Gail Waters, age l2, of Gastonia, NC. for her question:
How does yeast make bread rise?
The first bread was baked from a soggy paste of powdered grain. It was flat and it was heavy. It was hard to chew and hard to digest. Then the magic of yeast was discovered, and the dreary old dough became light and spongy. History has no record of the master baker who made the great discovery.
The happy trick of using yeast to make bread was discovered by accident and rediscovered many times. Ancient bakers learned that their bread often tended to be lighter if the pasty dough was left a while in the air before baking. Wads of the spongiest doughs were saved and mixed with the next batch of dough. In l857 a french scientist discovered that this bread rising trick was done by colonies of living, one celled fungus plants,
Large quantities and assortments of these yeasts swarm in the air and the soil. Some are bread making types, and the best of them are prepared and packaged for our baking jobs. Under a microscope you may see yeast as a colony of sausage shaped cells often linked like sausages in a chain. The single fungus plants are encased in cellulose cell walls and eager to grow by absorbing chemicals from their environment.
Bread making yeast thrives when surrounded by warm, moist starch and sugar. It then produces enzymes to ferment or change the starch and sugar into alcohol and carbon dioxide. The cells grow and multiply by budding new cells. When conditions are right, in one hour the bud from a mother cell may be a grown daughter ready to bud. As the colony doubles its numbers each hour, generations of mother and daughter cells often remain linked in long chains.
This is the process that makes bread rise. The baker mixes yeast with a dough of starchy flour and a little sugar and lets it work in a temperature of 80 to 85 Fahrenheit degrees. The yeast produces distase, zymase and invertase to use its environment in order to grow and multiply. These busy enzyme chemicals ferment some of the doughy starch and sugar, and one substance formed in the conversion process is carbon dioxide.
The pasty dough becomes riddled with bubbles of carbon dioxide gas. These puffy holes cause the bread to swell and rise. At the right time, the spongy dough is popped into a hot oven. The bubbles of carbon dioxide remain trapped, and the dough cooks around them.
The busy yeast enzymes also produce alcohol as they ferment the doughy starch and sugar. The alcohol is in the mixture when the dough goes into the oven. But alcohol has a low boiling point, and all of it evaporates as the bread cooks. And the bread stops rising when the oven heat destroys the yeast that created the spongy bubbles of carbon dioxide.