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Food tests – sugars, starch, protein and fats in unknown foods

BiologyCell BiologyAges 15–16

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Run the classic food tests in a virtual lab: Benedict's reagent in a hot water bath for reducing sugars, iodine for starch, biuret for protein, and Sudan III or the ethanol emulsion test for lipids. Test milk, a glucose drink, egg white, bread, cooking oil, potato and 2–3 randomly chosen unknowns, fill in the results table, estimate the sugar concentration from the Benedict's colour scale, and check whether you identified the unknowns.

Lesson: Food tests for biological molecules

What it shows

Food tests detect the main biological molecules with reagents that change colour. Benedict's reagent contains blue copper(II) ions; when heated with a reducing sugar they are reduced to orange-red copper(I) oxide, so the colour moves from blue through green, yellow and orange to brick-red as the sugar concentration rises. Iodine turns blue-black with starch, biuret reagent turns purple with the peptide bonds of proteins, and lipids take up the red Sudan III dye or form a cloudy emulsion when their ethanol solution is poured into water. Sucrose is not a reducing sugar, so it leaves Benedict's reagent blue.

How to use

Choose a Sample and a Test, then press the step buttons in order, for example Add Benedict's and then Heat in water bath. Read the observation and click the cells of the results table to enter + or −. Use Glucose standard to build your own colour scale for reducing sugar. Pick a food for each unknown and press Check my answer; New unknowns gives a fresh set.

Parameters you can change

  • Food sample Milk, Glucose drink, Egg white, Bread, Cooking oil, Potato, Unknown A, Unknown B, Unknown C, Glucose standard solution
  • Test Benedict's reagent + water bath (reducing sugar), Iodine solution (starch), Biuret reagent (protein), Sudan III (lipid), Ethanol emulsion test (lipid)
  • Number of unknowns 2–3 samples
  • Glucose standard concentration 0–2.5 %
  • Show Benedict's colour scale

Questions to explore

  1. Why must the tube be heated in a water bath for the Benedict's test?
  2. Why does a solution of table sugar (sucrose) stay blue with Benedict's reagent?
  3. A sample is positive only for biuret and the emulsion test. Which food could it be?

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