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How does yeast cause bread to rise?

I don't know about you, but I've been in the kitchen more than usual. I'm cooking new recipes and I have been baking A LOT of bread products. Which means, I've been using a lot of yeast. I've tried different brands and types of yeast, and they each do their own thing a little differently than the others, but ultimately, they all help my bread rise. But how exactly?

Yeast is a single-celled fungi. As fungi, yeast are related to the other fungi such as mushrooms, molds that ripen blue cheese, and the molds that produce antibiotics for medical and veterinary use.

Yeast cells are egg-shaped and can only be seen with a microscope. It takes 20 billion yeast cells to weigh one gram, or about 1/7th of a package of yeast (2.25 teaspoons) The scientific name for the yeast used by baker’s like myself is Saccharomyces Cerevisiae, or “sugar-eating fungus”. This species of yeast is very strong and capable of fermentation, the process that causes bread dough to rise.

Yeast cells digest food to obtain energy for growth (sound familiar?). Their favorite food is sugar (again, sound familiar?) in its various forms: sucrose (beet or cane sugar), fructose and glucose (found in honey, molasses, maple syrup and fruit), and maltose (derived from starch in flour).

The alcoholic fermentation process produces useful end products, carbon dioxide and ethyl alcohol. These end products are released by the yeast cells into the surrounding liquid in the dough. When baking bread, yeast ferments the sugars available in the flour (and/or from added sugar), and the carbon dioxide gas cannot escape because the dough is elastic and stretchable. As a result of this expanding gas, the dough rises. The ethyl alcohol (and other compounds) produced during fermentation produce the typical flavor and aroma of yeast-leavened breads.

Fermentation occurs naturally as well. One example is in the late fall berries break open when they are overripe and full of sugar. Natural yeast organisms lodge on the surface of these berries, which then become fermented and alcoholic.

When using yeast in baking, there are two types. Fresh yeast (also called wet, cake, and compressed yeast) comes in small square cakes that are made of fresh yeast cells. These blocks of fresh yeast are 70 percent moisture, and therefore are quite perishable.

There are also dry yeasts (the type I use). A key difference between active dry yeast and instant yeast is the size of the granules; active yeast has larger granules while instant has been ground into a finer texture. Active dry yeast is dormant until proofed, meaning dissolved in a small amount of lukewarm water (about 110 degrees Fahrenheit). Instant yeast gets its name because it can be added directly to other ingredients and does not need to be dissolved in water before using it as active dry yeast does.

As a side note, if you are wanting to bake and have some old yeast you are unsure is still alive, you can easily test it. Pour about 1/4 cup lukewarm water into a bowl and sprinkle a small amount yeast and a dash of sugar. Stir, and let sit for a few minutes; if it dissolves and the liquid bubbles, it means the yeast is active.

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