14 September 2026 · 6 min read
Food tests for AQA Biology: Benedict's, biuret, iodine and emulsion
Exact methods and positive results for every test in Topic 1, including the non-reducing sugar test students most often get wrong.
Reducing sugars — Benedict's test
Add an equal volume of Benedict's reagent to the sample and heat in a water bath at about 95 °C for five minutes. A positive result changes from blue through green and yellow to a brick-red precipitate. The more reducing sugar, the further along the colour sequence — making it semi-quantitative.
Non-reducing sugars
- First do the Benedict's test — it stays blue.
- Boil a fresh sample with dilute hydrochloric acid to hydrolyse glycosidic bonds into monosaccharides.
- Neutralise with sodium hydrogencarbonate (Benedict's does not work in acidic conditions).
- Repeat the Benedict's test: a brick-red precipitate now shows a non-reducing sugar such as sucrose was present.
Starch, lipids and proteins
- Starch: add iodine in potassium iodide solution — orange-brown turns blue-black.
- Lipids (emulsion test): shake the sample with ethanol, then pour into water — a milky-white emulsion is positive.
- Proteins (biuret test): add sodium hydroxide then copper(II) sulfate solution — blue turns purple/lilac, detecting peptide bonds.
Making tests quantitative
Colour judgements are subjective. For a more objective measure, use a colorimeter: filter out the precipitate after Benedict's and measure the absorbance or transmission of the remaining solution, then compare against a calibration curve made from known glucose concentrations.
Advertisement
Revise this properly
This guide covers part of Topic 1: Biological molecules. Inside A-Plus Biology you get 95 flashcards and 22 exam-style questions with mark schemes for this topic alone.