Food Tests

Food tests use a reagent (a test chemical) that changes colour when a particular nutrient is present. You must know four tests for the Double Award.

Nutrient	Reagent	Heat?	Negative (no change)	Positive result
Glucose (reducing sugar)	Benedict's solution	Yes (water bath)	stays blue	brick-red
Starch	iodine solution	No	stays orange-brown	blue-black
Protein	Biuret solution	No	stays blue	purple/lilac
Fat (lipid)	ethanol + water (emulsion test)	No	stays clear	cloudy white emulsion

Exam tip. To score the result mark you must give the colour before and after — e.g. "blue → brick-red", not just "it goes red". A bare final colour often loses a mark.

Watch out. Only Benedict's needs heating. Students who write "heat the Biuret/iodine" lose method marks.

Method

1. Add the food sample to a test tube (grind/dissolve solid food in water first to make a solution).
2. Add an equal volume of blue Benedict's solution.
3. Heat the tube in a water bath (about 80 °C) for a few minutes.
4. Observe the colour change.

Results

• Positive (reducing sugar present): the blue solution turns green → yellow → orange → brick-red as more sugar is present. A brick-red precipitate forms with lots of glucose.
• Negative (no reducing sugar): the solution stays blue.

Skill — semi-quantitative result. Benedict's is the one test that gives a rough idea of how much sugar is present: the redder the colour, the more reducing sugar. This is useful in questions comparing samples.

Safety

• Heat using a water bath, not a naked flame, so the solution does not spit or boil over.
• Benedict's solution is an irritant — wear eye protection.

Starch — iodine test

1. Place the food sample on a spotting tile or in a test tube.
2. Add a few drops of orange-brown iodine solution (iodine in potassium iodide).
3. Observe — no heating is needed.

• Positive (starch present): orange-brown turns blue-black.
• Negative: stays orange-brown.

Protein — Biuret test

1. Add the food sample (in solution) to a test tube.
2. Add Biuret solution (or add dilute sodium hydroxide then a few drops of copper sulfate solution).
3. Observe — no heating is needed.

• Positive (protein present): blue turns purple/lilac (mauve/violet).
• Negative: stays blue.

Watch out. Iodine on a negative sample still looks orange-brown — that is the reagent's own colour, not a result. The positive change is the dramatic blue-black.

Fat does not dissolve in water but does dissolve in ethanol. The test uses this to reveal fat as a cloudy mixture.

Method

1. Add the food sample to a dry test tube.
2. Add a few cm³ of ethanol and shake/mix to dissolve any fat.
3. Pour the liquid into a second tube containing an equal volume of cold water.
4. Observe.

Results

• Positive (fat present): a cloudy white emulsion forms (tiny droplets of fat suspended in the water).
• Negative (no fat): the mixture stays clear/colourless.

Why it works. Fat dissolved in ethanol comes out of solution when mixed with water, forming an emulsion — countless tiny fat droplets scattering light, giving the milky appearance.

Safety

• Ethanol is highly flammable — keep it well away from naked flames; this is why the fat test is never heated.
• Use a clean, dry tube — water already present can give a false cloudy result.

Paper 1 (4WSD0/1B) tests the practical reasoning behind these tests, not just the colours.

Preparing a food sample

• Solid foods must be ground up and dissolved/mixed in water to make a solution (or suspension) the reagents can react with.
• Use the same volume of food and reagent so results are comparable.

Controls and fair testing

• A control is the same test on a sample known to contain no nutrient (or on distilled water). It shows the negative colour, so you can tell a real positive from the reagent's own colour.
• To compare foods fairly, keep volumes, temperature and timing the same — only the food changes.

Choosing the right test

• "Is there sugar?" → Benedict's. "Is there starch?" → iodine. "Is there protein?" → Biuret. "Is there fat?" → ethanol emulsion test.

Safety summary

• Benedict's: heat in a water bath, not a flame; irritant — wear eye protection.
• Biuret / sodium hydroxide: corrosive/irritant alkali — avoid skin and eyes.
• Ethanol: flammable — no naked flames anywhere nearby.
• Wear eye protection throughout and handle hot glassware with care.