Food Tests — Nutrition & Digestion | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 16% of your exam marks.

stable

Very High

Stable16%

Enzymes in digestion and the role of digestive structures appear regularly across both papers.

Four standard food tests detect the four major nutrient groups. Each works by adding a reagent to a food sample and watching for a specific colour change.

Preparing a food sample

For solid food, before testing:

Grind the food into a paste using a pestle and mortar
Add a small volume of distilled water and stir
Filter through filter paper to give a clear solution to test

The four tests

Test for	Reagent	Method	Positive result
Glucose (reducing sugar)	Benedict's solution	Add a few drops; heat in a water bath for 5 minutes	Blue → brick-red (orange/yellow if less glucose)
Starch	Iodine solution	Add a few drops at room temperature	Yellow-brown → blue-black
Protein	Biuret solution	Add a few drops at room temperature	Blue → violet (lilac/purple)
Lipids	Ethanol (then water)	Mix the sample with ethanol, then add an equal volume of cold water	Clear → cloudy white emulsion

Notes for each test

The Benedict's test needs heat. Use a water bath rather than a Bunsen burner directly because the test tubes are easy to break and the Bunsen flame is hard to control. Switch the burner off once the water is hot.
The iodine test is quick and simple, with no heating needed.
Biuret solution contains copper sulfate and sodium hydroxide, both of which are irritants. Always wear goggles.
Ethanol is highly flammable. Keep it well away from any naked flame.