Food Tests and Enzymatic Reactions

Food Tests

Overview of Food Tests

These tests are used to identify the presence of different types of nutrients in food samples, including reducing sugars, starch, proteins, and fats.

Benedict's Solution - Tests for Reducing Sugars

  1. Sample Preparation:

    • Crush the solid food sample and mix it with water, placing it into a test tube.

  2. Addition of Reagents:

    • Add an equal amount of Benedict's solution to the food sample in the test tube.

  3. Heating:

    • Heat the test tube in a boiling (or near boiling) water bath.

  4. Observation:

    • If glucose (sugar) is present, the solution will change color from blue to:

      • Green: indicative of low sugar concentration

      • Yellow: indicative of moderate sugar concentration

      • Orange: indicative of high sugar concentration

      • Brick-red: indicative of very high sugar concentration

Iodine Solution - Tests for Starch

  1. Sample Preparation:

    • Place the food sample onto a spotting tile.

  2. Addition of Reagent:

    • Add a few drops of iodine solution onto the food.

  3. Observation:

    • If starch is present, the solution will change from brown to blue-black.

Biuret Solution - Tests for Protein

  1. Sample Preparation:

    • Crush the solid food sample and mix it with water, placing it into a test tube.

  2. Addition of Reagents:

    • Add 5 cm³ of Biuret reagent to the sample and shake gently.

  3. Observation:

    • If protein is present, the solution will turn from blue to purple.

Ethanol Test - Tests for Fats

  1. Sample Preparation:

    • Crush the food sample and mix it with ethanol.

  2. Filtering:

    • Filter the mixture to separate the solids from the liquid.

  3. Testing for Fats:

    • Pour the filtered solution into a test tube of water and shake the tube.

  4. Observation:

    • If fat is present, the water will turn cloudy.

Enzyme Investigations

Gastric Juice and Protease Enzymes

  • Gastric juice in the stomach contains protease enzymes and acid.

  • Arif's Experiment on Protein Digestion:

    • Setup:

      • Three test tubes with different pH levels:

      • pH 7: egg white (protein) and protease

      • pH 5: egg white (protein) and protease

      • pH 2: egg white (protein) and protease

    • Observation:

    • Initially, all test tubes looked cloudy. After 15 minutes in warm water, the liquid in two of the tubes became clear.

Questions Related to Arif's Experiment

  1. What nutrients do protease enzymes digest?

    • Protease enzymes digest proteins.

  2. Why did Arif put egg white in his test tubes?

    • Egg white serves as a source of protein for the experiment to observe the action of protease enzymes.

  3. What kind of acid is added to food in your stomach?

    • Hydrochloric acid is the primary acid in gastric juice.

Josie's Experiment with Fat

  • Setup:

    • Three test tubes with different contents:

      • Tube A: fat and water

      • Tube B: fat, water, and lipase

      • Tube C: lipase and bile

    • Special Chemical Test:

    • A chemical was added to all three tubes which turned pink in the presence of fat and white in the absence of fat.

Questions Related to Josie's Experiment

  1. What colour was the chemical at the beginning of Josie's experiment?

    • The chemical was pink, indicating the presence of fat in the test tubes.

  2. Lipase is an enzyme. Which nutrient does it digest?

    • Lipase enzymes digest fats.

  3. The liquid in tube D did not change colour at all. Why not?

    • Tube D likely did not contain any lipase or bile, or there might have been no fat present to be digested, hence no change in color.