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.
Reading it isn't running it.
Collect the apparatus, build the setup and take your own readings — as many times as it takes.