Question
Describe simple food tests to detect the presence of (a) starch, (b) fat, and (c) protein in a food sample.
Solution — Step by Step
Reagent: Iodine solution (iodine dissolved in potassium iodide)
Procedure:
- Take a small amount of the food sample in a test tube or on a white tile
- Add 2–3 drops of iodine solution
Observation:
- Positive result (starch present): The solution turns blue-black
- Negative result (no starch): Iodine remains its original yellow-brown colour
Why it works: Iodine molecules form a complex with the helical chains of amylose (a component of starch), producing the characteristic blue-black colour. This is a very sensitive test — even trace amounts of starch give a visible colour change.
Example: Rice, potato, bread → strong blue-black. Banana → mild colour. Sugar solution → no colour change (sugar is not starch).
Simple method — Paper spot test:
- Rub a small amount of food on a piece of white paper
- Allow the paper to dry
Observation:
- Positive result (fat present): A translucent (oily) spot remains on the paper after drying
- Negative result (no fat): Spot dries completely, leaving no translucent mark
Why it works: Fats are non-polar and cannot evaporate. They soak into the paper fibres, creating a permanent translucent region (oily spot) by filling the spaces between paper fibres, allowing light to pass through.
Chemical method — Sudan III test:
- Add Sudan III dye solution to the food sample in water
- Positive result: The Sudan III stains fat droplets red/orange, forming a layer on top of the aqueous phase
Example: Butter, ghee, fried chips → clear oily spot. Apple juice → spot dries and disappears.
Reagent: Biuret reagent (sodium hydroxide + copper sulphate)
Procedure:
- Take 2 mL of food sample (dissolved in water if solid)
- Add 2 mL of sodium hydroxide (NaOH) solution
- Add 2–3 drops of dilute copper sulphate solution
Observation:
- Positive result (protein present): Solution turns violet/purple
- Negative result (no protein): Remains blue (colour of CuSO₄ in NaOH, which gives pale blue)
Why it works: The Biuret reaction occurs with peptide bonds (−CO−NH−) in proteins. The Cu²⁺ ion coordinates with the nitrogen atoms of at least two peptide bonds, forming a violet-coloured complex. Amino acids give a different (blue) colour since they have only one peptide bond.
Simple alternative (heat test for protein — Millon’s test): Add Millon’s reagent (mercuric nitrate). Proteins containing tyrosine turn brick red on heating.
Example: Egg white, milk, lentils → violet colour. Glucose solution → no colour change (not a protein).
| Nutrient | Reagent | Positive Result | Negative Result |
|---|---|---|---|
| Starch | Iodine solution | Blue-black colour | Yellow-brown (unchanged) |
| Fat | Paper spot test | Translucent oily spot persists | Spot disappears on drying |
| Fat | Sudan III | Red/orange droplets | No colour |
| Protein | Biuret (NaOH + CuSO₄) | Violet/purple colour | Light blue |
Why This Works
These tests work by detecting specific molecular features:
- Starch: The helical structure of amylose physically traps iodine molecules
- Fat: Non-polar lipids cannot evaporate like water; they permanently coat the paper
- Protein: Peptide bonds coordinate with metal ions (Cu²⁺) to form coloured complexes
Understanding the mechanism behind each test is essential for CBSE Class 6–8 (where these are taught as practical activities) and for NEET (where the biochemical basis is tested).
Alternative Method
For the protein test in Class 6 context: the lead acetate test (simpler but less specific) can detect sulphur-containing amino acids. But the Biuret test is the standard for detecting proteins in general.
Common Mistake
The most common confusion is between the test for starch and the test for glucose (simple sugar). Iodine tests for STARCH, NOT for all carbohydrates. Glucose, sucrose, and fructose do NOT give a colour change with iodine. For reducing sugars (like glucose), Benedict’s test (blue → brick red precipitate on heating) is used. Never say “iodine tests for carbohydrates” — it specifically tests for starch.
For CBSE Class 6 practicals: practice describing the procedure in 4 steps (take sample, add reagent, observe, record result). Examiners want the exact colour change, not just “colour changes.” Always specify: blue-black for starch, violet for protein, translucent spot for fat.