1.13B - Food tests
Use chemical reagents to identify starch, reducing sugars, proteins and lipids. Connect each method to its observation and use controls, separate samples and appropriate precautions to make conclusions trustworthy.
1.13B — Evidence from chemical reagents
A food can contain several different biological molecules at once. Looking at it is not enough to identify them, so a chemical reagent is added to a prepared sample. If the target substance is present and the test conditions are correct, the reagent produces a characteristic visible observation.
Positive and negative results
A positive result is the specified observation showing that the target substance was detected. A negative result means that the substance was not detected under the test conditions; it does not prove that every type of carbohydrate, protein or lipid is absent.
For a solid food, crush a small mass with distilled water to make a suspension or extract. Use separate labelled portions for the different tests so that each receives only one reagent sequence. For the lipid test, extract a fresh portion of the food with ethanol first; use the clear ethanol extract for addition to water so undissolved food particles are not mistaken for an emulsion. Use a clean spatula or pipette for every food and every reagent: material carried from another tube could create a false positive.
The food identity is the input being compared. The dependent observation is the colour or cloudiness after the correct treatment. When foods are being compared, use the same mass of food, volume of water, volume of sample and volume of reagent. Food tests are Biology-only, but are required at both Foundation and Higher tier.
1.13B — The four food tests
Each test has four linked pieces: the substance, the reagent, any required treatment, and the observation. A colour name without the correct method does not provide reliable evidence.
| Target substance | Reagent and method | Negative observation | Positive observation and conclusion |
|---|---|---|---|
| Starch | Add a few drops of iodine solution to the food sample at room temperature. | Iodine remains yellow-brown or orange-brown. | It changes to blue-black, so starch is present. |
| Reducing sugar | Add Benedict's solution to a separate portion. Place the tube in a hot water bath at about 95-100 degrees C for several minutes. | The mixture remains blue. | A green, yellow, orange or brick-red precipitate forms, so reducing sugar is present. |
| Protein | Add potassium hydroxide solution, then add a few drops of copper sulfate solution and mix gently. This is the Biuret test; do not heat it. | The mixture remains blue or light blue. | It changes to lilac or purple, so protein is present. |
| Lipid (fat or oil) | Add ethanol to a separate sample and shake so any lipid dissolves. Add the ethanol mixture to water. | The mixture remains clear. | A cloudy white or milky emulsion forms, so lipid is present. |
Pearson specifies potassium hydroxide followed by copper sulfate for the core practical. Sodium hydroxide followed by copper sulfate is also a chemically valid Biuret test: both provide the alkaline conditions needed for the purple protein result. A question asking for the two reagents should credit either alkali unless it explicitly asks for Pearson's named recipe.
The order of the emulsion test matters. Ethanol is used first because lipid dissolves in it. When water is then added, the lipid forms tiny dispersed droplets, making the mixture look cloudy rather than clear.
Benedict's colours can range from green to brick red. In this core practical, any of those changes is used as a positive identification; an exact reducing-sugar concentration cannot be read reliably from an uncalibrated colour by eye. A Benedict's test that stays blue means no reducing sugar was detected, not that the food contains no carbohydrate of any kind.
Safety decisions belong to particular hazards. Wear eye protection against reagent splashes. Use a test-tube holder around the hot water bath, keep the tube opening pointed away from people, and take care because hot water and hot glass can scald. Ethanol is highly flammable, so keep it capped and away from Bunsen burners, burning food and other ignition sources. Do not taste laboratory food samples, and check food-allergy risks before practical work.
1.13B — Making food-test conclusions trustworthy
Food-test data are mainly qualitative: they record what was observed and whether the target was detected. Suppose an unknown gives these results:
| Test | Observation | Conclusion |
|---|---|---|
| Iodine | Remains yellow-brown | No starch detected |
| Benedict's after heating | Orange precipitate | Reducing sugar detected |
| Biuret | Purple | Protein detected |
| Emulsion | Remains clear | No lipid detected |
The conclusion comes from matching each observation to its own test. The four reagents must not be mixed in one tube because they could interfere with one another; use fresh portions of the same food extract.
A positive control containing the target substance checks that a reagent and method can produce the expected positive result. A negative control made with distilled water should stay negative; if it changes, contamination or a faulty procedure is likely. Controls are comparison setups. They are different from controlled variables such as sample volume, reagent volume, Benedict's water-bath temperature and heating time.
Repeat each test with a fresh portion. Agreement between repeats improves confidence in repeatability. If repeats disagree, investigate contamination, labelling, unequal volumes or an unclear endpoint rather than averaging colour names. Viewing tubes against a white background and using the same timing makes observations easier to compare, although a strongly coloured food can still mask a colour change.