Food tests
Introduced| English | Meaning |
|---|---|
| food tests/fuːd tests/ | food tests |
| iodine/ˈaɪədiːn/ | A halogen element; its solution is used to test for starch. |
| reducing sugar/rɪˈdjuːsɪŋ ˈʃʊɡə/ | A sugar capable of reducing a suitable oxidising reagent, producing a positive Benedict's or similar test under appropriate conditions. |
| Benedict's/ˈbenɪdɪkts/ | Benedict's |
| Biuret/ˈbɪjuːrət/ | Biuret |
| emulsion/ɪˈmʌlʃn/ | A mixture in which droplets of one liquid are dispersed in another liquid that does not mix with it. |
| control/kənˈtrəʊl/ | A comparison test missing only the variable being investigated. |
Testing for food molecules
- How do you prove a food contains starch, sugar, protein or fat? With simple food tests 食物检测 that change colour.
- Each test gives a clear, named colour change.
Food test lab
Follow how the test result reveals the nutrient in a sample.
A positive iodine test for starch gives a:
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Iodine turns blue-black with starch.
Benedict's solution, when heated with a reducing sugar, turns:
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Benedict's goes blue → brick-red with reducing sugar.
Starch and sugar
- Starch: add iodine 碘 solution → turns from orange-brown to blue-black.
- Reducing sugar 还原糖: add Benedict's 本尼迪克特试剂 solution and heat → turns from blue to brick-red.
Each food test gives its own colour change for a positive result
Match each food molecule to its test result.
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Each test gives a distinct colour change.
The ______ test turns purple when protein is present.
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Biuret detects protein (blue → purple).
Protein and fat
- Protein: add Biuret 双缩脲 solution → turns from blue to purple/violet.
- Fat: the ethanol (emulsion 乳浊液) test → a cloudy white emulsion forms.
Bread is rich in carbohydrates, one of the main food groups
The Benedict's test for reducing sugar must be heated.
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Benedict's needs heating in a water bath; the others are done cold.
Doing tests safely and fairly
- Benedict's test must be heated (in a water bath); the others are done cold.
- Use a control 对照 (a sample you know the result of) to check the test works.
Worked example. A student tests milk. Biuret turns purple (protein present) and the emulsion test goes cloudy (fat present) — so milk contains both protein and fat.
Carbohydrates and fats contain C, H and O; proteins also contain nitrogen
You've got it
- iodine → blue-black = starch; Benedict's (heated) → brick-red = reducing sugar
- Biuret → purple = protein; ethanol/emulsion → cloudy white = fat
- only Benedict's needs heating; use a control to check the test