Yeast Fermentation
Summary
Yeast eats sugar without oxygen and gives off carbon dioxide and ethanol — the fermentation behind bread, beer, and wine. You’ll be able to explain why yeast uses this inefficient pathway, why the CO₂ inflates a balloon while the ethanol stays dissolved, and why the catalyst1 enzymes are so fussy about temperature.
History
Fermentation is one of the oldest biotechnologies, practiced for at least 9,000 years for making beer and wine. Ancient peoples knew that grape juice became wine and grain became beer, but the mechanism remained mysterious until the 19th century.
Louis Pasteur2 proved in the 1850s that living yeast cells were responsible for fermentation - not spontaneous chemical action. This work founded microbiology. In 1897, Eduard Buchner showed that cell-free yeast extract could still ferment sugar, proving that enzymes (not life itself) catalyzed the reaction. He received the 1907 Nobel Prize.
The equation for fermentation appears simple, but represents one of the most important biochemical pathways: glycolysis followed by alcohol fermentation. Every cell in your body uses the first half of this process to extract energy from glucose.
Hazards & preparation
PPE: none required — this is a food-safe, low-hazard experiment.
- Use warm, not hot water (30–35°C) — water above ~45°C kills the yeast.
- Don’t seal the bottle rigidly; let the balloon take the gas so pressure can’t build.
Disposal: the spent, slightly alcoholic yeast slurry is harmless — pour it down the drain. See the Safety page.
Materials
- Baker’s yeast (active dry) - 7g (1 packet or 2 teaspoons)
- Dextrose or sugar - 25g (2 tablespoons)
- Warm water (30-35°C) - 250mL
- Balloon (medium size)
- Plastic bottle - 500mL
Procedure
- Pour 250mL warm water into the bottle (it should feel warm, not hot).
- Add 25g sugar and swirl to dissolve.
- Add 7g yeast and swirl gently to mix.
- Stretch the balloon’s opening over the bottle neck.
- Place in a warm spot (25-35°C).
- The balloon inflates noticeably within 30-60 minutes and fully in 2-4 hours.
- The solution turns cloudy and smells of bread/alcohol.
What you should see
Within an hour a froth of fine bubbles builds on the surface and the balloon starts to stand up; over a few hours it swells taut with CO₂. The liquid goes cloudy and gives off a warm, yeasty, faintly boozy smell — the ethanol staying behind in the bottle.
| Symptom | Likely cause | Fix |
|---|---|---|
| Balloon never inflates | Water too hot (killed yeast) or yeast dead | Use 30–35°C water and fresh yeast |
| Very slow | Too cold, or too little sugar/yeast | Move somewhere warmer; check amounts |
| Balloon inflates then stops | Sugar used up | Expected once fuel runs out; add more sugar to continue |
| Bottle bulges, no gas in balloon | Balloon not sealed on the neck | Fit the balloon snugly over the mouth |
The reactions
\[\ce{C6H12O6 -> 2 C2H5OH + 2 CO2}\]
glucose → ethanol + carbon dioxide gas (the CO₂ inflates the balloon; the ethanol stays in solution).
The Science
Yeast performs anaerobic respiration (fermentation) when oxygen is limited:
- Glycolysis: Glucose is broken down to pyruvate (releases some energy)
- Fermentation: Pyruvate is converted to ethanol and CO₂ (regenerates NAD⁺ so glycolysis can continue)
The process is much less efficient than aerobic respiration (2 ATP vs 36-38 ATP per glucose), but allows yeast to survive and grow without oxygen. Each glucose molecule produces two CO₂ molecules, and the gas has nowhere to go but into the balloon. The distinctive “yeasty” aroma comes from ethanol and various byproducts.
Questions to Explore
Why does yeast switch to fermentation when oxygen runs out? Fermentation yields only 2 ATP vs. 36–38 with oxygen. Why choose the inefficient path?
Hint / answer
Without oxygen, the efficient aerobic route is blocked entirely, so 2 ATP beats zero. Crucially, fermentation regenerates the NAD⁺ that glycolysis needs, letting the cell keep extracting some energy and stay alive until oxygen returns.
Why does CO₂ inflate the balloon while ethanol stays dissolved? Both are products — why the different fates?
Hint / answer
CO₂ is a gas at room temperature (its small molecules barely attract each other), so it bubbles out and fills the balloon. Ethanol is a liquid that mixes freely with water thanks to hydrogen bonding, so it stays dissolved in the bottle.
Why does temperature matter so much? Optimal is 30–35°C; above ~45°C yeast dies. What happens to the enzymes?
Hint / answer
Warmth speeds the enzyme reactions up to a point, but the enzymes are proteins with precise folded shapes. Too much heat unfolds (denatures) them permanently, so above ~45°C the machinery is wrecked and the yeast dies.
Why can’t yeast ferment salt or starch directly? Sucrose must first be split by invertase. What does that say about enzymes?
Hint / answer
Each enzyme fits only specific molecules, like a lock and key. Yeast has enzymes for simple sugars, so sucrose must first be cut into glucose and fructose; it has no enzyme for salt or intact starch, so it can’t touch them. Enzymes are highly specific.
How did Pasteur prove fermentation needs living cells? Design an experiment with boiled (dead) yeast — what would you predict?
Hint / answer
Set up two identical bottles, one with live yeast and one with yeast boiled first, and watch which inflates. The live one ferments; the boiled one doesn’t — showing living, functioning cells (their intact enzymes) are required, as Pasteur argued.
Going further
- Prove it’s CO₂. Pipe the balloon’s gas through limewater — it should turn milky, confirming carbon dioxide.
- Race the sugars. Compare glucose, sucrose, and fructose (and a sugar substitute) for how fast each inflates the balloon.
- Find the sweet spot. Run identical bottles at fridge, room, and warm temperatures to map yeast’s temperature response.
Footnotes
Catalyst — A substance that speeds up a reaction without being consumed by it.↩︎
Louis Pasteur — French chemist (1822–1895) who discovered molecular chirality by separating tartrate crystals by hand.↩︎