28 GENERAL SCIENCE 



so small that they cannot be seen. These floating yeast 

 plants are called wild yeast. When they fall into liquids 

 containing small quantities of sugar they start to grow 

 and soon spoil the liquids. While yeast is in the air or 

 on dry material, it remains inactive, just as do seeds that 

 are not planted. When it falls in places favorable for 

 growth, it throws off its protective covering and absorbs 

 the food. The conditions favorable for its growth are: 

 i. Temperature from 70 to 100 F. (Bread rises best 

 in a temperature of from 80 to 95 F.) 2. Moisture in 

 sufficient quantity to dissolve the food. 3. Food in 

 solution. When these three conditions are present a 

 few yeast plants will soon increase to thousands. While 

 they are growing, they give off two waste products. 

 The important one in bread-making is carbon dioxide 

 gas. This gas causes the dough to expand and become 

 spongy, thus making the bread light and porous after 

 baking. The only reason why yeast is used in baking 

 is because of this carbon dioxide gas which it forms in 



the dough. It also gives 

 off alcohol as a waste 

 product. But so little 

 of this is formed in the 

 dough while rising that 



it is all driven out by 

 ANCIENT GRINDING STONES ,. , . ,, 



evaporation during the 



process of baking. Alcohol boils at a lower temperature 

 than water and so will evaporate faster and more easily. 

 23. History of Bread Making. The ancients made 

 bread in a way somewhat similar to the method of the 

 savage tribes of today. They crushed the grain with a 

 hand tool of wood or stone and made a paste of the meal 

 with water and then baked it by holding it over a fire. 



