Food tests and the enzyme activity practical
Four simple tests tell you which nutrients a food contains. The enzyme practical uses one of them, the iodine test, to time how quickly amylase breaks down starch at different pH values.
Part 1 of 3: Learn it
In short
- Benedict's for reducing sugars (heat it), iodine for starch, biuret for protein, Sudan III for lipids.
- Grind the food with a little water first to make a solution or suspension.
- In the amylase practical, record the time until the iodine no longer turns blue-black.
Where this is in your specification
Spec points: AQA 4.2.2.1 (required practicals 4 and 5), Edexcel Topic 1 (core practical), OCR B1.4
| Board | Topic: Organisation: tissues, organs, digestion and circulation |
|---|---|
| AQA 8461 | 4.2 (8461); 4.2 (8464) |
| Edexcel 1BI0 | Topics 2 and 8 |
| OCR J247 | B2 and B3 |
| Eduqas C400 | 1.3, 2.2, 2.3 |
| Cambridge IGCSE 0610 | 4.1, 5.1, 7.1 to 7.5, 8.1 to 8.4, 9.1 to 9.4, 11.1 |
| National 5 C807 75 | Cell biology: 4 Proteins; Multicellular organisms: 5 Transport systems, 7 Absorption of materials |
The four tests
| Testing for | Reagent | Method | Positive result |
|---|---|---|---|
| Reducing sugars, such as glucose | Benedict's solution | add, then heat in a water bath at about 75 °C | blue to green, yellow or brick red |
| Starch | iodine solution | add a few drops | orange-brown to blue-black |
| Protein | biuret reagent | add and shake | blue to purple (lilac) |
| Lipids | Sudan III | add and shake | a red-stained oily layer forms on top |
The Benedict's colour gives a rough idea of how much sugar there is: green means a little, brick red means a lot. Ethanol can also test for lipids: shake the food with ethanol, pour into water, and a cloudy white layer shows fat.
Safety
- Wear eye protection: biuret reagent contains a corrosive alkali.
- Heat Benedict's tests in a water bath, not over a flame.
- Sudan III and ethanol are flammable: keep them away from open flames.
The amylase and pH practical
- Put a drop of iodine in each dip of a spotting tile.
- Mix starch solution, amylase and a buffer of one pH in a test tube kept in a water bath at a fixed temperature.
- Every 30 seconds, put a drop of the mixture onto a fresh drop of iodine.
- Stop when the iodine stays orange-brown: the starch has gone. Record that time.
- Repeat with buffers at other pH values.
The rate is 1 ÷ time. Control the temperature, the volumes and the concentrations, so that pH is the only thing changing.
Which reagent tests for protein, and what is a positive result?
Show the answer
Biuret reagent; it turns from blue to purple.
Part 2 of 3: See it worked
Worked examples
Example 1
A food sample is tested. Biuret stays blue, iodine turns blue-black, and Benedict's turns green after heating. What does it contain?
- Biuret stayed blue: no protein
- Iodine turned blue-black: starch present
- Benedict's green: a small amount of reducing sugar
Answer: Starch and a little reducing sugar, but no protein.
Example 2
At pH 4 the starch has gone after 240 s, and at pH 7 after 60 s. Compare the rates.
- pH 4: 1 ÷ 240 = 0.0042 s⁻¹
- pH 7: 1 ÷ 60 = 0.0167 s⁻¹
- The time at pH 4 is 240 ÷ 60 = 4 times as long, so the rate is a quarter
Answer: The reaction is 4 times faster at pH 7.
Common mistakes
- Forgetting to heat the Benedict's test.
- Describing the iodine colour change the wrong way round: it starts orange-brown and goes blue-black if starch is there.
- Saying the amylase works fastest at the pH with the longest time. Longest time means slowest rate.
- Not controlling temperature in the enzyme practical.
Why is a buffer solution used in the amylase practical?
Show the answer
To keep each tube at one fixed pH, so the effect of pH can be measured.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Which reagent tests for protein, and what is a positive result?
Biuret reagent; it turns from blue to purple.
Why is a buffer solution used in the amylase practical?
To keep each tube at one fixed pH, so the effect of pH can be measured.
What colour does iodine stay once all the starch has been broken down?
Orange-brown.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses