on this cheat sheet
Everything on the GCSE Biology Food Tests poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
Starch – Iodine Test
- Add a few drops of iodine solution to the food sample.
- Positive result: the solution turns blue-black.
- Negative result: the solution stays brown/orange.
Iodine binds to starch, producing the blue-black colour. The test works on solid and liquid samples.
Reducing Sugars – Benedict's Test
- Mix the sample with Benedict's reagent (blue).
- Heat in a water bath.
- Positive result: a coloured precipitate forms – green, yellow, orange or brick red depending on the concentration of reducing sugar.
- Negative result: the solution stays blue.
Brick red indicates a high concentration of reducing sugars such as glucose. The test does not detect non-reducing sugars such as sucrose unless they are first broken down.
Protein – Biuret Test
- Add Biuret reagent (or sodium hydroxide followed by copper sulfate) to the sample.
- Positive result: the solution turns purple.
- Negative result: the solution stays blue.
The purple colour shows that peptide bonds in protein are present.
Lipids – Ethanol Emulsion Test
- Dissolve the sample in ethanol, then pour into water.
- Positive result: a milky white emulsion forms on top of the water.
- Negative result: the solution stays clear.
Lipids are not soluble in water but form a cloudy emulsion when ethanol is added to water. Ethanol is highly flammable – keep it away from naked flames and use a water bath when heating other tests nearby.