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Fermentation of glucose using yeast

Class practical

Beer and wine are produced by fermenting glucose with yeast. Yeast contains enzymes that catalyse the breakdown of glucose to ethanol and carbon dioxide. In this experiment, a glucose solution is left to ferment. Students then test for fermentation products.
 

Lesson organisation


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

Chemicals

Glucose, 5 g

Yeast (as fast acting as possible), 1 g

Limewater 

Refer to Health & Safety and Technical notes section below for additional information.

Apparatus

Eye protection

Each pair of students requires:

Conical flask (100 cm3)

Boiling tube

Measuring cylinder (50 cm3)

Access to a balance (1 dp)

Cotton wool

Sticky labels

Warm water 30–40 °C (Note 1)

Health & Safety and Technical notes


Read our standard health & safety guidance

Wear eye protection. 

Glucose C6H12O6(s) - see CLEAPSS Hazcard.

Limewater, Ca(OH)2(aq) - see CLEAPSS Hazcard and Recipe book.

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.

 

Procedure


Lesson 1
a Put 5 g of glucose in the conical flask and add 50 cm3 of warm water. Swirl the flask to dissolve the glucose.

b Add 1 g of yeast to the solution and loosely plug the top of the flask with cotton wool.

c Wait while fermentation takes place.

d 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.

Apparatus set-up

e Gently swirl the limewater in the boiling tube and note what happens.

f Replace the cotton wool in the top of the flask.

Lesson 2
a Remove the cotton wool and note the smell of the solution.

b 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 burnt or poured away immediately. It must not be kept or used.

Apparatus set up 2

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.

Health and Safety checked September 2007

 

Weblinks


GCSEScience - for general information on glucose fermentation.

 

Page last updated on 31 July 2012