Science While You Dine

How Bread Works

If you examine a slice of bread closely, you will see that it is full of air holes.

This makes it spongy and soft. Notice, too, that bread is moist.

If you let a slice of bread sit out on a table for a day you will realize just how moist fresh bread is!

Bakers use two facts to make soft, spongy, moist bread:

	First, yeast (a single-cell fungi) will eat sugar, and from the sugar create alcohol and carbon dioxide gas as waste products. The carbon dioxide gas created by yeast is what gives bread its airy texture. The alcohol, which evaporates during baking, leaves behind an important component of bread’s flavor.
	Second, wheat flour when mixed with water and kneaded becomes very elastic. The mixture becomes stretchy like a balloon because of a protein in wheat known as gluten. Gluten gives bread dough the ability to capture the carbon dioxide produced by yeast in tiny flour balloons.

Those Gas-y Yeast Cells!

Try this experiment to make carbon dioxide from yeast.

What You Need:

	One large Ziplock Freezer Bag
	One envelope Rapid-Rise Active, Dry Yeast
	One cup lukewarm Water (When you stick your finger in it, it should feel neither warm nor cold).
	½ cup Sugar

What You Do:

	In a bowl, mix the water and the yeast. When you pour the yeast granules into the water, you activate the yeast cells.
	Mix in sugar.
	Pour the entire mixture into the plastic bag. Squeeze and push as much air as possible out of the bag.
	Seal the bag tightly.
	Put the plastic bag in a warmish place. Come back in about an hour.

When you come back to your experiment, you’ll notice that yeast cells do a really good job of creating carbon dioxide. You will see that the bag has partially filled with the gas, and that the liquid is full of carbon dioxide bubbles that the yeast produced. A yeast cell can digest approximately its own weight of sugar (glucose) per hour. From the glucose, yeast produces two molecules each of carbon dioxide and alcohol (ethanol). Although yeast cells are very small, a packet of yeast contains billions of them. You should be able to see a noticeable amount of puffiness in you bag after 2 hours. It will get quite puffy if you leave it sit overnight.

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