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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.

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.

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.

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 used to test detect lipids.

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.

Hazards

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