What does iodine solution test for in foods?

• A. Fats
• B. Starch
• C. Proteins
• D. Sugars

Answer: (B)

Iodine solution detects starch in foods because starch’s long, helical molecules trap iodine inside their coils. When iodine is added, it slips into those helices and forms a blue-black complex, so the sample turns blue-black if starch is present. If there’s no starch, the colour stays the usual yellow-brown of iodine solution.

This is a handy test to tell starchy foods apart from others. It’s not used for fats, proteins, or simple sugars—different tests are used for those nutrients (for example, Benedict’s or Fehling’s tests for sugars, Biuret test for proteins, and Sudan III or emulsion tests for fats).