Practical: Test for 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.

How Benedict's test detect sugarsThe results from testing for sugars

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
  • take care when heating in a boiling water bath

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 peptide bonds in proteins.

A diagram showing the presence of protein in a solution

Add Biuret solution A to a solution of the food being tested and mix carefully. Then trickle a little Biuret solution B down the side of the tube. Look for a purple colouration where the solutions meet.

Biuret reagent is sometimes available as a single 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

Sudan III test

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

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

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