Food tests: starch, sugars, protein and fat
Four classic bench tests — iodine, Benedict's, biuret and the emulsion test — run on real food samples to detect the major nutrients.
WHAT YOU'LL LEARN
- ✓The reagent, method and positive result for each of the four tests
- ✓Which tests need heat (Benedict's) and which must never see it (biuret)
- ✓Semi-quantitative reading: Benedict's traffic-light scale from green to brick red
ON YOUR BENCH
- □Test tubes, rack and holder
- □Water bath (beaker + Bunsen or electric)
- □Iodine solution, Benedict's solution, biuret reagents (NaOH + CuSO₄), ethanol
- □Food samples: ugali, glucose solution, egg white solution, cooking oil, and a mystery sample
- □Droppers and a white spotting tile
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Starch — iodine test
Put a piece of ugali on the spotting tile and add 2 drops of iodine solution. Blue-black = starch present. Yellow-brown = negative.
- 02
Reducing sugars — Benedict's test
Add 2 cm³ of glucose solution and an equal volume of Benedict's solution to a test tube. Heat in a boiling water bath for 3–5 minutes. Watch the colour climb the scale: blue → green → yellow → orange → brick red.
- 03
Protein — biuret test
Add 2 cm³ of egg white solution, then an equal volume of dilute NaOH, then 2 drops of dilute copper sulfate. Shake gently. Purple/violet = protein. No heating — heat denatures the result.
- 04
Fat — emulsion test
Shake a few drops of cooking oil with 2 cm³ of ethanol until dissolved, then pour into a tube of water. A milky-white emulsion = fat present.
- 05
The mystery sample
Run all four tests on the mystery sample and build its nutrient profile. Write a conclusion naming the food you think it is.
Positive results at a glance
- →Benedict's is semi-quantitative: brick red means lots of reducing sugar, green means a trace.
- →A cloudy biuret result usually means too much copper sulfate — 2 drops is genuinely enough.
| Test | Nutrient | Positive result |
|---|---|---|
| Iodine | Starch | Yellow-brown → blue-black |
| Benedict's (heated) | Reducing sugar | Blue → green → orange → brick red |
| Biuret | Protein | Blue → purple/violet |
| Emulsion | Fat | Clear → milky-white emulsion |
⚠ BEFORE YOU START
- •Point heated test tubes away from everyone, including yourself.
- •Ethanol is flammable — keep the stock bottle away from the Bunsen and use the water bath for all heating.
- •Iodine stains skin and clothes; use the droppers provided.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.