Steven Gregory, age 10, of Seattle, Washington, for his question:

What exactly is yeast?

Yeast is an invisible servant that works for us behind the scenes. Its most important fob is to make our daily bread light, spongy and digestible. It ;;urns grapes into wine and wine into vinegar. Men have known how to put yeast to work for thousands of years. But not until the Age of Science did we know how it worked or what it looked like.

We call a single yeast a cell and we need a microscope to see it. Twelve average yeast cells equal the thickness of a hair from your head. The thickness of a new dime is equal to about 200 yeast cells. They look like oval blobs under the microscope, usually strung together like sausages.

Yeast belongs to the plant kingdom. It is the one‑celled member of the fungus family. This makes it a relative of the mushroom. The big,, round puff ball is the giant of this family, the little yeast cell is the midget. The fungi are unlike other plants because they have no green chlorophyll. It is chlorophyll that enables a plant to make food from air, water and sunlight. Hence fungus plants need a richer diet than green plants.

Yeast cells devour sugar, nitrogens, ammonias and other chemicals. In the process the sugar is broken down into alcohol and carbon dioxide. But yeast calls are delicate and choosy. They need moisture before they can carry on this chemical process. They must be warm, but nit too warm.

When conditions are dust right a colony of yeast cells doubles its weight in two hours. As a cell becomes well fed it sprouts into a little bud. The bud grows into a daughter cell. Usually it remains attached to the mother cell. When food is plentiful it takes about an hour for a cell to bud and produce a full grown daughter. During the next hour both mother and daughter produce a new cell apiece and the original little sausage becomes a chain of four linked sausages.

Bread dough is a mixture of flour, water, sugar and, maybe, a few flavoring ingredients.. It is flat and heavy. Yeast, mixed in warm watery is added to the dough. The little cells start at once to feast. The sugar is broken down into alcohol and carbon dioxide gas. The gas becomes tripped in the dough. It forms little bubbles and the dough swells up. It becomes light and puffy.

At the right time the spongy dough is put into a hot oven. The heat destroys the yeast cells and dries out the moist dough. The alcohol in the dough evaporates and the gas bubbles are baked into the bread.

Bread yeast is a very special variety, grown in huge tanks. But there are countless other varieties in the world. Yeast tolls are everywhere, in  the soil and on the skin of every ripe, juicy fruit. On the skin of a ripe grape there are maybe 300,000 yeast cells. If the grape has been bitten by a bird or a wasp there may be millions of yeast cells. They have fed and multiplied on the sugar freed from the pulp of the fruit.