Apparatus & materials:
- Apparatus :
2. Materials :
Procedure:
- Preparation of extract from the plant structures
- 10 mL of distilled water was measured with a measuring cylinder.
- 10g of plant tissue was cut into small pieces by means of scalpel.
- The pieces are crushed in a mortar by pressing them with a pestle until they disintegrate. Then a small amount volume of water was added and the pieces are grinded for several times or until the crushed pieces of plants material change into paste. After that, the rest of the water was added thus viscosity was reduced and grinding was continued.
- The suspended plant fibres were removed by passing the extract through the muslin cloth. The extract was collected by using a boiling tube.
- Chemical Tests
- Benedict’s test for reducing sugars
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2 cm3 of extract solution were added to a test tube by using a 2 mL pipette and a pipette filler.
- 2 mL Benedict’s solutions were added to the extract by using a measuring cylinder.
- The mixture was than shaken and boiled by using a boiling water bath.
- The final colour was then recorded
- Iodine test for starch
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2 cm3 of extract solution was added to a test tube by using a 2 mL pipette and a pipette filler.
- A few drops of iodine solution were added
- The final colour was recorded.
- Biuret test for protein
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2 cm3 of extract solution were added to a test tube by using a 2 mL pipette and a pipette filler.
- 2 mL of sodium hydroxide solution was added to the extract by using a measuring cylinder and the solutions were mixed.
- A few drops of 1% copper sulphate solution were added.
- The final colour was recorded.
Result:
The results of the Benedict’s test of the above two plant material samples
The results of the Iodine test of the above two plant materials samples
The results of the Biuret test of the above two plant materials samples
Discussion:
From the result of the Benedict’s test, the mixture of apple juice and Benedict’s solution changed from light blue to brick-red colour. It showed a positive result which means that reducing sugar was present. For the mixture of the green bean extract and Benedict’s solution, there was no any change in the solution. Solution remains in blue colour. It showed a negative result which reflected that reducing sugar was absent in green bean.
From the result of the Iodine test, the mixture of apple juice and the iodine solution remained brown in colour. It showed a negative result which implied the absence of starch in the apple. While the mixture of the green bean extract and iodine solution had a positive result, which the solution turned from brown to blue-black colour. The colour revealed the presence of starch in the green bean.
From the result of the Biuret test, the mixture of apple juice and the alkaline copper (II) sulphate solution gave a negative result to the test since the solution didn’t have any colour change and it still remained in blue colour. That meant no proteins were presented in apple. The mixture of green bean extract and the alkaline copper (II) sulphate solution changed from blue to purple colour, it gave a positive result which indicated proteins were present in the green bean.
The three chemical tests are not sensible enough to show the small amount of sugars proteins and starch present in the food samples. In addition, we lost some materials during the grinding process. Although we used 10g of both the apple and green bean and added 10 mL of water into them for the experiment, the final volume of extract we used in testing would not be the same.
In the Benedict’s test, the result was depended on the colour change but the extract obtained from the apple and bean were coloured. This would affect the colour intensity of the solution. Therefore, our determinations of the amount of reducing sugar in each sample by referring to the colour of solution would be greatly affected. Moreover, the green bean extract after mixed with the testing solution still contained some precipitates inside. So, these precipitates would affect our determination on the amount of the reducing sugars. Accuracy of the test thus reduced because the amount of precipitate formed would imply the amount of reducing sugars in the sample. In order to improve the test by making it more accurate, centrifuge should be used for the experiment. It could make all the precipitates to deposit at the bottom of test tube. Then, a liquid portion could be fully obtained by this method. No precipitates from the green bean would leave in the solution and it thus wouldn’t affect our determination of the amount of reducing sugars form during the test. The result obtained at the end would be more accurate.
In the Iodine test, we discovered that no starch was present in the apple. In fact, apple actually contains a few amount of starch. However, starch is not very soluble in water. So that little amount could not make the solution to change the colour since only a few could dissolve into the solution. That is why we could not get a positive result of apple on the Iodine test even though it contains starch. To improve the test, iodine solution should be added into the bean and a slice apple. The result could be obtained immediately and even a little amount of starch in apple could also be detected by a colour change.
Reducing sugar is present in the apple because the sugar can attract insects for the pollination. Starch and protein are stored inside the bean as the nutrients that should be provided for growth of a plant.
This experiment also has some problems. First is the incomplete extraction, we need to extract several times by adding water to the residue and filter again. Second is the insufficient grinding, we should grind for a long time, or use a blender instead. For sufficient extraction of starch, we may boil the foodstuff before grinding. Then the result of this experiment can be more accurate.
For further investigation, we may investigate the vitamin C content in fruits by using DCPIP solution. During the experiment, we can test how many drops of example is required to decolorize a specific volume of DCPIP solution. The less drops of the juice to turn this solution colourless, the higher the concentration of vitamin C in the solution.
Conclusion:
Apple contains reducing sugars but there is no starch or protein inside. Green beans contain starch, protein but no reducing sugar.