Why do I need yeast to make bread?

Julie Douglas

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Imagine the first loaf of bread you ever fell in love with. Maybe it was an unassuming sourdough or a roguish rye. Or perhaps it was a freshly baked baguette, and the first time you tore into its stiffened peaks you discovered its soft underbelly steaming from the heat of a hearth oven -- sugar, salt and a slight acidity commingling on your tongue.

Enough romanticizing. Let's talk yeast. It's alive! And technically it's not required for all types of bread. But if you want a hearty, barrel-shaped loaf of fluffy goodness, you'll unleash this single-celled organism belonging to the group of organisms called fungi into your bread bowl.

The most common type of baking yeast is known as Saccharomyces cerivisiae. But there are numerous strains. And these strains are bred for gas production and fermentation speeds, depending on the flavors that the baker or baking company is trying to tease out of the dough. Most home bakers are familiar with active dry yeast cells, which spring into action when mixed with water. According to "How Baking Works" by Paula Figoni, yeast cells are so tiny that one pound of compressed yeast contains 15 trillion cells.

So what's happening inside that humble lump of dough languishing on the counter? It's teeming with organic chemical activity -- a veritable fireworks display of fermentation and leavening.

After the ingredients for bread are mixed together, fermentation occurs when the yeast cells break down large starch molecules into sugars for energy. They use this energy for survival and reproduction. The sugars digested by the yeast "burp out" carbon dioxide and ethyl alcohol into existing air bubbles in the dough. And this causes the dough to rise.

In the meantime, while you're working with the dough these bubbles of carbon dioxide and alcohol burst, allowing for two proteins in the flour, glutenin and gliadin, to glom onto water particles. As they tango they become an elastic-like mass of molecules known as gluten. And the more gluten, the stronger your bread becomes, and the more it can act as a dome to keep in the symphony of organic chemicals that cause the dough to exponentially rise. All of which results in the delightful crater-like terrain of the finished product.

When the dough is left to rest it gives the gluten bonds a chance to relax, and, presumably, reflect on their journey ahead. Byproducts like organic acids and amino acids, along with sugar, salt and bacteria contribute to the developing flavor profile of the bread.

During the early stages of baking alcohol evaporates to a gas and helps to leaven the dough. The end result is a mouth-watering aroma wafting off the crust. An aroma so influential that researchers at the University of Southern Brittany in France found that its mere presence amped up altruism in strangers passing by a bread shop. Turns out they were far more willing than their non-bread-shop countertops to help someone pick up a handkerchief or glove that had gone amiss.

Behold, the power of bread.