Food tests required practical

Food tests use chemical reagents to show which nutrients a food contains. There are four standard tests: Benedict's test for sugars (after heating, the blue reagent changes to green, yellow, orange or brick red), the iodine test for starch (orange-brown turns blue-black), the biuret test for protein (blue turns purple or lilac) and the ethanol emulsion test for lipids (a cloudy white layer forms). In AQA's GCSE Biology this is required practical 4, and in Combined Science Trilogy it is required practical 3.

Food tests: results at a glance

The colours in this table are the ones AQA's mark schemes accept. A 2021 Combined Science mark scheme accepts "purple / lilac" for biuret, "black / blue-black / dark blue" for iodine, and "(brick) red / orange / brown / green / yellow" for Benedict's after heating in a water bath. A 2023 GCSE Biology mark scheme gives purple as the colour biuret turns with protein.

Aim

AQA's specification sets the aim: use qualitative reagents to test for a range of carbohydrates, lipids and proteins, to include Benedict's test for sugars, the iodine test for starch and biuret reagent for protein. Qualitative means the tests show whether a nutrient is there, not exactly how much.

Equipment

AQA's required practicals handbook lists:

• a sample of the food to be tested, and a pestle and mortar to grind it;
• a stirring rod, a filter funnel and filter paper;
• beakers, a conical flask, test tubes, labels and distilled water;
• a water bath (electric, or a beaker of water heated with a Bunsen burner) and a thermometer;
• pipettes;
• Benedict's solution, iodine solution (0.01 mol/dm³), ethanol and biuret solution.

Preparing the food sample

1. Grind a small amount of the food with a pestle and mortar.
2. Add some distilled water and stir to dissolve as much of the food as possible.
3. Filter the mixture to remove solid lumps, collecting the solution in a flask.
4. Split the solution between labelled test tubes, one for each test.

Benedict's test for sugars

1. Put some of the food solution into a test tube.
2. Add a few drops of Benedict's solution. It is blue.
3. Put the test tube in a water bath at a minimum of 80 °C for about 5 minutes, as AQA's handbook describes.
4. Record any colour change.

If a reducing sugar is present, the blue solution turns green, then yellow, orange or brick red. The more sugar there is, the further along that sequence it goes, so the colour gives a rough idea of how much sugar is present. AQA's handbook notes that glucose, lactose and maltose are reducing sugars and give a positive test, but sucrose (table sugar) is a non-reducing sugar and does not.

The iodine test for starch

1. Put some of the food solution into a test tube, or a drop onto a spotting tile.
2. Add a few drops of iodine solution. It is orange-brown.
3. Record the colour.

If starch is present, the colour changes to blue-black. If not, it stays orange-brown. No heating is needed. AQA's handbook warns that grated cheese bought in packs often contains starch, which can confuse results.

Test for protein: the biuret test

1. Put some of the food solution into a test tube.
2. Add biuret solution. AQA's handbook suggests making it up as two solutions, biuret A and biuret B, added one after the other. Biuret reagent contains copper sulfate and sodium hydroxide.
3. Shake gently and record the colour.

If protein is present, what colour will biuret change to? Purple, or lilac if there is only a little protein. The reagent starts blue, and stays blue if there is no protein.

Test for lipids: the emulsion test

1. Put some of the food (it does not need to be dissolved in water first) into a dry test tube.
2. Add ethanol and shake well, so any lipid dissolves in the ethanol.
3. Pour the ethanol into a test tube of water.
4. Look for a cloudy white layer.

A cloudy white layer, an emulsion, shows lipids are present: the fat dissolved in ethanol but comes out as tiny droplets when it meets water. AQA's handbook mentions a simpler alternative: rub the food on filter paper, and if the paper goes translucent (see-through), lipid is present.

Variables and a fair test

Food tests are qualitative, but comparisons between foods are only fair if you keep other things the same:

• the same volume of food solution in each tube;
• the same volume of reagent;
• for Benedict's, the same water bath temperature and heating time.

A control helps too: a tube of distilled water tested in the same way shows what a negative result looks like, so you can tell a small colour change from no change.

Risks

Results table

AQA's handbook gives a table like this to record results, with a row for each food:

Testing pure glucose and starch solutions alongside your foods gives you known positive and negative results to compare colours against. Processed foods often contain added starch or sugar, so their results can surprise you.

Here is a quick check.

Errors and improvements

• Judging colours is subjective. Small colour changes are easy to misread. Compare against a control tube and a known positive, and ask a partner to agree the colour.
• Benedict's not hot enough. Below about 80 °C the colour change may be weak or missing. Check the water bath with a thermometer.
• Contamination. Use clean pipettes for each reagent and each food, or a trace of one sample can give a false positive in another.
• Only a rough amount. To compare sugar content more precisely, use the same volumes and heating time and compare colours against a set of standard glucose solutions, or use a colorimeter at A-level.

Food tests at A-level

AQA's A-level Biology specification extends these tests: Benedict's solution for reducing sugars and non-reducing sugars, iodine in potassium iodide for starch, the emulsion test for lipids and the biuret test for proteins. A non-reducing sugar such as sucrose gives a negative Benedict's test at first. Breaking it down by boiling with dilute acid, then neutralising and repeating Benedict's test, gives a positive result if a non-reducing sugar was present.

Common mistakes

• Heating the biuret test. Only Benedict's test needs heating.
• Saying iodine turns "blue". AQA's mark scheme accepts black, blue-black or dark blue, and ignores just "blue" or "purple".
• Saying Benedict's tests for "sugar" in general. It detects reducing sugars; sucrose does not give a positive result.
• Using ethanol near a flame. Ethanol is flammable.
• Forgetting the starting colour. A result is a change: say what colour it was and what it turned.

Practise biology in a short timed paper

GCSE Biology Foundation and GCSE Biology Higher each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so required practicals and digestion turn up regularly but not in every paper.

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