Identification of reducing sugar carbohydrates and starch in different materials using biochemical tests
Introduction:
Organic compounds are associated to living organisms and contain carbon and hydrogen. Main organic compounds in living organisms are carbohydrates, proteins, nucleic acids and lipids.
Carbohydrates are organic molecules that supply energy for cell processes. Sugars and starches are carbohydrates that cells use for energy. Some carbohydrates also are important parts of cell structures. For example, a carbohydrate called cellulose is an important part of plant cells
Particularly, carbohydrates are organic compounds that have carbon, hydrogen and oxygen. They can be part of cell structures and a source of energy for the cell. Sugar, starch and cellulose are examples of carbohydrates.
One can classify carbohydrates as monosaccharide, disaccharide and polysaccharide. Monosaccharides are the simplest carbohydrate and they include glucose. A link of two monosaccharides form disaccharides; an example of these is sucrose, which is formed by the union of glucose and fructose. The combination of more than two monosaccharides form the polysaccharides, such as the starch and cellulose (Campbell, 1996).
Some monosaccharides have the ability to reduce due to a free aldehyde group. This ability reduces weak oxidized agents, such as cooper in the Benedict’s test. Particularly, fructose and glucose are reducing sugar monosaccharides.
In experiments, controls are important; especially, a positive and a negative control help to determine if the unknown reactions are running appropriately. Particularly, when one tries to identify organic compounds through biochemical tests, the inclusion of controls is important in order to validate the results.
In this experiment, we tested the composition of some carbohydrates and the ability of some biochemical tests to identify and unknown compounds. Thus, if a material contains a sugar reducing agent or starch, then it will be positive for the Benedict’s test or iodine reaction, respectively, changing the initial color of the solution.
Methods and Materials:
For Benedict’s test, used to demonstrate the sugar reduction, we added 10 drops of different materials (apple juice, potato juice, sucrose solution, glucose solution, distilled water, sugar reducing solution and starch solution), each in a different test-tube. Then, we added 2 mL of Benedict’s solution in each tube and boiled them for 3 minutes. We let the tubes cool at room temperature and recorded the color of the solution.
For the iodine test used to identify starch, we added 10 drops of all the materials mentioned before in different tubes. Then, we added 3-5 drops of iodine in each tube and recorded the color of the tube’s content.
Results:
In this experiment, we found changes in glucose solution and sugar reducing solution in the Benedict’s test, demonstrating the presence of sugar reduction. There was no color change for the other solution, as was expected (Table 1). When we use the iodine solution, we saw change in color of the solution for potato juice and starch solution, demonstrating a positive reaction (Table 2).
Discussion:
In this experiment, we determined the ability of some carbohydrates to reduce sugar and the starch detection ability of some materials. Using Benedict’s test, we demonstrated that glucose is a sugar- reducing monosaccharide. Reducing-sugar solutions act as a positive control and we found the activity of the Benedict’s solution to detect what we expected. On the other hand, distilled water acted as negative control, demonstrating how a negative reaction occurs. The identification of this sugar by Benedict’s test is possible by the presence of a complex of cupric ion (copper) and citrate in the Benedict’s reagent in alkaline conditions. When sugar reduction is present, as in in the case of the glucose and reducing sugar solution, cupric ions (Cu2+) transformed into to cuprous oxide. In that way, we saw a change in the solution from blue to green to reddish orange. In the case of sucrose, disaccharides, we did not see any change because it is not a reducing sugar.
We also determined the presence of starch by doing the iodine test. We saw positive results for potato juice and starch solution. A positive result in this test demonstrates the ability of the iodine to react with coiled polymers, such as starch. We observed the initial color of iodine, yellowish brown, changed to a bluish black color, indicating a positive reaction. Starch solution and distilled water acted as positive and negative control, respectively, indicating that the test responded correctly. Although one would think that apple juice could have starch, we did not detect it using the iodine test. Carrin et al., 2004, did not detect starch in juices obtained from apples that were not ripe and which was stored for three weeks at room temperature.
Our hypothesis was right, since the materials containing reducing sugar agents (glucose solution and reducing sugar solution) came out positive with the Benedict’s solution and those having starch (potato juice and starch solution) came out negative after performing the iodine reaction.
References:
Campbell, N (1996). Biology. 4th Ed. The Benjamin/Cummings Publishing Company Inc. CA. Pg, 65-69.
Carrin, M. E., Ceci, L. N., Lozano, J., E. (2004). Characterization of starch in apple juice and its degradation by amylases. Food Chemistry, 87, 173-178.
Table 2: Color Reaction of Different Materials for Iodine Test For Starch
