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Biological molecules and food tests
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Carbohydrates, proteins and lipids are large molecules built from smaller units. Simple tests show which ones are in a food.
Elements and building blocks
Food tests
Doing the tests safely
Grind solid food with a little water first.
Use a water bath to heat Benedict's tests, not a naked flame, and keep ethanol away from flames because it is flammable.
A negative result keeps the starting colour: Benedict's stays blue, iodine stays orange-brown, biuret stays blue.
Use a water bath to heat Benedict's tests, not a naked flame, and keep ethanol away from flames because it is flammable.
A negative result keeps the starting colour: Benedict's stays blue, iodine stays orange-brown, biuret stays blue.
A food sample turns biuret solution purple and Benedict's solution stays blue after heating. What does it contain?
- Purple with biuret means protein is present.
- Benedict's staying blue means no glucose.
Answer: Protein, but no glucose
Carbohydrates and lipids contain C, H and O; proteins also contain N. Starch and glycogen are made of sugars, proteins of amino acids, lipids of fatty acids and glycerol. Benedict's for glucose, iodine for starch, biuret for protein, ethanol emulsion for fat.
The interactive lesson includes the diagrams for this topic.
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