Specified practical 1.3B

Qualitative identification of starch (iodine), glucose (Benedict's reagent) and protein (biuret)

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

Method

How Benedict's test detect sugars

Results

The results from testing for sugars

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. The colour you’ll see is likely to be simply red or brown. 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 available as a single solution.

A diagram showing biuret solution being added to food in a test tube

Method

  1. Add 1 cm3 of biuret solution A to the food solution.

Results

A diagram showing the presence of protein in a solution

Hazards

  • Wear safety goggles.
  • Biuret solution is corrosive.
  • Avoid contact with skin and eyes.