Food tests
| English | Chinese | Pinyin |
|---|---|---|
| food tests | 食物检测 | shí wù jiǎn cè |
| iodine | 碘 | diǎn |
| reducing sugar | 还原糖 | huán yuán táng |
| Benedict's | 本尼迪克特试剂 | běn ní dí kè tè shì jì |
| Biuret | 双缩脲 | shuāng suō niào |
| emulsion | 乳浊液 | rǔ zhuó yè |
| control | 对照 | duì zhào |
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:
Iodine turns blue-black with starch.
Benedict's solution, when heated with a reducing sugar, turns:
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.
Each test gives a distinct colour change.
The ______ test turns purple when protein is present.
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.
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