148 PLANT BIOLOGY 



a pale blue flame. And so we learn that yeast, as it grows in the 

 molasses mixture, changes the sweet substances into carbon dioxid and 

 alcohol, a process that is known as alcoholic fer-men-ta f tion. 



157. Uses of yeast. When bread is made, water (or milk), 

 butter, salt, sugar, and yeast are added to flour. After the mixture 

 has been stirred together, a sticky mass of dough is formed, which 

 in a warm place begins to rise. This is due to the fact that the yeast 

 cells change the sugar into alcohol and carbon dioxid. Bubbles of 

 gas are thus imprisoned in the sticky dough. While expanding and 

 seeking to escape, they make the solid mass porous. After the bread 

 has risen sufficiently, it is kneaded in order to break up the large 

 bubbles and in order to distribute the gas throughout the dough. 

 When the bread is baked, the alcohol and carbon dioxid' pass off 

 into the air, leaving the bread light and digestible. These minute 

 organisms are also of great commercial importance in the manufac- 

 ture of alcohol and of all kinds of liquors. It is known that 

 yeast cells are found commonly in the air. As different kinds of 

 fruits ripen, they are usually more or less covered with yeast or 

 its spores. When, therefore, grapes are gathered and their juice is 

 pressed out, the sweet liquid is soon alive with the busy cells, and 

 fermentation begins at once. In this way wines are produced. 

 Cider is produced by the fermentation of apple juice. 



In the manufacture of beer and of other malt liquors, barley is 

 commonly used. The grain is soaked and allowed to sprout for 

 a short time, until the starch is changed to grape sugar. The barley 

 kernels are then killed by heat to prevent further changes, and the 

 grain is then known as malt. When this is put into water, the sugar 

 is extracted. Yeast is then added, and the mass ferments. The 

 beer thus formed contains 2 to 5 per cent of alcohol. 



Distilled liquors, or spirits, are obtained from wines and other 

 fermented liquors by the process of distillation, the principles of 

 which have already been explained. Brandy is made by distilling 

 wine, whisky is obtained from fermented corn and rye, and rum is 

 manufactured from molasses. All of these liquors contain a large 

 percentage of alcohol (40 to 50 per cent). 



