If a weak sugar solution is exposed to the air, in several days a light, frothy scum appears on the surface and the liquid begins to smell of alcohol. This change takes place because tiny plant cells called yeast have settled from the air into the liquid. They have found conditions favorable to their growth.

Man has long known that this process takes place and he has used it for thousands of years to make alcoholic beverages of all kinds. Sugar solutions made from molasses, potatoes, rye, corn, malt and hops, apples, and grapes have been exposed to the air to make alcohol, whiskey, beer, ale, cider, wine, and other beverages.

Probably through accident, it was also discovered that if bread dough were allowed to stand for some time before baking, very often a peculiar change took place. The flat lump of dough began mysteriously to swell and rise. It developed a strange but pleasant odor. When this dough was baked, instead of making a flat, heavy slab, it made a light, porous, soft bread!

In 1857, Louis Pasteur announced that he had discovered the explanation for these changes. He said they were due to the presence of tiny, one-celled plants called yeast. Yeasts belong to the fungi family, -and are tiny rounded colorless bodies. They are larger than most bacteria, but still so small that it would take from 1,200 to 1,600 of them laid side by side to make a centimeter. Yeast cells reproduce by budding. This means they send out projections which become cut off from the parent cell by a cell wall. Finally, these projections grow to full size. As they grow, they form substances called zymase and invertase.

These substances are called enzymes, and they have the power to ferment starch to sugar, and sugar to alcohol and carbon dioxide. As fermentation takes place, carbon dioxide is formed and rises to the top. Then it escapes, leaving the alcohol. Beer, ale, wine, and cider are fermented beverages in which yeast has changed some of the sugar to carbon dioxide and alcohol.

In bread-making, the carbon dioxide collects in bubbles in the dough, which makes it rise. Heat later drives off the carbon dioxide? thus making the bread porous and light.