Testing for biological molecules
Introduced| English | Meaning |
|---|---|
| carbohydrate/ˌkɑːbəʊˈhaɪdreɪt/ | An organic compound such as a sugar, starch or cellulose, made primarily from carbon, hydrogen and oxygen. |
| lipid/ˈlɪpɪd/ | A member of a group of largely water-insoluble biological molecules, including fats, oils, phospholipids and steroids. |
| reducing sugar/rɪˈdjuːsɪŋ ˈʃʊɡə/ | A sugar capable of reducing a suitable oxidising reagent, producing a positive Benedict's or similar test under appropriate conditions. |
| emulsion/ɪˈmʌlʃn/ | A mixture in which droplets of one liquid are dispersed in another liquid that does not mix with it. |
| semi-quantitative/ˈsemi ˈkwɒntɪteɪtɪv/ | Providing approximate amounts, ranges or comparisons rather than precise numerical measurements. |
| acid hydrolysis/ˈæsɪd haɪˈdrɒləsɪs/ | acid hydrolysis |
Which molecules are in your food?
- Living things are built from carbohydrates 碳水化合物, lipids 脂质, proteins and nucleic acids.
- Simple chemical tests reveal which are present in a sample.
- Each test has a clear colour change.
The four food tests
| Test | Finds | Positive result |
|---|---|---|
| Benedict's (heat) | reducing sugar 还原糖 | blue → green → orange → brick-red |
| iodine | starch | orange-brown → blue-black |
| emulsion 乳浊液 (ethanol then water) | lipid | white, cloudy emulsion |
| biuret (room temperature) | protein | blue → purple |
Benedict's test: blue turns green, then orange, then brick-red as more reducing sugar is present.
A sample is heated with Benedict's solution and turns brick-red. This shows the presence of:
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Benedict's test (with heat) detects reducing sugars; a positive result goes blue → green → orange → brick-red.
Orange-brown iodine solution turns blue-black. This detects:
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The iodine test detects starch — the orange-brown solution turns blue-black.
In the biuret test, a positive result for protein turns the solution from blue to ______.
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Biuret solution turns blue → purple when protein is present (no heating needed).
A positive emulsion test for a lipid gives a clear blue solution.
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A positive emulsion test gives a white, cloudy emulsion — not a colour change.
Match each test to the molecule it detects.
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Benedict's → reducing sugar; iodine → starch; emulsion → lipid; biuret → protein.
Semi-quantitative 半定量 Benedict's
- The normal Benedict's test only says yes/no. A semi-quantitative version gives a rough amount.
- First standardise it on known concentrations, then estimate an unknown by the time to the first colour change (more sugar → faster), or by comparing the final colour to standards.
Testing for non-reducing sugars
- Sugars like sucrose are non-reducing — they give a negative Benedict's test.
- To detect them: do a normal test (stays blue) → add dilute hydrochloric acid and heat (acid hydrolysis 酸水解 splits the sugar) → cool and neutralise → repeat Benedict's. A brick-red result now shows a non-reducing sugar was present.
A non-reducing sugar gives a negative test until acid hydrolysis splits it — then Benedict's turns red
- Step through it below.
Testing for a non-reducing sugar
Step through the trick: a non-reducing sugar stays blue, so you hydrolyse it with acid, neutralise, then re-test.
To test for a non-reducing sugar such as sucrose, you first:
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Acid hydrolysis breaks the non-reducing sugar into reducing sugars; after neutralising, Benedict's now gives brick-red.
Put the non-reducing sugar test in order.
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You must hydrolyse the non-reducing sugar into reducing units first, neutralise the acid, then the second Benedict's test works.
You've got it
- Benedict's (heat) → reducing sugar (brick-red); iodine → starch (blue-black)
- emulsion → lipid (white cloudy); biuret → protein (purple)
- semi-quantitative Benedict's estimates amount (time to change / colour standards)
- non-reducing sugar: hydrolyse with acid + heat, neutralise, then retest