How Bread Works

By: Marshall Brain

Experiment 1

One thing you'll learn in this experiment is that yeast does, in fact, produce carbon dioxide gas. To perform this experiment, you will need:

  • One large Ziploc-type freezer bag - The plastic bag should be able to hold between a couple of quarts to a gallon of water. (Usually the box that the bag comes in will state the bag's capacity.)
  • One envelope of "rapid-rise, active, dry yeast" from the grocery store
  • 1 cup (.24 L) lukewarm water (about 100 degrees F, 37.7 C) - When you stick your finger in it, it should feel neither warm nor cold.
  • 1/2 cup (.12 L) sugar

Let's get started!


  1. Take the 1 cup lukewarm water and mix the package of yeast into it. When you pour the yeast granules into the water, you allow the yeast cells to become active.
  2. Mix in your sugar.
  3. Pour the entire water-sugar-yeast mixture into the plastic bag. Push as much air as possible out of the bag and then seal it tightly shut.
  4. Put the plastic bag in a warmish place (see this section to find out how to turn your oven into 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 has produced. A yeast cell can process approximately its own weight of glucose (sugar) per hour, and from the glucose (C6H12O6), yeast produces carbon dioxide (CO2) and ethanol (C2H5OH) (two molecules of each). Although yeast cells are small, there are billions of them available from the packet of yeast. You should be able to see a noticeable amount of puffiness in your bag after two hours. You may want to go to bed and let the bag sit overnight -- it will get quite puffy if you let it.