Jill Hilton, age 15, of Montgomery, Ala., for her question:
HOW DOES YEAST WORK?
Yeast is a substance that bakers put in dough to make it rise. This type of yeast is made up of a mass of tiny, one celled plants called "yeasts." Yeasts are among the simplest kinds of plants. Like mushrooms, they belong to a group of plants called "fungi."
Yeasts increase very rapidly and the tiny plants float in the air almost everywhere. Some yeasts form new plants by a process called "budding." A small part of the cell wall swells out and a wall of cellulose soon shuts off this new growth from the parent plant. It becomes an independent cell and grows other buds.
Sometimes all the cells cling together in chains that later break up. Some yeast plants increase simply by dividing in two. This process is called "fusion."
It was in 1857 that French scientist Charles Cagnaird de Latour discovered that yeasts are living plants which increase by budding. He also found that these plants can act on sugar to change it to alcohol.
The important chemical change of yeast is part of a process of fermentation. It is the result of the way yeast plants get their food. While each cell is growing, it produces substances known as enzymes or ferments.
Yeasts may form three enzymes: diastase, zymase and invertase. Enzymes cause fermentation by breaking down starch and sugar in solution. A diastase breaks down starch; invertase causes one sugar to form another sugar. Zymase breaks down the sugar.
Bakers use either dry or compressed yeast. Dry yeast is made by mixing yeast mass and corn meal into cakes and drying them. In this form the yeast cells are inactive or dormant and become active only when mixed with the right material. Compressed yeast contains enough starch and moisture to start fermentation in a short time.
Mixing yeast with dough to ferment it is called "leavening" the dough.
Enzymes from the yeast cells attack the starch in the flour and change it to sugar.
In the baker's process, the sugar is then changed to alcohol and carbon dioxide gas. The gas bubbles up through the mixture, forming the familiar bubbles in bread dough, and making the mass light and porous.
When the bread is baked, the alcohol evaporates and the yeast plants are destroyed.
If bread is baked properly, it should have no taste of alcohol or yeast. Sometimes the dough is left to rise too long, and the fermentation forms acid. This condition results in sour bread.
Fermentation by yeast is also important in making alcoholic beverages, such as beer and ale.
In the days before yeast cakes were sold in stores, people had to make their own. They prepared a batter of flour, potato water, salt and sugar, and left it uncovered for several hours. Yeast cells in the air furnished the enzymes but this process was uncertain because types of yeast not suitable for bread sometimes lodged in the batter.