Objectives : 1. To test for the present of carbohydrate in food samples by using Benedict’s reagent. 2. To determine the present of starch in food samples by using iodine reagent. 3. To investigate the present of protein in food samples with Biuret solution. 4. To determine the present of lipid in food samples using brown paper.
Introduction : We had learned about that all living organisms are made up of molecules, each molecule serve its own special purpose. Molecule can be divided into two types, macromolecules and micro molecules. The molecules that we are studying in this experiment are macromolecules. In this case, we are studying on 3 types of macromolecules, carbohydrates, protein and lipid that present in different food. Carbohydrate can be divided into monosaccharide, disaccharide and polysaccharide. Monosaccharide is the simplest sugar that appears in food, the most common example of this is glucose. Disaccharide is a double sugar, which means by it linked two monosaccharides together to form a disaccharide. The examples for disaccharide are maltose, galactose and sucrose. The example of polysaccharide is starch. Protein is build up by the monomers, amino acid. Each protein has it own special sequence that is different from others. The Biuret reagent is used to determine the presence of protein as it turns the original blue colour of the solution to purple when it react with the protein, specifically between the bonds of amino acid. Lipids are oily or greasy in nature and it is insoluble in water. It only can be dissolve with organic solvent such as ether or chloroform. It can store energy in the cell for long term usage. Its presence can be tested by rubbing some food samples on a brown paper and hold under the light.
Materials : * Benedict’s Reagent * Starch solution * Sucrose solution * Iodine reagent * Concentrated KOH * 0.5% CuSO4 * Potato * Onions * Bread * Instant noodle * Peanut butter * Apple juice * Soft drink * Soya milk
Apparatus : * Test tubes * Water bath * Pipette * Measuring cylinder * Cheesecloth * Mortar and pestle * Brown paper
Method :
Part 1: Carbohydrate 1. The solid and semi-solid food was grinded with mortar and pestle while adding in distilled water. 2. The food samples were strained into separate test tubes by using cheesecloth. 3. A little of the 8 food samples were placed into different test tubes and 5 ml of Benedict’s reagent was added. 4. The test tubes were heated for 2 to 3 minutes in the hot water bath. 5. The control set was prepared by adding 1 ml of sucrose solution with 5 ml of Benedict’s reagent into another test tube. 6. The test tube was heated for 2 to 3 minutes in the hot water bath. 7. The changes in the test tubes were observed.
Part 2: Starch 1. A little of the 8 food samples were placed into different test tubes. 2. The control set was prepared by adding 1 ml of starch solution in another test tube. 3. A few drops of iodine reagent was added into each test tubes. 4. The observations were recorded.
Part 3: Protein 1. 1 ml of the 8 food samples were placed into different test tubes. 2. 2 ml of the concentrated KOH was added into each test tubes follow by 1 ml of CuSO4 solution. 3. The observations were recorded.
Part 4: Lipid 1. A drop of each food samples were put onto 2 pieces of rectangular brown paper. 2. The spots were allowed to dry. 3. When the spots were dried, the paper was held up to the light. 4. The observations were recorded.
