YEASTS AND WHAT THEY DO 



117 



from the air which is in contact with the food. As a matter 

 of fact this is true. The tiny plants of the home yeasts, 

 molds, and bacteria are always present in the air, al- 

 though we cannot see them, and when they lodge on food 

 and grow, they cause certain changes to take place in the 

 food. These changes cause the food to have an unpleasant 

 taste and odor. In fact, its quality is so changed that it is 

 no longer suitable to eat. 



Yeasts and what they do. All boys and girls who live 

 in the country know that apple juice or grape juice, if 

 allowed to stand exposed to the 

 air, will very soon ferment, that 

 is, little bubbles will appear in 

 the liquid, and a prickly, pun- 

 gent taste is noticeable. The 

 yeast changes the sugars in the 

 fluid to alcohol and carbon di- 

 oxide is formed. The escaping 

 bubbles of this gas may be seen 

 as the liquid ferments. Yeasts 

 are tiny plants made up of a 

 single cell or unit of building material. They grow very 

 rapidly under favorable cond'tions. These conditions can 

 be easily shown by a home experiment. 1 Yeasts are useful 

 in commercial industries which make use of fermentation. 

 They are of great value also in the home in causing bread 

 to rise. 



Experiment. How and why yeast is used in bread making. 



Materials: Flour. Water. Sugar. Salt. Lard. Yeast cake. Bak- 

 ing tins. 

 Method: Make a dough by mixing the flour, sugar, salt, lard, and water 



1 See Hunter's Civic Biology, page 137. 



Yeast plant. 



