From the previous two experiments, you can see that yeast cells produce plenty of carbon dioxide. The reason why bread bakes up so airy is because the bread dough captures and holds the carbon dioxide that the yeast produces. It does this because flour contains a protein called gluten. To see gluten in action, try this experiment:
- Mix 1/2 cup water and 1/2 cup flour in a bowl.
- Stir the mixture with a fork to wet the flour. What you will have initially is a lumpy, grainy mass.
- Lift the fork out of this mass. You will find that the mass is quite watery.
- Now keep stirring for about five minutes (set a timer for five minutes -- it is a long time when you are stirring!). Over time, the batter will smooth out.
- Keep stirring, and a funny thing will happen when you lift the fork slowly from the bowl: The batter will have become quite elastic! Not elastic like a rubber band, but elastic enough that you'll be able to pull away up to a 1-inch-long thread of batter with the fork. This mixture is now extremely smooth and not watery at all.
That elasticity is caused by the gluten in the flour. Gluten is a protein that forms thread-like chains. By stirring (or more commonly, kneading) the dough, the gluten develops into long, interlaced chains. Kneading is better for developing these chains because kneading is gentle -- it does not cut the chains up. When you knead bread dough, you are creating gluten chains. If you were to skip the kneading part, your bread would not rise very well -- all the carbon dioxide in the yeast would bubble up to the top and escape, rather than being captured inside the elastic dough.