438 FOODS 



Sometimes other substances besides yeast are used to 

 generate the carbon dioxide necessary to raise the dough. 

 In Experiment 26, it was found that the action of an acid 

 on certain substances liberated carbon dioxide. Often in 

 making biscuits and cake, soda and sour milk are used. 

 The gas is liberated by the action of the acid in the sour 

 milk upon the baking soda. Baking powder, which usually 

 consists of baking soda and cream of tartar mixed with corn- 

 starch, is also used. When the baking powder is mixed 

 with flour and moistened, the cream of tartar acts like an 

 acid upon the soda, liberating carbon dioxide and thus 

 causing the dough to rise. As in bread, the gas is expanded 

 by the heat of the oven, making the cake or the biscuits 

 more porous. 



Most of the minute plants which cause changes in food 

 render it unfit for man's use. We have found that decay, 

 which is caused by bacteria, is on the whole a friendly pro- 

 cess. But we look upon it as an unfriendly process when 

 it results in the souring of milk, the tainting of meat, the 

 spoiling of eggs, and the rotting of vegetables all of 

 which are due to the activities of bacteria. 



The decay in fruit, the mold on bread, the corn smut, 

 the smut on oats and barley, the potato blight, the scabs 

 of apples and potatoes, the rusts on grains, and many other 

 common plant diseases are simply fungous plant growths. 

 The wheat rust alone costs the United States many millions 

 of dollars each year. Thousands of feet of timber are de- 

 stroyed yearly by the wood-destroying fungi. Dry rot of 

 timber, as it is called, is due to a fungous growth. The fight 

 against these harmful fungi costs millions of dollars each year. 



Experiment 144. Place a slice of freshly boiled potato in each 

 of six clean, 4-ounce, wide-mouthed bottles. Close the mouths of 



