Core practical: food tests

Key concepts in biology · Biological catalysts, food tests and energy in food · note 2 of 3

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Core practical: food testsSpec 1.13B

In short

Food tests use reagents to identify biological molecules by a colour change. Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a water bath, turns from blue to green, yellow, orange or brick-red with reducing sugars. Biuret reagent turns from blue to purple with protein, and ethanol forms a cloudy white emulsion with fats.

You can use chemical reagents to identify the main biological molecules in a food. If the food is a solid, crush it with a little water and use the liquid; for the fat test, shake the food with ethanol instead. Always wear eye protection.

Core practical:

A negative result means the reagent keeps its original colour. Learn the start colour as well as the positive result.

Food tests
Biological molecule	Reagent	Method	Positive result
Starch	Iodine solution	Add a few drops of iodine solution to the sample	Orange-brown to blue-black
Reducing sugars	Benedict's solution	Add Benedict's solution and heat in a hot water bath for a few minutes	Blue to green, yellow, orange or brick-red, depending on the amount of sugar
Protein	Biuret reagent	Add biuret reagent to the sample and mix	Blue to purple (lilac)
Fats	Ethanol	Shake the sample with ethanol, then pour the mixture into water	A cloudy white emulsion forms

In a negative result, iodine stays orange-brown, Benedict's solution and biuret reagent stay blue, and the ethanol test stays clear. Ethanol is flammable, so keep it away from naked flames, and Benedict's solution is heated in a water bath, not directly over a flame.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

Frequently asked questions

Why are enzymes important in the body?

Enzymes are important because chemical reactions in living things would be far too slow to keep us alive without them. As biological catalysts, they speed up reactions in cells. They are needed both to break down large molecules such as proteins, carbohydrates and lipids, and to build them up from smaller molecules.

What are carbohydrates, proteins and lipids broken down into?

Carbohydrates are broken down into sugars, proteins into amino acids, and lipids into fatty acids and glycerol. Each reaction is catalysed by a different enzyme, because enzymes are specific. The same small molecules are also used to synthesise the large carbohydrates, proteins and lipids that the body needs.

How do you test for protein in food?

Triple only To test for protein, add biuret reagent to a liquid sample of the food and mix. If protein is present, the colour changes from blue to purple (lilac). If there is no protein, it stays blue. Solid food is first crushed with a little water. Always wear eye protection during food tests.

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