2. Foods Test

Food Tests Practical: Identifying Biological Molecules in Foods

Aim

To test a range of food samples for the presence of:

  • Starch (iodine test)
  • Reducing sugars (Benedict’s test)
  • Proteins (biuret test)
  • Fats/oils (ethanol emulsion test)

Key Idea

Each reagent causes a visible change if a particular nutrient is present. Using controls and careful method helps make the results reliable.

Safety

  • Wear goggles and lab coat.
  • Benedict’s test requires heating: use a hot water bath (not a flame), and handle hot tubes with tongs/test-tube holder.
  • Ethanol is flammable: keep away from heat sources, recap bottles, and clean spills immediately.
  • Do not taste any samples. Wash hands after the practical.

Materials

  • Test tubes + rack, droppers/pipettes, measuring cylinder
  • Spotting tile (optional)
  • Hot water bath (beaker of hot water on hot plate/kettle water)
  • Distilled water
  • Iodine solution
  • Benedict’s solution
  • Biuret reagents (NaOH + CuSO₄, or premixed biuret)
  • Ethanol
  • Food samples (suggestions below)
  • Suggested Samples (pick 6–10)

Bread, potato, rice water, apple juice, milk, egg white solution, cooked beans, vegetable oil, butter solution, honey solution.

Preparing Food Samples (quick method)

For solids:

  • Add a pea-sized amount of food to a test tube.
  • Add ~10 mL water, mash/shake.
  • Let settle; use the liquid for testing.

For liquids:
Use directly (dilute thick liquids with a little water if needed).

Controls (important!)

Set up these known samples to check your tests:

  • Starch positive control: starch solution or bread extract
  • Reducing sugar control: glucose solution
  • Protein control: egg white solution
  • Fat control: vegetable oil
  • Negative control: distilled water

Procedures (Four Tests)

Test 1: Iodine Test for Starch

  1. Place 2 mL of food solution in a test tube (or a well on a spotting tile).
  2. Add 2–3 drops of iodine solution.
  3. Record the colour change.

Positive result: brown → blue-black
Negative result: stays brown/yellow-brown

Test 2: Benedict’s Test for Reducing Sugars

  1. Add 2 mL of food solution to a test tube.
  2. Add 2 mL Benedict’s solution.
  3. Place tube in a hot water bath for 2–3 minutes.
  4. Record the final colour.

Positive result: blue → green/yellow/orange/brick-red (more sugar = more red)
Negative result: remains blue

Test 3: Biuret Test for Proteins

  1. Add 2 mL of food solution to a test tube.
  2. Add 2 mL sodium hydroxide (or biuret reagent part 1).
  3. Add 2–3 drops copper sulfate (or biuret reagent part 2). Gently mix.
  4. Record the colour.

Positive result: blue → lilac/purple
Negative result: stays blue

Test 4: Ethanol Emulsion Test for Fats/Oils

  1. Add 2 mL of food solution to a test tube.
  2. Add 2 mL ethanol, cap and shake well.
  3. Add 2 mL water and shake again.
  4. Observe and record.

Positive result: cloudy white emulsion
Negative result: remains clear

Data Collection

Table 1: Results Summary

Food Sample Iodine (Starch) colour Starch? (Y/N) Benedict’s (Reducing sugar) colour Reducing sugar? (Y/N) Biuret (Protein) colour Protein? (Y/N) Ethanol emulsion appearance Fat? (Y/N)
Distilled water (negative control)                
Known starch control                
Known glucose control                
Known protein control                
Known fat control                
Sample 1:                
Sample 2:                
Sample 3:                
Sample 4:                

Table 2: Benedict’s Semi-Quantitative Scale (optional)

Final colour Estimated reducing sugar level
Blue None
Green Low
Yellow Medium
Orange High
Brick-red Very high

Analysis Questions (Student Responses)

  1. Which tests required a colour change without heating, and which test required heating?
  2. Pick one food sample and explain how you know which nutrients it contained using your results.
  3. Why are controls important in this practical? Give two reasons.
  4. Identify two possible sources of error in this investigation and how to reduce them.
  5. Some foods contain non-reducing sugars (like sucrose). Predict what Benedict’s test might show and explain why. (Extension)
  6. Which nutrient(s) would you expect to find in milk? Do your results match your prediction?

Conclusion Prompt

Write a short conclusion (4–6 sentences) answering:

  • What nutrients were detected in each sample?
  • Which sample had the widest variety of nutrients?
  • How confident are you in your results, and why?

Notes 

  • If you want reliable “protein” positives, egg white solution works great.
  • For a clear fat positive, use oil. For a “hidden fat” food, crushed nuts or chips extract often works.
  • Use a water bath around 70–90°C for Benedict’s.