Joanne Matsco, age 13, of Allentown, Pa.,‑ for her question:

What is yeast?

In olden days, a baker saved a wad of dough before baking his batch of bread. He mixed this wad of so‑called sourdough with the next batch to make it light and spongy. If he lost car forgot his sourdough, he could get a new one merely by leaving a doughy paste bP° flours water and sugar out in the air. Yeast spores of some kind would be sure to settle, set up housekeeping and work their magic to make the bread light and digestible.

The yeasts are midgets of the fungus plant worlds somewhat larger than the dainty diatoms and the busy bacteria.  Science places them in the pant class Ascomycetes, along with molds, mildews; truffles and morels. They are separated from their diminutive  order Saccharomycetales, meaning the sugar‑eating fungi. They are single celled plants and our world abounds with teeming varieties of these little sugar eaters.

Yeast cells come in a variety of sizes, though on the average it takes about 5,000 of them to measure an inch. A yeast cell is most often sausage‑shaped, though some varieties are round or rod‑shaped. Each midget plant is able to process its own food., grow and multiply. As a rule, it multiplies by budding, A well‑fed mother yeast cell starts a bud along one side. It grows until it becomes the proper size to form a unit of plant life.

The young cell may remain loosely attached to the mother cell and soon produces a daughter of its own. The microscope often shows a dab of yeast to be a loosely linked colony of mother and daughter cells. Beasts also may produce spores, tiny seedlets which float through the air hoping to find likely places to set up new colonies of yeast settlements. However, these roving yeast spores easily are destroyed by heat, cold or disinfectant.

A likely place for a yeast settlement must be warm and moist and abounding in food supplies containing sugars, A wad of pasty flour left in the open is sure to be invaded by roving yeast spores ‑ which change it to sourdough. When conditions are Just right, a yeast cell will consume its own weight in food every hour ‑ and also produce a daughter. The food is absorbed through the cell walls and put to use. In the process, sugar is converted into carbon dioxide gas and alcohol. In bread dough, the gas oozes out and punctures the tacky paste with springy little bubbles, and so the dough rises, When the loaf is baled, the alcohol In the mixture evaporates, the protein‑rich yeast cells are cooked and the gassy bubbles trapped in the dough.

We can trap wild yeasts from the air to make sourdough and use them to make bread, but some yeasts are more useful than others. Through the ages, we have selected the best bread‑makers and a modern bakery uses these tried and true tame yeasts to make our daily' bread. Wine and beer also are made with yeasts. In these items, the alcohol is retained in the finished product.