This practical involves students preparing a glucose-yeast mixture, leaving it to ferment and then testing the fermentation products, namely ethanol.
The practical may be conducted in two parts as the fermentation process may take a long time. Some teachers may ask technicians to keep an eye in the fermentation process in between lessons. As long as the temperature is kept within the correct range and the yeast is fresh and ‘in date' then there is not too much to go wrong in this practical.
Equipment required (per set)
Students may add the 5g of glucose to 50ml of warm (not too hot) water in the conical flask until dissolved (see Fig 1). The yeast is added and the cotton wool bung placed in the neck of the flask. The reason a cotton wool bung is used instead of a rubber bung is that the yeast-glucose mixture produces carbon dioxide as the fermentation process takes place which could cause a rubber bung to become dislodged.
- glucose powder (low hazard) approx 5g
- yeast powder (fast acting) approx 1g
- small conical flask
- cotton wool bung
- measuring cylinder
- boiling tube
- accurate balance (per group)
The mixture can be left in a warm place or in a water bath at round 30°C. Read the recommended temperature for the yeast you use as these temperatures vary with yeast variety.
Yeast powder can be purchased in supermarkets (for break making) or brewing shops (used in the production of beer and wines) Brewer's yeast comes in a variety of types requiring differing temperatures to achieve a good ethanol yield. Fast acting yeast cuts down the fermentation time and is recommended.
Once the mixture has fermented, students may test the gas produced in the conical flask using limewater (see Fig 2) by pouring the gas from the flask into the boiling tube and shaking. The limewater should turn cloudy if there is enough carbon dioxide present.
A teacher may want to demonstrate the distillation of the fermentation product using a Liebig condenser. Firstly the product needs to be filtered and poured into the distillation flask. Ethanol boils at 78°C and so collecting the fraction between 77-82°C should produce some ethanol. This should be burned or disposed of as soon at is produced.
Yeast contains an enzyme called zymase and this catalyses the fermentation process, the formula of which is below:
Glucose zymase → Ethanol + carbon dioxide
C6H12O6 (aq) → 2C2H5OH(aq) + 2CO2(g)