Linda Ferris, age 11, of Wichita, Kansas, for her question:
What makes yeast grow?
Yeast grows by multiplying. It makes the bread dough grow by breathing out little bubbles of carbon dioxide. A single yeast is no more than one cell, too small for our eyes to see. There are countless varieties of these small members of the plant kingdom. The microscope shows that they range in shape from a round ball to an oval sausage. Small as it is, a yeast cell equals the weight of 150 average bacteria.
The multiplication of cells begins when food and temperature are just right. While the supplies last, the cells devour their own weight in food every few hours. Since the individual cells can grow no bigger, these must produce new cells. A bacterium multiplies by simply dividing in two.
It pinches in. at the waist, then parts and the two halves go their separate ways. If conditions are right, each may be full gown and ready to multiply by dividing in an hour or so. few yeasts also use this method of cell division. But the bulk of the; family multiplies by budding and also by spores. A well fed yeast cell soon sprouts a bump along one side. This is a bud which rapidly grows into a full sized yeast. The daughter cells remain attached to the mother cell and each is ready to sprout a new bud.
In a short time, the first yeast has multiplied into a colony of daughters and grandaughters. Under the microscope, they look like strings of sausages. This budding goes on at a great rate when the yeasty climate is warm, moist, full of sugar and maybe a little ammonia, Then a new generation is produced, one every hour or so. In six,. hours, an ounce of well fed yeast cells can grow into 64 ounces.
Once in a while;, a yeast cell develops spores. From one to four bitsy seeds grow inside the cell. The cell wall dissolves and the spores are cast out into the world. Ordinary air teems with countless types of microscopic yeast spores. They are ready to settle and start life on the surface of our food. Hence, we do not leave food standing around in the air, we cover left‑overs, or put them in the refrigerator where the climate is too cold for yeasts to grow.
Nothing happens to yeast. while it is sealed in a dry package. To make bread, we just soak the dry yeast in warm water. It comes to life at once. We mix it with the dough and it starts to feed and grow. It feeds on sugar, converting it into alcohol. Meantime it breathes out bubbles of carbon dioxide. This gas, trapped in little pockets, makes the dough spongy. It rises and grows with the yeast.
When the bread is spongy enough, it is put into a hot oven. The heat kills the yeast and evaporates the alcohol they made. But the little bubbles trapped in the dough remain to make it spongy and digestable.