Core Practical: Food Tests

Iodine Test for Starch

• Place one spatula of the food sample or 1 cm³ if the sample is liquid onto a dish.
• Using a dropper, add a few drops of iodine solution to the food sample.
• Observe and record any colour change.
• Positive Result: If starch is present, the iodine solution will turn blue-black.

Benedict's Test for Reducing Sugars

• Place two spatulas of the food sample or 1 cm³ of a liquid sample into a test tube.
• Add 1 cm³ of water and stir to mix.
• Add an equal volume of Benedict's solution.
• Place the test tube in a water bath at 95°C for a few minutes.
• Observe and record the colour of the solution.
• Positive Result: If reducing sugars are present, the Benedict's solution will change from blue to cloudy orange or brick red.

Sugars - Benedicts reagant test results

Biuret Test for Proteins

• Place one to two spatulas of the food sample or 1 cm³ of liquid sample into a test tube.
• Add 1 cm³ of water and stir to mix.
• Add an equal volume of potassium hydroxide solution and stir.
• Add two drops of copper sulfate solution and stir for two minutes.
• Observe and record the colour of the solution.
• Positive Result: If proteins are present, the solution will turn mauve or purple.

Proteins - biuret test results

Emulsion Test for Lipids

• Place two spatulas of the food sample or 1 cm³ of a liquid sample into a test tube.
• Add 2 cm³ of ethanol and shake vigorously.
• Allow the mixture to settle.
• Pour the liquid from the top into a test tube half-filled with distilled water.
• Observe and record whether the water becomes cloudy or remains clear.
• Positive Result: If lipids are present, a milky-white emulsion will form.

Lipids - emulsion test results

Results

Food	Iodine Test (Starch)	Benedict's Test (Reducing Sugars)	Biuret's Test (Proteins)	Emulsion Test (Lipids)
Beef	Orange-brown	Blue	Purple	Milky white
Vegetable Oil	Orange-brown	Blue	Blue	Milky white
Rice	Blue-black	Blue	Blue	Colourless
Breakfast Cereal	Blue-black	Orange	Blue	Milky white

Conclusion

The tests show the presence of starch, reducing sugars, proteins, and lipids in different food samples:

• Beef contains proteins and lipids.
• Vegetable oil contains lipids but no proteins or starch.
• Rice contains starch but no reducing sugars, proteins, or lipids.
• Breakfast cereal contains starch, reducing sugars, and lipids.