In short: Yeast is a single-celled fungus that breaks down sugar and produces carbon dioxide gas. In this simple experiment, you trap that gas in a balloon to see fermentation happen in real time.
What is yeast, anyway?
Yeast is a fungus—but not the kind that grows on soil or logs. It's a single-celled fungus called Saccharomyces cerevisiae, also known as baker's yeast. You can buy it at any grocery store, and it's the same tiny organism that bakers use to make bread fluffy and brewers use to make beer.
Yeast is so small that one cell measures only about 5–10 micrometers across—far too small to see without a microscope. Even though it's a fungus, it's nothing like a mushroom. It lives on its own, floating around waiting for food. In nature, yeast thrives on rotting fruit, tree sap, and other decaying plant material where it helps break down organic matter.
How does this experiment work?
When yeast finds sugar, something amazing happens: it eats the sugar and releases two things—ethanol (the same alcohol in beer) and carbon dioxide gas (CO₂). This process is called fermentation, and it's a form of cellular respiration—the way yeast breathes and gets energy.
Here's the setup:
-
Gather your materials:
- 1 tablespoon of active dry yeast
- 1 teaspoon of sugar
- 1 cup of very warm water (105–115°F, or about 41–46°C)
- One small balloon
- One clean bottle or jar
-
Mix it together: Combine the yeast, sugar, and warm water in your bottle. Stir it gently.
-
Seal the balloon: Stretch the balloon's mouth over the bottle's opening. Make sure it's snug so gas can't escape.
-
Wait and watch: Place the bottle in a warm spot—a sunny windowsill works great. Within 30 to 45 minutes, you'll see the balloon start to inflate. The CO₂ gas produced by the yeast fills the bottle and pushes the balloon outward. It's fermentation in action.
Why does the balloon inflate?
The balloon fills because of gas pressure. As the yeast eats sugar, it produces lots of CO₂ gas. This gas has nowhere to go—the bottle is sealed and the balloon is the only outlet. The pressure builds up inside, pushing air into the balloon and making it grow. You're literally watching a fungus breathe.
This is the exact same process that makes bread rise in an oven. The CO₂ bubbles get trapped in dough, making it expand and turn soft and spongy. In this experiment, instead of dough, you're using a balloon to catch and show the gas.
What does this teach us?
Yeast is often thought of as something people use, but this experiment proves yeast is a living, respiring organism with its own biology. It eats, it produces waste (gas), and it grows. Just like animals breathe oxygen and release carbon dioxide, yeast takes in sugar and releases CO₂. The balloon is proof.
This is why yeast is so valuable: for thousands of years, humans have harnessed this tiny fungus to make bread, beer, wine, and more. But yeast was doing this long before we discovered it—it was busy fermenting sugar in nature all along.
Safety first: Dry yeast is completely safe to handle and non-toxic. Never taste the mixture or drink the water afterward. If you grow mushrooms from a kit, only eat them when they're fully grown, cooked, and from your kit—never pick or eat wild mushrooms; look-alikes are impossible for non-experts to tell apart, and some are poisonous.
The short version
Yeast is a single-celled fungus that eats sugar and produces CO₂ gas. In this experiment, you mix yeast with warm sugar water and seal a balloon over the bottle. Within 30 to 45 minutes, the gas builds up and inflates the balloon, showing you fermentation in real time. It's the same process that makes bread rise and beer bubble—proof that even tiny fungi are alive and respiring.
Curious questions
Is yeast alive?
Yes. Yeast is a single-celled fungus that eats, grows, and reproduces. The balloon experiment proves it's alive because it produces CO₂ gas as it ferments sugar—just like respiration.
Why do I need warm water, not cold?
Yeast cells work faster when it's warm, just like most living things. Warm water (around 105–115°F) helps the yeast eat the sugar quickly and produce CO₂ faster. Cold water would slow everything down.
Is the gas in the balloon dangerous?
No. The gas is CO₂, the same gas plants breathe in and that's in carbonated drinks. It won't hurt you, but do not deliberately inhale from the balloon.
Can I repeat this experiment?
Yes, many times. Use fresh yeast and sugar each time. Try changing one thing—water temperature, sugar amount, or balloon size—and see how it affects the results.
How is this the same as bread-making?
In bread dough, yeast eats sugar and produces CO₂ bubbles. These bubbles get trapped in the dough, making it rise and puff. In your experiment, the balloon catches the gas instead of dough, so you can see exactly what's happening.
Sources & further reading
- Science World (Canada) — Yeast-Inflated Balloons
- University at Buffalo News — Meet baker’s yeast, the budding, single-celled fungus that fluffs your bread
- NIH/PMC — History and Domestication of Saccharomyces cerevisiae in Bread Baking
A note for growers: only grow and eat mushrooms from our kits or from species you know are safe, and always check with an adult. Never eat a wild mushroom unless an expert adult says it’s safe.
