Yeast Fermentation

Watch yeast produce CO₂ and alcohol

Difficulty: Easy | Time: 2-24 hours | Visual Impact: Medium-High

Historical Context

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 Pasteur 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.

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

  1. Pour 250mL warm water into bottle (test: should feel warm, not hot)
  2. Add 25g sugar, swirl to dissolve
  3. Add 7g yeast, swirl gently to mix
  4. Stretch balloon opening over bottle neck
  5. Place in warm location (25-35°C)
  6. Balloon inflates noticeably within 30-60 minutes, fully inflated in 2-4 hours
  7. Solution becomes cloudy and smells of bread/alcohol

Reaction

\[\ce{C6H12O6 -> 2 C2H5OH + 2 CO2}\]

Glucose → Ethanol + Carbon dioxide

The Science

Yeast performs anaerobic respiration (fermentation) when oxygen is limited:

  1. Glycolysis: Glucose is broken down to pyruvate (releases some energy)
  2. 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.

Why the balloon inflates: Each glucose molecule produces two CO₂ molecules. The gas has nowhere to go but into the balloon.

The smell: The distinctive “yeasty” aroma comes from ethanol and various byproducts.

Variations

  • Compare different sugars (glucose, sucrose, fructose)
  • Test different temperatures
  • Compare yeast amounts

Resources