To prepare and show the production of alcohol by yeast
Procedure
(i) To perpare alcohol using yeast
- Boil glucose solution to remove oxygen present.
- Add yeast and glucose to conical flask A.
- Add glucose to flask B. This acts as a control .
- Add a layer of vegetable oil to each flask to minimise oxygen exposure.
- Attach a fermentation lock to each flask. Tubing could also be used to connect flask to a tube of limewater.
- Place both flasks in the incubator at 30 oC overnight.
(ii) To show the presence of alcohol: Iodoform test for alcohol
- Filter the contents of each flask into test tubes.
- To each test tube, add potassium iodide solution
and sodium hypochlorite solution.
- Transfer to a water bath for 4-5 minutes.
- Allow to cool.
- Record and compare results.
- Replicate the investigation.
Sample Questions
2005 Q11 C
2004 Q7
Sample Answers
2005 Q11 C
1. Diagram showing explained anaerobic conditions eg. airtight lock on flask, layer of vegetable oil, bioling the glucose solution to ekiminate dissolved oxygen.
2. C6H1206 + O2 --------------------------> CO2 + H20
3. Idoform test - Potassium iodide & sodium hypochlrorite added to solution and warmed.
The colour is initially yellow brown as iodine is liberated and then becomes colourless. The formation of yellow crystals occurs if alcohol is present.
4. Carbon dioxide
5. No more bubbles are given off.
6. Alcohol kills the yeast/ all sugar is used up and reaction can no longer occur.
2004 Q7
(a) Yes - rate of enzyme reactions are affected by temperature
(b)
1. Diagram including labels and notiing of anaerobic conditions, eg. boiled glucose solution, layer of vegetable oil.
2. Sugar/ Glucose or starch
3. Carbon Dioxide
4. Same aparatus but with no yeast or boiled, denatured yeast in flask
A control is used as a comparison for the actual experiment to prove alcohol is produced by the yeast only.
5. No more bubbles given off
6. to test for the presence of alcohol in the product
7. Water
