Tyrosine + oxygen Melanin + Water
When an inhibitor, such as ethanol, is added the browning of the sliced apples or potatoes can be stopped. The oxidation process can also be stopped by other factors. These are the dependant variables.
Dependent variables:
- Temperature
- Acids and Bases (pH-level)
- Amount of oxygen
- Preservative chemicals
- Time
The dependent variables should be kept constant (be controlled) or else they will affect the outcome. If the sliced apples get heated they will denature the enzyme and this will have an affect on our result. Therefore, the temperature must be kept constant. This will be done by using a thermometer making sure the room temperature and the sliced apples are not too cold or too warm. In our case the temperature will be kept constant at 20 degrees Celsius. By adding lemon juice or another acid we would lower the pH-level of the sliced apples and this would also inactivate the enzyme. Thus, we should keep the pH-level constant by adding an indicator that would show if the pH-level gets changed. By reducing the level of oxygen, for instance by putting the sliced apples under water or vacuuming them, the outcome will also be affected. Therefore we should make sure the level of oxygen is constant. Adding certain preservatives such as sulphur dioxide will stop the oxidation process. These preservatives can only affect the outcome if they are added by us – they don’t occur naturally in apples or potatoes. Of course, we will avoid adding preservatives. When more time is used, the more will the oxidation process take. Therefore we should use a stop watch to be aware of the time spent for the experiment.
Independent variable:
We will vary the concentration of ethanol; therefore this is the independent variable. The darker the colour of the pieces, the more oxidation has taken place. From this we can see which concentration is the most effective for inhibition of tyrosinase. We will use a spectrophotometer to see which piece has the most intense colour. A spectrophotometer is a device that can measure the light intensity.
Material list and Procedure:
Materials:
-
6 pieces of sliced apples (1 cm3)
- Ethanol (1 %, 5 %, 10 %, 15 %, 20 %)
- Spectrophotometer
- Thermometer
- Pipette for measuring exactly 1.0 mL
- Indicator
- Stop watch
- Piece of paper and a pen
Procedure:
- Place the sliced apples on the piece of paper – number them from 1 to 6
- Add a drop of indicator to the different pieces.
- Add 1.0 mL of ethanol to each of the pieces, except number 1 as this will be the control piece.
- Remember to start the stop watch as soon as adding the ethanol and remember to stop after 15 min.
- Stop the experiment after min. and compare the colour change of the different pieces by using a spectrophotometer.
Recording Data:
Table showing the relationship between the concentration of ethanol and the measured light intensity:
- Which concentration gave the best inhibition of tyrosinase?
- Explain why a certain concentration gives a better inhibition.
- Make a graph showing the relationship between the concentration of ethanol and light intensity.
Light intensity (A)
Concentration of inhibitor (%)
