOOD 179 



see what it means to produce a loaf of wholesome bread. 

 Flour contains much starch, some sugar, some mineral 



substances known as phosphates, a large quantity of 

 gluten (a protein), and some bacteria (tiny plant-, see 

 Chapter XXIV) which may or may not be of value in 

 making bread. When water is added to the flour, it 

 becomes tough and sticky, this being a characteristic 

 of gluten, and the most important one, so far as tin- 

 making of bread is concerned. A small bit of yeast (a 

 small plant, see Chapter XXIV) is added to the water 

 used in making bread, and the dough is placed where it 

 will be neither too hot nor too cold (70°-80° F.). 



The yeast begins to grow rapidly, feeding on the 

 proteins of the flour, and as the yeast grows, it acts od 

 the sugar. A substance called zymase (zim'as), secret ed 

 by the yeast plant, breaks the sugar up into carbon 

 dioxide, alcohol, and a small quantity of glycerin. The 

 gas tries to escape, but is held in by the sticky dough. If 

 the yeast plant is well distributed, the gas collects in 

 many small bubbles, and the loaf is fine-grained. The 

 alcohol keeps other plants from growing there, and also 

 helps to soften the gluten. 



When the loaf is put into the oven, the heat kills the 

 yeast plant, drives off the carbon dioxide, and causes the 

 alcohol to evaporate. The heat changes the gluten into 

 a substance more easily digested and of a more pleasant 

 taste. In "salt rising' bread bacteria from the air, 

 instead of yeast cells, form the gas which makes the 

 bread light. When a batch of bread "sours," it is 

 usually because harmful bacteria get into the dough ami 

 grow more rapidly than the yeast plants. Sometimes 

 other kinds of yeasts than the helpful ones employed in 

 bread-making accidentally get into the batch of bread and 

 it spoils as a result. 



