WHERE DOES YEAST COME FROM? 155 



that fermenting liquids contain an army of minute plant organ- 

 isms which not only live there, but which actually grow and 

 multiply within the liquid. For growth and multiplication, 

 food is necessary, and this the tiny plants get in abundance 

 from the fruit juices ; they feed upon the sugary matter and as 

 they feed, they ferment it, changing it into carbon dioxide and 

 alcohol. The carbon dioxide, in the form of small bubbles, 

 passes off from the fermenting mass, while the alcohol remains 

 in the liquid, giving the stimulating effect desired by users 

 of alcoholic drinks. The unknown strange organisms were 

 called yeast, and they were the starting point of the yeast cakes 

 and yeast brews manufactured to-day on a large scale, not only 

 for bread making but for the commercial production of beer, 

 ale, porter, and other intoxicating drinks. 



The grains, rye, corn, rice, wheat, from which meal is made, 

 contain only a small quantity of sugar; on the other hand, 

 they contain a large quantity of starch which is easily convertible 

 into sugar. Upon this the tiny yeast plants in the dough feed, 

 and, as in the case of the wines, ferment the sugar, producing 

 carbon dioxide and alcohol. The dough is thick and sticky 

 and the gas bubbles expand it into a spongy mass. The tiny 

 yeast plants multiply and continue to make alcohol and gas, 

 and in consequence, the dough becomes lighter and lighter. 

 When it has risen sufficiently, it is kneaded and placed in an 

 oven ; the heat of the oven soon kills the yeast plants and 

 drives the alcohol out of the bread ; at the same time it expands 

 the imprisoned gas bubbles and causes them to lighten and 

 swell the bread still more. Meanwhile, the dough has become 

 stiff enough to support itself. The result of the fermentation 

 is a light, spongy loaf. 



Where does yeast come from? The microscopic plants 

 that we call yeast are widely distributed in the air, and 

 float around there until chance brings them in contact with a 



