WHERE DOES YEAST COME FROM? 235 



bubbles and causes them to lighten and swell the bread still 

 more. Meanwhile, the dough has become stiff enough to sup- 

 port itself. The result of the fermentation is a light, spongy 

 loaf. 



216. Where does Yeast come From ? The microscopic 

 plants which we call yeast are widely distributed in the air, and 

 float around there until chance brings them in contact with a 

 substance favorable to their growth, such as fruit juices and 

 moist warm batter. Under the favorable conditions of abun- 

 dant moisture, heat, and food, they grow and multiply rapidly, 

 and cause the phenomenon of fermentation. Wild yeast 

 settles on the skin of grapes and apples, but since it does not 

 have access to the fruit juices within, it remains inactive 

 very much as a seed does before it is planted. But when the 

 fruit is crushed, the yeast plants get into the juice, and feed- 

 ing on it, grow and multiply. The stray yeast plants which 

 get into the sirup are relatively few, and hence fermentation 

 is slow ; it requires several weeks for currant wine to ferment, 

 and several months for the juice of grapes to be converted 

 into wine. 



Stray yeast finds a favorable soil for growth in the warmth 

 and moisture of a batter ; but although the number of these 

 stray plants is very large, it is insufficient to cause rapid 

 fermentation, and if we depended upon wild yeast for bread 

 raising, the result would not be to our liking. 



When our remote ancestors saved a pinch of dough as 

 leaven for the next baking, they were actually cultivating 

 yeast, although they did not know it. The reserved portion 

 served as a favorable breeding place to the yeast plants within 

 it ; they grew and reproduced amazingly, and became so 

 numerous, that the small mass of old dough in which they 

 were gathered served to leaven the entire batch at the next 

 baking. 



