Jack Hathaway, age 10, of Amarillo, Texas, for his question:
HOW DOES YEAST WORK?
Yeast is a substance that bakers put in dough to make it rise. This yeast contains a mass of tiny, one celled plants called yeasts. Yeasts are among the simplest kinds of plants.
Enzymes from the yeast cells attack the starch in 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 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 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.
Bakers use two forms of commercial yeast: dry and the 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 indefinitely without spoiling and become active only when they are mixed with the right material.
Compressed yeast contains enough starch and moisture to start fermentation in a short time. It cannot be stored in the house very long without spoiling and must be kept cool until it is used.
Mixing yeast with dough to ferment it is called leavening the dough. With dry yeast, the baker must first make a sponge. This is a mixture of yeast, flour and water. Sugar may be added to hasten the fermentation.
The sponge is allowed to stand a few hours. Then it is mixed with flour and more liquid, such as water, milk, potato water or whey. This makes a dough which must be kneaded thoroughly, covered, and then set aside to "rise."
With compressed yeast, the baker does not have to prepare.a sponge.
In the olden days before yeast cakes were sold in stores, housewives had to make their own yeast. 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.
This process was uncertain because types of yeast not suitable for bread sometimes lodged in the batter.
Commercial yeast is prepared 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.
Next, a culture of the bacteria which turn milk sour is added. The mash is then filtered. The liquid, called wort, is then ready to serve as food for living yeast cells.
The yeasts increase rapidly in this liquid and when the fermentation of the wort is violent, the yeast is skimmed off. It is pressed to free it from water and finally the mass is molded and cut into cakes.