The purpose of any leavener is to produce the gas that makes bread rise. Yeast does this by feeding on the sugars in flour, and expelling carbon dioxide in the process.
While there are about 160 known species of yeast, Saccharomyces cerevisiae, commonly known as baker's yeast, is the one most often used in the kitchen. Yeast is tiny: Just one gram holds about 25 billion cells. That amount of fungi can churn out a significant amount of carbon dioxide, provided it has the simple sugars it uses as food. Fortunately, yeast can use its own enzymes to break down more complex sugars—like the granulated sugar in the activity below—into a form that it can consume.
Make a yeast-air balloon to get a better idea of what yeast can do.
Did You Know?
What do I Need?
1 packet of active dry yeast
1 cup very warm water (105° F-115° F)
2 tablespoons sugar
a large rubber balloon
a small (1-pint to 1-liter) empty water bottle
Kids, please don t try this at home without the help of an adult.
What do I do
Stretch out the balloon by blowing it up repeatedly, and then lay it aside.
Add the packet of yeast and the sugar to the cup of warm water and stir.
Once the yeast and sugar have dissolved, pour the mixture into the bottle. You ll notice the water bubbling as the yeast produces carbon dioxide.
Attach the balloon to the mouth of the bottle, and set both aside.
After several minutes, you ll notice the balloon standing upright. If you don t see anything happen, keep waiting. Eventually, the balloon will inflate.
What's Going On?
As the yeast feeds on the sugar, it produces carbon dioxide. With no place to go but up, this gas slowly fills the balloon.
A very similar process happens as bread rises. Carbon dioxide from yeast fills thousands of balloonlike bubbles in the dough. Once the bread has baked, this is what gives the loaf its airy texture.
What Else Can I Try?
Try the same experiment, but this time use about a tablespoon of baking powder instead of yeast, and leave out the sugar. What differences do you notice? Which leavener takes longer to fill up the balloon?
Also, try the same experiment using hotter and colder water. Use a thermometer to measure the temperature of the water. At what temperature is the yeast most active? At what temperatures is it unable to blow up the balloon?