IGCSE Biology / Food and digestion

Testing for starch, sugars, protein and fat

Edexcel International GCSE Biology (4BI1), chapter 4: Food and digestion, specification point 2.9. Free lesson.

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Food tests use specific reagents and colour changes to identify starch, reducing sugar, protein and fat. To describe a positive result fully, give the reagent, the starting and final colours, and whether heating is needed.

How to test for starch

Iodine solution tests for starch. The positive result is a clear colour change from orange-brown to blue-black. This test does not need heating. A negative result stays orange-brown.

How to test for reducing sugar

Benedict’s solution tests for reducing sugars, including glucose. It starts blue, and this is the only one of these four tests that needs heating. The final colour gives a rough indication of how much reducing sugar is present. Blue means no reducing sugar was detected. As the amount rises, the result changes through green, yellow and orange to brick red. Use a water bath at about 80 °C for a few minutes, not a naked flame. Benedict’s solution can spit if heated directly.

How to test for protein

Biuret solution tests for protein. It starts blue and turns purple or lilac when protein is present. No heating is needed. A negative result stays blue.

How to test for fat

The emulsion test uses ethanol followed by cold water. Fat dissolves in ethanol but not in water. When the mixture is poured into water, the fat forms tiny droplets that scatter light, making the mixture look cloudy white. A cloudy white emulsion means fat is present. If the water stays clear, fat was not detected. Use cold water and a clean, dry test tube. A wet tube can make the ethanol go cloudy by itself, giving a false positive.

How to describe results fully

A colour change is not a complete answer unless you state the starting colour, the final colour and the condition needed for the test. For Benedict’s test, simply saying that it turns red leaves out the heating step and the range of possible results. For a large amount of reducing sugar, say that blue Benedict’s solution turns brick red after heating in a water bath. For starch, give orange-brown to blue-black; for protein, blue to purple or lilac; for fat, describe a cloudy white emulsion after ethanol is mixed with cold water.

More lessons in Food and digestion

• Carbohydrates, proteins and lipids
• The seven parts of a balanced diet
• What changes how much energy you need
• The parts of the digestive system
• Teeth, chewing and peristalsis
• Amylase, protease and lipase
• Bile, the liver and emulsifying fat
• The villus and absorption
• Practical: measuring the energy in a food sample