Lisa Cunningham, age 13, of Jackson, Miss., for her question:
WHAT IS YEAST AND HOW IS IT MADE AND USED?
Yeast is a substance that bakers put in dough to make it rise. The yeast contains lots of tiny, one celled plants called yeasts. Yeasts are among the simplest kinds of plants and like mushrooms, they belong to the group of plants called fungi.
A French scientist named Charles Cagniard de Latour discovered in 1857 that yeasts are living plants which increase by budding. He also found that these plants can act on sugar or change it to alcohol. This chemical change is part of a process called fermentation, which yeasts produce in organic substances. It is a result of the way yeast plants get their food.
Yeasts increase very quickly. Bakers use two forms: dry and compressed. 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. They will keep for a long time without spoiling and they become active only when they are mixed with the right ingredients.
Compressed yeast has enough starch and moisture added to start fermentation in a short time. It cannot be stored too long without spoiling and must be kept in a cool place until used.
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. 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 bread is baking, the alcohol evaporates and the yeast plants are destroyed. If bread is baked properly, it should have no taste of alcohol or yeast. Sometimes dough is left to rise too long, and the fermentation forms acid. This produces sour bread.
In the days before yeast cakes were sold in stores, people had to make their own yeast. They had to make a batter of flour, potato water, salt and sugar and then left it uncovered for several hours. Yeast cells in the air furnished the enzymes. This process, however, was uncertain because types of yeast not suitable for bread sometimes lodged in the batter.
Commercial yeast is made by grinding corn and rye to a mash and mixing it with filtered water. Sprouted barley, or malt, is then added. The malt changes the starch in the grain to malt sugar. After a culture of bacteria has been added, the product is filtered.
When commercial yeast is made, it is called wort in its liquid form. When the wort ferments, the yeast is skimmed off. It is then pressed to free it from water.
Finally, the mass is molded and cut into cakes. Starch is usually added to compress yeast before the pressing, but it must be mentioned on the label if this process is used.