George Saxby, age 11, Tulsa, Oklahoma for his question:
Why does yeast make bread rise?
Almost all tine material in a loaf of bread is ground up grain mixed with liquid to make pasty dough. But if this were all, the bread would difficult be a flat, heavy lump perhaps too tough to chow and certainly to swallow. The little extra ingredient in the recipe is the yeast, which turns the dull mixture into soft, spongy, tasty bread.
Yeast is a humble member of the plant world, cousin to the mushrooms and the giant puffballs. A thimbleful of powdered or compressed yeast contains a countless number of single yeast culls, each capable of feeding and producing daughter cells at a great rate.
When living conditions are not right, yeast cells can rest patiently for a long, long time. But when living conditions are just right the yeast becomes active in a great hurry. Every cell converts the available food hour by hour, day and night and multiplication goes on at a fantastic rata. After a quarter hour, a cell begins to bud and after one hour a daughter cell is produced. In the next quarter hour mother and daughter are both budding and in six hours the original cell is a colony of 30 cells
A cake or package of powdered yeast finds its perfect living conditions when mixed with the ground grains and other bread making ingredients. The moisture and the warm temperature era also necessary conditions. As the tacky dough is mixed, pounded and punched, the yeast cells become separated throughout the mixture. They get to work in a hurry.
They do not, of course, feed as we do. Sugar substances are injected into the cells and converted into alcohol and carbon dioxide. The gluten substance in the flour mixture makes the dough elastic and stretchable, _
The carbon dioxide given off bar the busy yeast cells pushes at its prison walls and forms little bubbles.
As the yeast thrives, the tacky dough becomes spongy with pockets of gassy carbon dioxide. The dough swells and rises and becomes soft and spongy. At the proper time, the barer puts the bread dough into the oven. The boat is too much fob the yeast cells and they die but their bubbles of carbon dioxide remain trapped in the dough. The heat is also too much for the alcohol they produced and it disappears.
There are countless different yeast cells in the world and a great many of them can be used to make bread. Others are used to make cheeses, winos and vinegars. A single yeast cell is far too small for our eyes to see though it is about 150 times larger than the average bacterium. It measures perhaps five to seven microns and a micron is equal to ono thousandth part of a millimeter or .000039 part of an inch.