The Science Behind the Invention
"The Oldest Organism Cultivated by Humans" (continued)

Yeasts are single-celled organisms called fungi that exist naturally in the soil and air. Scientists have identified more than 700 different species of yeast. Saccharomyces cerevisiae is the yeast that the Fleischmann Brothers turned into a commercial product, and the most common kind used in baking. Yeasts also are used to make beer, wine, and other alcoholic beverages. Not all serve humanity. Some cause infections in humans, and others make food spoil.

Yeasts play key roles in fermentation, a process in which complex molecules are broken down into simpler ones. Fermentation is at the heart of a huge industry that produces not just bread, beer, and wine but millions of gallons of ethanol for "gasohol," antibiotics, vitamins, and chemicals used to make many other products. That’s because different kinds of microbes can break down, or metabolize, different substances. In doing so, they produce different products that can be harvested and used.

In bread making, S. cerevisiae breaks down flour’s big starch molecules into sugars. Yeast uses the sugars as its own food to grow, releasing carbon dioxide gas. The tiny bubbles of CO₂ actually inflate bread dough, creating gas pockets that give bread a lighter texture. Those little holes in a slice of bread are gas pockets. Yeast doesn’t form much alcohol in the stages of fermentation that occur in bread making.

Yeast does a lot more. For instance, it helps produce a host of molecules that give bread a wonderful flavor and aroma. Yeast also helps change the two main protein molecules in wheat flour - gliadin and glutenin – into gluten. Gluten is an elastic substance, consisting of long, thread-like chains, that gives bread a nice chewy texture. Gluten chains also help trap those carbon dioxide bubbles that give baked goods a light, airy texture.