Results : | Carbohydrate | Starch | Protein | Lipid | Potato | Blue colour solution turns to orange solution. | Brownish yellow solution turns to blue black solution. | Blue colour solution with precipitate. | No changes. | Onions | Blue colour solution turns to orange solution. | Brownish yellow solution remains unchanged. | Solution turns to bluish green colour. | No changes. | Instant noodle | Blue colour solution remains unchanged. | Brownish yellow solution turns to blue black solution. | Solution turns light blue colour with precipitate. | No changes. | Bread | Blue colour solution turns to orange solution. | Brownish yellow solution turns to blue black solution. | Blue colour solution turns to purple colour solution. | No changes. | Peanut butter | Blue colour solution remains unchanged, with a little of grey precipitate. | Brownish yellow solution turns to blue black solution. | Blue colour solution remains unchanged. | Translucent | Apple juice | Blue colour solution turns to orange solution. | Brownish yellow solution remains unchanged. | Blue colour solution with green colour precipitate. | Translucent | Soft drink | Blue colour solution turns to orange solution. | Brownish yellow solution remains unchanged. | Blue colour solution remains unchanged. | Translucent | Soya milk | Blue colour solution remains unchanged. | Brownish yellow solution remains unchanged. | Blue colour solution turns topurple colour solution. | No changes. |

Discussion : In this experiment, 8 kinds of different food samples is prepared to test for the presence of macromolecules in each food sample. The 8 food samples are potato, onions, instant noodle, bread, peanut butter, apple juice, soft drink and soya milk. Each food samples is tested with Benedict’s reagent, iodine reagent, Biuret solution and brown paper. The solid food such as potato, onions, instant noodle and bread were grinded with mortar and pestle and added with some distilled water to make it liquefied. The food samples is then strained and poured into separate test tubes. In the first part of the experiment, Benedict’s reagent was used to test the presence of reducing sugar. If there is the presence of reducing sugar in the food samples, the blue colour solution will turn orange colour after the test tubes are heated in the hot water bath for 2 to 3 minutes. Sugar is classified into 2 groups, reducing sugar and non-reducing sugar. Only reducing sugar will gives rise to the positive result to the Benedict’s test which is turn the blue colour solution into orange colour solution. This is because the reducing sugar act as a reducing agent when it reacts with the Benedict’s reagent. It oxidizes itself by donating an electron in the redox reaction. There are two types if functional groups in the sugar chain, which are aldehyde functional group and ketone functional group. The aldehyde functional group is the groups that act as the reducing agent in the test. Reducing sugar is the simplest sugar which included all the monosaccharide and most of the disaccharide. The examples of monosaccharide are glucose, fructose and lactose. The examples of the reducing sugar for disaccharide are galactose and maltose. Sucrose is a non-reducing sugar which will not give rises to orange colour to the Benedict’s reagent when heated in the hot water bath. The food samples that contain carbohydrates are potato, onions, bread, apple juice and soft drink. For part 2 of this experiment, the presence of starch is tested by using iodine reagent. Starch is an example of polysaccharide, which is built by many monomers, glucose to form a complex structure. In this experiment, starch solution is used as the positive control set to react with the iodine reagent. The brownish yellow colour of the solution will turn to blue black colour whenever there is presence of starch. Each food samples were added with a few drops of iodine solution to test for the presence of starch in it. Once the food samples with starch presence in it, it will turn to blue black colour immediately without any heating process. This is because a starch-iodine complex is formed. The transferring of electron between starch and iodine caused the spacing between the orbitals to be changed follow by the different absorption of light wavelength. The negative control set will not be having a colour changes as the starch-iodine complex will not form within water molecules and iodine molecules. The food samples that contained starch are potato, instant noodle, bread and peanut butter. In the third part of this experiment, Biuret solution is used to test for the presence of protein in the food samples. Biuret solution is a mixture of concentrated potassium hydroxide and copper (II) sulphate. When there is presence of protein, the blue colour of the Cu2+ ions will turn to purple colour. This is because the Biuret solution reacts with the peptide bonds that appeared between the protein monomers, amino acid. The presence of protein can be also tested by using Fehling’s solution A and B or sodium hydroxide and copper (II) sulphate solution. The food samples that contain protein are bread and soya milk. In the test tubes of both of the food samples, the blue colour of the solution turns to purple colour, indicating the presence of protein in these two food samples. For the fourth part of this experiment, the presence of lipids in food is tested. By rubbing a drop of food samples onto the brown paper and let it dry, after that hold it up under the light, the presence of lipids can the determine. Lipids have the same organic compound with carbohydrate which are carbon, hydrogen and oxygen but lipid have a larger ratio compare to carbohydrate. It is used to store energy and is oily in nature. It is not soluble in water but soluble in organic solvent such as ether. The food examples that contain lipid are peanut butter, apple juice and soft drink. When this three food samples are rubbed onto the brown paper and let to dry, they showed a translucent mark on the brown paper under light. The other food samples that do not contain any lipids will not leave any mark on the brown paper.
Conclusion :
When there is present of carbohydrates in the food samples, the blue colour of the solution will turn to orange colour. On the other hand, when there is starch present in the food samples, the brownish yellow colour solution will turn to blue black colour. For the Biuret test, the blue colour of the solution will turn purple when there is the present of protein in the food samples. When the spots on the brown paper appear as translucent when hold up to light, it means that lipids is present in the food sample.
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