This experiment Iodine reagent was used to determined the presence of starch in the solution (or food). When iodine reagent added in the starch presence solution, the iodine ion will integrate in the polysaccharide chain and result in a dark blue colour, the positive result showed.
Whereas when the solution showed yellow colour, which is the iodine reagent initial colour, the negative result showed.According to Table 2, for the starch solution, the iodine test showed positive result, because the starch solution itself definitely contain of starch molecules. As for the distilled water, it serves as a negative control of this experiment, so, it showed a negative result.
While, for the food sample, only the potato juice and instant noodle showed positive result which indicates the presence of starch, the rest all showed a negative result.But the potato juice and instant noodle did not showed the same colour, because the initial colour of the food sample will affect the final colour of the test.This experiment use Benedict’s reagent and Iodine’s reagent to test the hydrolysis of sucrose and starch by using hydrochloric acid. For Benedict’s test, the positive result for the theoretically should be showed in test tube 2 for Benedict’s test of sucrose and 7 for Benedict’s test of starch.
For the starch Iodine test, the test tube 4, and 6 should gives positive result, and the test tube 8 should give a negative result.According to Table 3, the Benedict’s test of sucrose, test tube 2 showed positive result in the Benedict’s test, the reason of the test tube 2 will showed the positive result but not test tube 1 is, the sucrose need to boiled for few minutes in order to hydrolyze the disaccharide into monomer (fructose and glucose), then the monomer formed only will give positive result. For the Benedict’s test of starch, test tubes 3, and 5 showed a negative result, indicate that no presence of starch, but the test tube 7 showed unexpected negative result, that test tube 7 are absence of reducing sugar, which mean that the starch in the tube 7 lack of time to hydrolyze starch (polysaccharide) to simple reducing sugar.
From Table 3, the test tube 4 and 6 get the expected positive result, which determined that at the first 5 minutes boiling, the starch did not hydrolyze into monomer and dimmer, iodine test still give positive result. But test tube 8, it showed unexpected result, that test tube 8 still presence of starch, because lack of time to hydrolyze starch. Lipid was test by using brown paper, and divided into 8 parts which each part have different droplet of solution (or juice).The positive result showed when the paper was dry and holds under the light, and the droplet became transparent, if the paper doesn’t have any changes on the paper, it showed negative result. The oil droplet showed a positive result which the whole droplet became transparent, distilled water was also act as negative control showed no changes on the paper.
For the food sample, only apple juice, onion juice, and the instant noodle show slightly positive result, because when observing the paper, there is slightly transparence in outer part of the droplet. The other food sample droplets, which showed no change on the paper, negative result showed.Conclusion:1. The Benedict’s test only able to determined the reducing sugar in a given solution, which show precipitate at the bottom part of the solution, the precipitate can be yellow, green, yellowish red, or red colour that all this colour of precipitate indicate the positive result of the test. 2. The Iodine test are able to determined the starch compound in a given solution, which show a dark blue solution that indicate the positive result of the test.3.
When added the non-reducing sugar, sucrose with hydrochloric acid, and heated for few minutes, sucrose able to hydrolyze to form simply reducing sugar, fructose and glucose that able to detect by Benedict’s test. 4. When added the polysaccharide, starch with hydrochloric acid, and heated for a longer time (longer then the time needed to hydrolyze sucrose), starch able to hydrolyze to form simple reducing sugar, that able to detect by Benedict’s test.5. The Biuret test able to determined the presence of protein in a given solution, which show dark blue solution that indicate the positive result of the test. 6. The lipid test by using brown paper, able to determined the presence of lipid in a given solution, which the changes the transparency of brown paper indicate the positive result of the test. 7.
The apple juice provided was tested, then found out that apple juice consist of carbohydrate, and also little amount of lipid, did not consists of protein and starch. 8. The potatoes juice provided was tested, then found out that potatoes juice consist of carbohydrate, starch, protein, did not consist of lipid.9. The soya bean provided was tested, then found out the soya bean consist of carbohydrate, and protein, did not consist of lipid and starch. 10. The soft drink provided was tested, then found out the soft drink consist of carbohydrate, and protein, did not consist of lipid and starch. 11.
The onion juice provided was tested, then found out the onion juice consist of carbohydrate, protein and slightly lipid, did not consist of starch. 12. The instant noodle provided was tested, then found out that instant noodle consist of carbohydrate, starch, protein, and slightly lipid.
Reference:1. Eldra PS, Linda RB, Diana WM, 2005 Biology 8th edition, Thomson Brooks/Cole2. Taylor DJ, 1997 Biological Science, Cambridge: Cambridge University Press3.
Stanton, Rosemary, 1995 Rosemary Station’s complete book of food and nutrition, East Rosevill, N.S.W.: Simon ; Schuster.4. 2004 A Dictionary of Biology, Oxford: Oxford University Press