4.2.2.1 Food tests: required practical — Human digestive system — AQA Combined Science Trilogy 8464

Describe the human digestive system as a complex organ system where multiple organs work cooperatively to digest and absorb food.
Describe the nature of biological enzyme molecules and explain how external changes in temperature and pH fundamentally affect their catalytic activity.
Carry out accurate rate calculations for chemical reactions catalysed by enzymes.
Explain how enzymes catalyse specific biological reactions in living organisms due to the unique complementary shape of their active site.
Apply the 'lock and key theory' as a simplified model to correctly illustrate and explain enzyme action.
Identify the primary production sites and describe the specific catalytic actions of amylase, proteases, and lipases within the body.

What food tests tell us

Foods can contain a mixture of carbohydrates, lipids and proteins. Their appearance alone does not reliably tell us which substances are present. In this required practical, you use qualitative reagents: chemicals that produce an observable change when a particular substance is detected.

A qualitative test identifies presence rather than measuring an exact amount. A positive result means the substance has been detected. Different tests answer different questions: Benedict’s solution detects reducing sugars, while iodine detects starch. Both sugars and starch are carbohydrates, but one test cannot replace the other.

Preparing and organising the samples

You need food samples, labelled test tubes and a rack, pipettes, distilled water and the appropriate reagents. Benedict’s test also needs a hot water bath and a thermometer. A measuring cylinder allows you to measure the Biuret solutions.

For solid foods being tested for sugars, starch or protein, break up a small sample using a pestle and mortar. Transfer it to a test tube, add distilled water and stir with a glass rod. Where needed, filter the mixture through filter paper in a funnel and collect the liquid. This makes the sample easier to mix with a reagent and observe. Keep a separate portion of the food for the lipid test.

Use a fresh portion for each test, rather than adding all the reagents to one tube. Label the tubes so that each observation can be matched to the correct food and test. Use clean equipment and separate pipettes for reagents to avoid transferring substances between samples.

Benedict’s test: detecting reducing sugars

Benedict’s solution starts blue. It tests for reducing sugars, including glucose, lactose and maltose. Sucrose is a non-reducing sugar and does not give a positive result in this test.

Put some prepared food sample into a test tube.
Add a few drops of Benedict’s solution.
Place the tube in a water bath at at least 80 °C for about five minutes. Check the water temperature with a thermometer.
Record the resulting colour.

A positive result is a change from blue towards green, yellow, orange or brick-red. If the solution remains blue, reducing sugar has not been detected. Although the range of colours can give a rough indication of the amount present under comparable conditions, this qualitative test does not give an exact concentration.

The water bath heats the mixture without putting the test tube directly into a Bunsen flame. It can be made using a beaker of water heated by a Bunsen burner, or an electric water bath can be used.

Iodine test: detecting starch

Iodine solution starts orange-brown and turns blue-black when starch is present.

Put some food sample into a clean test tube and add a few drops of iodine solution. Record the colour. A blue-black result indicates starch; remaining orange-brown is a negative result. No heating is needed.

This distinction matters when interpreting carbohydrates: a negative Benedict’s result does not rule out starch, and a negative iodine result does not rule out reducing sugars.

Ethanol test: detecting lipids

Lipids include fats and oils. In the ethanol test, the positive result is a change in appearance rather than a coloured product: a milky or cloudy emulsion forms.

Put a separate portion of the food sample into a clean test tube.
Add ethanol and shake gently to dissolve or extract any lipids into the ethanol.
Allow solid food particles to settle, then transfer the ethanolic liquid into another test tube containing distilled water, leaving the solids behind.
Mix gently and record the appearance.

Lipids dissolve in ethanol but do not dissolve in water. When the ethanolic extract is added to water, any lipids form tiny droplets dispersed through the water, producing a milky or cloudy emulsion. This is a positive result. If no cloudy emulsion forms and the liquid remains clear, the result is negative. The order matters: extract with ethanol first, then add the extract to water.

Ethanol is highly flammable. Keep it away from flames and turn off the Bunsen burner before working with it. This test does not need heating.

Biuret test: detecting protein

The Biuret test changes from blue to lilac or purple when protein is present. The reagent can be supplied as two solutions, Biuret A and Biuret B.

Put some prepared food sample into a clean test tube.
Measure and add 1 cm³ of Biuret solution A, followed by 1 cm³ of Biuret solution B.
Shake gently to mix.
Record the colour: lilac or purple is positive; blue is negative.

No heating is needed. Biuret solutions contain copper sulphate and sodium hydroxide. Handle them carefully: sodium hydroxide is corrosive, and copper sulphate is harmful. Wear eye protection throughout the practical, wash off splashes on skin immediately and deal with spills as instructed by your teacher. Take care with hot water and hot glassware during Benedict’s test.

Turning observations into conclusions

Record each food’s actual colour or appearance before deciding which substances were detected.

Test	Positive result	Negative result
Benedict’s, with heating	Green, yellow, orange or brick-red	Remains blue
Iodine	Blue-black	Remains orange-brown
Ethanol	Milky or cloudy emulsion	No cloudy emulsion; remains clear
Biuret	Lilac or purple	Remains blue

For example, suppose an unknown sample turns iodine blue-black and Biuret purple, but Benedict’s stays blue and no cloudy emulsion forms in the ethanol test. These results indicate that starch and protein were detected, while reducing sugars and lipids were not detected.

A food can test positive for more than one substance because foods are mixtures. Interpret each test independently, then combine the conclusions to describe the sample.

Match the test to the substance

Substance	Reagent and essential step	Positive result
Reducing sugar	Benedict’s solution; water bath at least 80 °C for about five minutes	Blue → green, yellow, orange or brick-red
Starch	Add iodine solution	Orange-brown → blue-black
Lipid	Shake sample with ethanol, then transfer the ethanolic liquid into distilled water	Milky/cloudy emulsion
Protein	Add 1 cm³ each of Biuret A and B; mix	Blue → lilac/purple

Essential distinctions

Only Benedict’s test requires heating.
Glucose, lactose and maltose are reducing sugars; sucrose is not.
A qualitative test detects presence, not an exact amount.
Use a fresh portion for each test. Record observations, then draw conclusions.

Safety

Wear eye protection. Keep flammable ethanol away from flames. Handle Biuret solutions carefully and wash off skin splashes immediately. Take care with hot water and glassware.

For each test, state the reagent, any heating needed and the positive result—not just ‘it changes colour’.

Benedict’s solution tests for reducing sugars, not all sugars. Sucrose does not give a positive result in this test.

Only Benedict’s test needs heating. Include a hot water bath in its method.

Record the observation before the conclusion: for example, ‘iodine turned blue-black, so starch was detected’.

A negative result means the substance was not detected by the test; it does not prove that absolutely none is present.

A test that indicates whether a substance is present in a sample rather than measuring its exact amount.

A chemical used to test for another substance, usually by producing an observable change.

A sugar that gives a positive result when heated with Benedict’s solution. Examples include glucose, lactose and maltose.

An observable change indicating that the substance being tested for has been detected.

A mixture in which tiny droplets of one liquid are dispersed through another. A cloudy emulsion is a positive result in the ethanol test for lipids.