Required Practical - Enzymes in Digestion
Science: KS3, topic 1.4.13
Testing for Carbohydrates, Lipids and Proteins
Reagents can be used to test for the presence of various food substances. The first step is to grind up the food and add distilled water to dissolve some of the food. You can then test for the food substances:
Starch
- To test for starch, add iodine solution.
- It will turn blue-black if starch is present.
Sugar
- To test for sugar, add Benedict’s reagent and heat for about two minutes.
- It will turn any of green, yellow or red if sugar is present.
- The colour depends on the concentration.
Proteins
- To test for proteins, add Biuret solution.
- It will turn mauve or purple if proteins are present.
Lipids
- To test for lipids, add Sudan III (a fat-soluble dye).
- If lipids are present, a red-stained oil layer will float on the water surface.
- The presence of lipids can also be tested by adding ethanol.
- The solution is added to water and shaken. If the solution turns cloudy, lipids are present.
Test yourself on Required Practical - Enzymes in Digestion
Select the correct reagent and substance pairing in each row
What colour does iodine solution turn when starch is present?
What colour does Biuret solution turn when protein is present?
A food sample is tested with Sudan III, but no oily layer forms on the water surface. What does this result show?
A student tests a food sample for lipids using Sudan III, then tests another sample of the same food using ethanol. Both tests give a positive result. What can the student conclude?