156 YEAST AND BREAD MAKING 



substance favorable to their growth, such as fruit juices or 

 moist warm batter. Under 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 fermentation is often 

 slow; it requires several weeks for currant juice 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 fer- 

 mentation, 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 veast, al- 

 though 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. 



As soon as man learned that yeast plants caused fermenta- 

 tion, he realized that it would be to his advantage to 

 cultivate yeast and to add it to bread and to plant 

 juices rather than to depend upon accidental and slow fer- 

 mentation from wild yeast. Shortly after the discovery of 

 yeast in the nineteenth century, man commenced his attempt 

 to cultivate the tiny organisms. Their microscopic size added 

 greatly to his trouble, and it was only after years of careful 





