Fermentation of glucose using yeast

This experiment takes time. The solution needs to ferment between lessons, especially if you are distilling the final solution to produce ethanol.

Equipment

Apparatus

• Eye protection
• Conical flask (100 cm³)
• Boiling tube
• Measuring cylinder (50 cm³)
• Access to a balance (1 decimal place)
• Cotton wool
• Sticky labels
• Warm water 30–40 °C (note 1)

Chemicals

• Glucose, 5 g
• Yeast (as fast acting as possible), 1 g
• Limewater

Apparatus notes

1. A source of warm water is required. Larger conical flasks can be used, but this dilutes the carbon dioxide concentration, and makes testing for carbon dioxide with limewater more difficult.

Health, safety and technical notes

• Read our standard health and safety guidance
• Wear eye protection. 
• Glucose C₆H₁₂O₆(s) – see CLEAPSS Hazcard HC040c.
• Limewater, Ca(OH)₂(aq) – a saturated solution of calcium hydroxide in water – see CLEAPSS Hazcard HC018 and Recipe book RC020.

Procedure

Lesson 1

1. Put 5 g of glucose in the conical flask and add 50 cm³ of warm water. Swirl the flask to dissolve the glucose.
2. Add 1 g of yeast to the solution and loosely plug the top of the flask with cotton wool.
3. Wait while fermentation takes place.
4. Remove the cotton wool and pour the invisible gas into the boiling tube containing limewater. Take care not to pour in any liquid as well.
5. Gently swirl the limewater in the boiling tube and note what happens.
6. Replace the cotton wool in the top of the flask.

Lesson 2

1. Remove the cotton wool and note the smell of the solution.
2. The solution may be retained for a teacher demonstration of distillation.

Teaching notes

Class results can be pooled to demonstrate distillation.

If you want to do this, carefully decant or filter the solution into your distillation flask. (Significant quantities of yeast will produce foaming and this can be carried over into the product.)

Collect the fraction between 77–82 °C. (Ethanol boils at 78 °C.) This fraction should burn easily compared with the non-flammable original solution.

The ethanol must be poured away immediately. It must not be kept or used.

If fermentation is not rapid because of the yeast used, then the whole experiment can be carried over to the second lesson.

Yeast has an enzyme called zymase and this catalyses the fermentation process.

Glucose zymase → Ethanol + carbon dioxide

C₆H₁₂O₆ (aq) → 2C₂H₅OH(aq) + 2CO₂(g)

Student questions

Here are some possible questions to ask students:

• How do you know fermentation is taking place?
• Which gas does limewater test for?
• Suggest other methods for measuring the speed of this reaction.