Required practical 3 · Biology · Biology Paper 1, both tiers
Food tests
Use qualitative reagents to test a range of foods for carbohydrates (sugars and starch), lipids and proteins.
The exam trap. Learn each test as reagent, method and positive colour, and do not leave out the lipid test.
Variables
- Independent (you change)
- The food sample tested
- Dependent (you measure)
- The colour change seen with each reagent
- Control (you keep the same)
- The volume of reagent added
- The amount of food sample
- The temperature and time for the Benedict's test
Apparatus
- Food samples, ground with a pestle and mortar and mixed with distilled water
- Test tubes and rack
- Benedict's solution and a water bath at about 75 °C
- Iodine solution and a spotting tile
- Biuret reagent
- Sudan III stain (or ethanol for the emulsion test)
- Pipettes and filter funnel
Method, and why each step matters
Grind the food with distilled water and filter it to make a food solution.
Why The reagents need the nutrients in solution to react.
Sugars: add Benedict's solution and heat in a water bath for a few minutes. A positive result goes from blue to green, yellow or brick red.
Why Benedict's reacts with reducing sugars only when heated; the colour shows roughly how much sugar.
Starch: add a few drops of iodine solution. Orange-brown turns blue-black if starch is present.
Why Iodine forms a blue-black complex with starch.
Protein: add biuret reagent. Blue turns purple (lilac) if protein is present.
Why Biuret reacts with the peptide bonds in proteins.
Lipids: add a few drops of Sudan III and shake. A red-stained layer separates on top if lipid is present.
Why Sudan III dissolves in lipids, which float on water.
Record the colour before and after for every test.
Why A colour change is only meaningful compared with the starting colour.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Hot water bath can scald. | Use a water bath, not a naked flame; handle tubes with a holder. |
| Biuret reagent contains sodium hydroxide, which is corrosive. | Wear eye protection and wash skin at once. |
| Sudan III and ethanol are flammable and stain. | Keep away from flames; wear gloves. |
A worked set of results
| Food | Benedict's | Iodine | Biuret | Sudan III |
|---|---|---|---|---|
| Glucose drink | brick red | orange-brown | blue | no red layer |
| Potato | blue | blue-black | blue | no red layer |
| Egg white | blue | orange-brown | purple | no red layer |
| Olive oil | blue | orange-brown | blue | red layer |
- Each test is qualitative: it shows whether a nutrient is present, not how much (except roughly for Benedict's colours).
- A negative result is the reagent keeping its own colour.
Mistakes that cost marks
Mixing up the colours: purple is protein, blue-black is starch, brick red is sugar.
Instead Learn each test as reagent, method, positive colour.
Forgetting to heat the Benedict's test.
Instead Benedict's only reacts when heated in a water bath.
Leaving out the lipid test because only three reagents are named.
Instead Lipid testing is still required: use Sudan III or the emulsion test.