Required practical activity

Use qualitative reagents to test for a range of carbohydrates, lipids and proteins

Qualitative tests for foods

There are several qualitative tests for food chemicals. These can be used to detect the presence of food chemicals, but not how much is present.

Test for sugars

Benedict's test is used to detect sugars.

Method

How Benedict's test detect sugarsHeat in a water bath

Result

The results from testing for sugarsAfter heating, it may go through stages - green, yellow, orange, red or brown - depending on how much glucose is present

Sugars classed as reducing sugars will react with Benedict's solution on heating for a few minutes. Glucose is an example of a reducing sugar.

Reducing sugars give a red-brown precipitate with Benedict's solution. The precipitate takes a while to settle in the tube – you're more likely to see simply a red or brown colour. If there's not much glucose present, the final colour may be green or yellow, or orange if there's a little more.

Hazards

  • Wear safety goggles.
  • Benedict's solution is an irritant.
  • Avoid contact with skin and eyes.

Test for starch

Add iodine solution to the food being tested.

Image of a potato slice and a pipette of iodine being placed on it

Foods containing starch will turn a blue-black colour.

The iodine test can also be used with a microscope to stain starch grains in plant cells.

Hazards

  • Wear safety goggles.
  • Iodine solution is an irritant.
  • Avoid contact with skin and eyes.

Test for proteins

The biuret test is used to detect proteins.

Biuret reagent is sometimes available as a single solution.

Method

  1. Add 1 cm3 of biuret solution A to the food solution.
  2. Mix the liquids.
  3. Add 1 cm3 of biuret solution B and shake.

Results

A diagram showing the presence of protein in a solution

Hazards

  • Wear safety goggles.
  • Biuret solution A is corrosive.
  • Biuret solution B is an irritant.
  • Avoid contact with skin and eyes.

Test for fats

The Sudan III test

The Sudan III test is one test used to test for lipids.

Method

A diagram showing two test tubes containing food and water and Sudan III

Results

A diagram showing a test tube. Inside it there is a red layer.

Hazards

  • Wear safety goggles.
  • Sudan III solution is flammable as it is dissolved in alcohol.
  • Avoid contact with skin and eyes.

The emulsion test

The emulsion test is an alternative test for lipids.

Diagram showing how the emulsion test can detect lipids in a solution

Hazards

  • Wear safety goggles.
  • Ethanol is flammable.
  • Avoid contact with skin and eyes.