THE SCIENCE OF THE KITCHEN 99 



Effect of heat on foods. Heat hardens or coagulates 

 protein foods such as the white of an egg. Fats are little 

 effected by low temperatures, except to be melted; but at 

 high temperatures they are decomposed and form irritating 

 substances which hinder digestion. The chief effect of 

 heating starch in water is to cause a swelling of the starch 

 granules. Dry heat changes the starch into dextrin and 

 glucose, as seen in the crust of bread. 



Making bread light. One important quality of good 

 bread and similar foods is that they shall be light and porous. 

 This is brought about by mixing some gas with the dough ; 

 for as the dough is cooked these gases escape and leave the 

 bread porous. Sometimes this result is produced by me- 

 chanically forcing air into the dough by beating. Eggs are 

 sometimes added to render the dough more capable of hold- 

 ing the air. But usually something is put with the dough 

 to produce a gas, which rises through it and makes it porous. 

 Gas is produced in dough by using baking powder or yeast. 

 In both cases the same gas is produced, carbon dioxid. 



A baking powder is a mixture of two compounds of such 

 a nature that when water is added the two chemicals act 

 on each other and carbon dioxid is formed. One of these 

 compounds is usually baking soda, the other may be cream 

 of tartar, phosphate, or alum. With these a little starch is 

 usually mixed to prevent the powder from becoming moist. 

 In place of baking powder, soda and sour milk may be used. 



Yeast is, also used to make bread light. Yeast is a very 

 small plant ; a single one is too small to be seen by the eye 

 without the aid of a microscope. As a result of the action 

 of the yeast on the dough, carbon dioxid is formed and acts 

 in the same way as that formed by baking powders in making 

 the bread light. At the same time alcohol is formed, but 

 in the oven this passes off from the bread. When war 

 breads were being made by the use of substitutes for wheat, 

 it was frequently found that the bread was heavy and un- 



