I 



YEASTS AND BACTERIA 339 



Bread Making. — ■ Most of us are familiar with the process of 

 bread making. The materials used are flour, milk or water or 

 both, salt, a little sugar to hasten the process of fermentation, or 

 " rising, '^ as it is called, some butter or lard, and yeast. 



After the materials are mixed thoroughly the bread is put aside 

 in a warm place (about 75° Fahrenheit) to " rise." If we ex- 

 amine the dough after a few hours, we find many holes in it, 

 which give the mass a spongy appearance. The yeast plants, 

 owing to favorable conditions, have grown rapidly and made 

 bubbles of carbon dioxide. Alcohol is present, too, but this is 

 evaporated when the dough is baked. The baking cooks the 

 starch of the bread, drives off the carbon dioxide and alcohol, 

 and kills the yeast plants, besides forming a protective crust on 

 the loaf. 



Sour Bread. — In the ^' rising'^ of bread, bacteria always do 

 part of the work of fermentation. Certain of these plants form 

 acids after fermentation takes place. The sour taste of the 

 bread is usually due to this cause and may be prevented by 

 baking the bread before the acids form, by having fresh yeast, 

 good fresh flour, and clean vessels with which to work. 



Importance of Yeasts. — Yeasts in their relation to man are 

 for the most part useful. They may get into canned substances 

 put up in sugar and cause them to " work," giving them a peculiar 

 flavor. But they can be easily killed by heating to the temperature 

 of boiling. On the other hand, yeast gives us leavened bread, 

 which has become a necessity to most of mankind. 



Bacteria cause Decay. — The phenomenon of decay is one of 

 the numerous ways in which we can detect the presence of bacteria. 

 All organic matter, in whatever form, is sooner or later decomposed 

 by the action of untold millions of bacteria which live in water and 

 soil. These bacteria are most numerous in rich damp soils con- 

 taining large amounts of organic material. They are useful 

 because they feed upon dead bodies of plants and animals which 

 otherwise would soon cover the surface of the earth to the exclu- 

 sion of everything else. Bacteria may thus be considered scav- 

 engers. They oxidize organic materials, changing them to 

 compounds that can be absorbed by plants and used in building 

 protoplasm. Without bacteria and a few of the fungi it would be 



