Science with Tara

What is Fermentation?

Fermentation is the process where enzymes in yeast convert glucose into ethanol and carbon dioxide.

Chemical Equation: C₆H₁₂O₆ (aq) → 2C₂H₅OH (aq) + 2CO₂ (g)

Wine and beer are the products of fermentation. Wine is produced by fermenting sugars in grapes. Beer is produced by fermenting malt. Fermentation is a slow process.

A simple fermentation set up is shown in the image.

Procedure (Simple Laboratory Set-up)

1. Mix glucose solution with yeast (catalyst) in a conical flask.
   
2. Maintain the temperature at 37°C (optimum temperature).
   
3. Seal the flask with a rubber bung and connect a test tube containing limewater to prevent air from entering the set-up.
   
4. Carbon dioxide gas produced will turn limewater milky (white precipitate).

The product is a dilute solution of ethanol (up to about 15% concentration).

Important Conditions & Notes

• Temperature: Optimum is 37°C.
  • Lower temperature → slower reaction
  • Higher temperature → enzymes are denatured. The enzyme structure in yeast is destroyed by heat and no longer acts as catalyst.
    
• Oxygen: Fermentation is anaerobic (must occur in the absence of oxygen). If oxygen is present, ethanol is oxidised to ethanoic acid (vinegar), making the drink sour. Fermentation can only take place in air tight apparatus. The rubber bung is secured tightly to the flask. The limewater in the test tube prevents air from entering the apparatus.
  
• Alcohol Concentration: The product of fermentation is a dilute solution of ethanol. It contains up to about 15% of ethanol. A higher alcohol content is not produced as it causes the yeast to die and fermentation to stop.
  
• Separation of Ethanol: The ethanol is separated from the water by fractional distillation. The ethanol will be distilled first as it has lower boiling point of 78^oC than water, which has boiling point of 100^oC. This allows for higher concentrations of alcoholic beverages or pure ethanol to be produced.