2. Cut them into smaller pieces by the razor blade so that crushing can be done easily.
3. Crush the banana stored for 1 day thoroughly with mortar and pestle to extract their juice as much as possible. Add in 20ml of distilled water (measured by a measuring cylinder) to make the crushing easier.
4. Pipette up 0.5ml of the extracted solution and transfer it to a test tube labeled “1 day”.
5. Pipette up 0.5ml of the extracted solution and transfer it to a spot on the spotting tile labeled “1 day”.
6. Repeat steps (3) to (5) for the banana stored for 3 days and banana stored for 5 days. Label the test tube/spot as “3 days” and “5 days” respectively.
7. Pipette up 0.5ml of the distilled water to a test tube and a spot which labeled “water” as control setup.
8. Add 2 drops of iodine solution to each tile with solution, mix them well.
- Benedict’s test for reducing sugar
- Add 10ml of Benedict’s solution to test-tube labeled “1 day”. Mix thoroughly.
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Heat the test-tube and its content in a hot water bath of 80oC for 5 minutes.
- Repeat the steps (1) to (2) with test-tube labeled “3 days”, “5 days” and “water” respectively.
4. Remove the tubes from the hot water bath and put them back into the test-tube rack, allow them to cool down and let the precipitate its content in a the (if any) in the tubes to settle foe 15 minutes.
9. Record the result and tabularized it.
(b) Method of changing the independent variable
Independent variable: presence of reducing sugar in the extracts, presence of starch in the extracts.
Though we cannot measure the amount of reducing sugar accurately, we can still compare their result, like the amount of precipitate formed and color intensity, so a rough ranking can be shown. It is the same case for the iodine test, by comparing the color intensity change, we can get a rough ranking.
(c) Way of measuring the dependent variable
Dependent variable: Result recorded from the experiment, like the amount of precipitates formed and the change in colors.
We can first set up a scale and each time compares the result under this same scale. Like we can first define the scale of the amount of precipitates formed thus we can get a more factual result but not only by objective observation.
(d) Need for the controlling other variable
Other variable: Volume of the extract of banana stored for 1 day, 3 days, 5 days and distilled water, time allowed for the test tube to take a hot bath, temperature of though out the experiment.
This is to ensure the variation of the results in different setup in only due to the independent variable, but not other environmental factor.
(e) Precaution
(I) Benedict’s test for reducing sugar
-Wear safety spectacles.
- Use Water bath for heating process, avoid direct heating.
- Before lighting Bunsen burner, check that the gas tubing is properly connected, and
ensure that the gas top being turned on is the one connected to the burner to be
lightened
- During heating, point the mouth of the test-tube away from anyone, including
ourselves.
(f) Diagram of experimental setup
(g) Explanation of the control setup
Control setups: Glucose solution, distilled water
It can be used to show the negative result and compare with the result in the experimental setup, which help proving the result is due to the chemical but not other factor
Results
Table showing the result of the Iodine test and Benedict’s test on the 4 solution sample
( where + represent the present of blue-black spot/ brick red precipitate and
– represent the absence of blue-black spot/ brick red precipitate)
Discussion
In iodine test, the I3- ion in the iodine solution would be trapped in the centre of the helical starch molecules to from a blue-black polyiodide complex. Thus we can observe a turn of color from yellow to blue-black if starch is present. Therefore, we can add in iodine solution to the sample solution so that we can account that starch is involved in the biochemical change.
In Benedict’s test, the reducing sugar can reduce the soluble blue copper (II) sulphate in the Benedict’s solution to form insoluble red-brown copper (I) oxide. The latter is observed as a precipitate, which appears green to yellow to orange to brick-red with increasing amount of reducing sugars. Therefore, by noticing the formation of brick red participate, we can deduced the presence of reducing sugar in the banana after storing and thus its role in causing the sweet taste of the banana.
As my hypothesis is that the biochemical change is starch was broken down to a certain kind of reducing sugar. By comparing the difference in amount of starch and reducing sugar during the different stage of storage, we can tell that the decrease of starch and the increase of reducing sugar are related.
In the experiment, we can observe the formation of blue-black spots in the extract of banana stored 1 day and 3 days and a little of blue-black spots in the extract of banana stored 5 days and brick red precipitate in the extract of banana stored 3 days and 5 days and a little of orange precipitate in the extract of banana stored 1 day.
During the storage, the banana starts to ripe. Bananas produce a plant hormone called ethylene when ripe, which indirectly affects the flavor. Ethylene stimulates the formation of amylase, an enzyme that breaks down starch into sugar, influencing the taste of bananas. The greener, less ripe bananas contain higher levels of starch and, consequently, have a "starchier" taste. On the other hand, yellow bananas taste sweeter due to higher sugar concentrations.
Out of expectation, the extract of banana stored for 1 day shows positive result of Benedict’s test and the extract of banana stored for 5 days shows positive result of iodine test.
For the case of banana stored for 1 day, this may due to the heating process of the Benedict’s Test. During this 5 minutes heating, the extract of banana stored 1 day are being heated and the glycosidic bond in the starch are broken down, thus glucose is detected in the solution and a relatively smaller amount of brick red precipitate is formed. What is more, this may because the ripening process has begun during the 1 day. Thus ethylene was produced and formation of amylases is stimulated, thus some of the starch has been broken down into sugar.
This error shows the limitation of this experiment, which is we can not test for extract of raw substances since they will still be cooked in the heating stage of the Benedict’s solution and this error can not be avoided.
For the case of banana stored for 5 days, this may because the ripening process is still taking place in the banana. The starch stored in the banana has not been completely broken don into sugar, thus starch can still be found in the extract, which causes the formation of the blue-black spot.
This error involving the prediction can be avoided if we make a relative prediction in steed. For example, we can make the prediction by comparing the results, like: “The banana stored for 1 day should show a more positive result of iodine test and more negative result of Benedict’s test than the banana stored for 3 days.” Thus a more objective prediction can be made.
Further experiment can be hold to test the exact type of reducing sugar present in the solution. For example, Clinitix paper to test for glucose.
Conclusion
In conclusion, the biochemical changes involved is starch was broken down to a certain kind of reducing sugar, which is probably glucose. Yet a more accurate result can be carried out with a further experiment. Though the hypothesis can be supported by this experiment, due to the limitation of this experiment, the result on some samples are out of expectation. Therefore, improvements on this experiment are needed.
