| Aim | To perform characteristic tests on pure samples of carbohydrates, fats and proteins, and to detect their presence in given foodstuffs. |
| Apparatus Required | Test tubes Droppers Bunsen burner Test tube holder Pipettes Beakers Heating plates |
| Theory | Carbohydrates, fats, and proteins are essential nutrients required by the human body to function properly. These macronutrients can be found in a wide range of foodstuffs, including fruits, vegetables, meat, dairy, and grains. Carbohydrates are the primary source of energy for the body. They are classified into monosaccharides, disaccharides, and polysaccharides. The monosaccharides glucose and fructose are commonly found in fruits, while lactose and sucrose are disaccharides found in milk and table sugar, respectively. Polysaccharides such as starch and cellulose are found in grains and vegetables. Fats are also a source of energy, but they are more calorie-dense than carbohydrates. Fats can be saturated or unsaturated, and are typically found in animal products, nuts, and oils. Proteins are the building blocks of the body, and are necessary for growth and repair. They are made up of amino acids, and can be found in meat, dairy, legumes, and grains. |
| Procedure | I. Carbohydrates: Benedict’s test for reducing sugars: a. Take a small amount of sample solution in a test tube. b. Add an equal amount of Benedict’s solution. c. Heat the mixture in a water bath for 5 minutes. d. Observe the color change. Observation: If reducing sugars are present, the solution changes from blue to green, yellow, orange, or red, depending on the amount of reducing sugar present. Molisch’s test: a. Take a small amount of sample solution in a test tube. b. Add a few drops of Molisch’s reagent (alpha-naphthol in ethanol). c. Add an equal amount of concentrated sulfuric acid by letting it run down the side of the test tube. d. Observe the color change. Observation: If carbohydrates are present, a purple ring will appear at the junction of the two liquids. II. Fats: Sudan III test: a. Take a small amount of sample solution in a test tube. b. Add a few drops of Sudan III solution. c. Shake the tube vigorously. d. Allow the mixture to settle. Observation: If fats are present, the solution will separate into two layers, with a red layer on top. Acrolein test: a. Take a small amount of sample solution in a test tube. b. Add a few drops of glycerol and concentrated sulfuric acid. c. Heat the mixture by holding the test tube over a flame. d. Observe the color change. Observation: If fats are present, a pungent smell of acrolein is produced, and a black residue may be formed on the test tube. III. Proteins: Biuret test: a. Take a small amount of sample solution in a test tube. b. Add an equal amount of Biuret reagent (copper sulfate and sodium hydroxide). c. Mix well and let it stand for 5 minutes. d. Observe the color change. Observation: If proteins are present, the solution changes from blue to violet. Xanthoproteic test: Take a small amount of the protein sample in a test tube. Add a few drops of concentrated nitric acid to the sample. Mix the contents of the test tube by gently swirling it. Observe the color change. |
| Observation and Result | Carbohydrates, fats, and proteins are macronutrients that are essential for human nutrition. They are present in various foods in different proportions. To detect these macronutrients in foodstuffs, various tests can be performed based on their characteristic properties. Here are some of the tests and observations for the detection of carbohydrates, fats, and proteins in pure samples and foodstuffs: Carbohydrates: Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen in a 1:2:1 ratio. The following tests can be used to detect carbohydrates: a) Benedict’s test: This test is used to detect reducing sugars. When Benedict’s reagent (a solution of copper sulfate and sodium citrate) is added to a solution containing reducing sugars and heated, a brick-red precipitate is formed. The intensity of the color of the precipitate is proportional to the amount of reducing sugar present. b) Iodine test: This test is used to detect the presence of starch. When iodine solution is added to a solution containing starch, the color changes from brown to blue-black. Fats: Fats are a type of lipid that is insoluble in water. They are composed of fatty acids and glycerol. The following tests can be used to detect fats: a) Solubility test: Fats are insoluble in water but soluble in organic solvents such as ether, benzene, and chloroform. b) Sudan III test: This test is used to detect the presence of lipids in a food sample. When Sudan III stain is added to the food sample, it stains the lipids red. Proteins: Proteins are complex macromolecules composed of amino acids. The following tests can be used to detect proteins: a) Biuret test: This test is used to detect the presence of proteins in a sample. When Biuret reagent (a solution of copper sulfate and sodium hydroxide) is added to a protein-containing sample, the color changes from blue to violet. b) Ninhydrin test: This test is used to detect the presence of amino acids in a sample. When ninhydrin is added to a protein-containing sample and heated, a purple color is formed. To detect carbohydrates, fats, and proteins in foodstuffs, these tests can be performed on the food sample directly or after extracting the macronutrients using appropriate solvents. For example, to detect lipids in a food sample, the sample can be extracted with an organic solvent and then subjected to the Sudan III test. Similarly, to detect reducing sugars in a food sample, the sample can be extracted with water, and then the Benedict’s test can be performed on the extract. |