WJEC Wales Biology A Level
SP 1.1: Food tests
Practical notes
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Introduction
Qualitative tests can be used to identify the presence of a substance in food. You need to be able
to use chemical tests for five biological molecules:
● Reducing sugars
● Non-reducing sugars
● Proteins
● Starch
● Lipids (fats and oils)
Equipment
● Test solution
● Benedict’s reagent
● 0.5 mol dm-3 hydrochloric acid
● Dilute sodium hydroxide solution
● Biuret reagent
● Iodine-KI reagent
● Ethanol
● Distilled water
● 5 boiling tubes
● 2 test tubes
● 3 dropping pipettes
● 4× 5 cm3 syringes
● Water bath
Risk assessment
Hazard Risk Precaution Emergency
Benedict’s Irritation Avoid contact with Flood eye(s) with tap
reagent to eyes eyes; wear safety water; seek medical
goggles assistance
Hydrochloric Irritation Wear gloves when Remove contaminated
acid to skin handling HCl clothing; run the affected
area under cold water;
seek medical assistance
Irritation Wear safety goggles Flood eye(s) with tap
to eyes water; seek medical
assistance
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Biuret reagent Irritation Wear safety goggles Flood eye(s) with tap
to eyes water; seek medical
assistance
Corrosive Wear gloves when Remove contaminated
handling reagent clothing; run the affected
area under cold water;
seek medical assistance
Iodine-KI Irritation Wear safety goggles Flood eye(s) with tap
reagent to eyes water; seek medical
assistance
Irritation Wear gloves when Remove contaminated
to skin handling solution clothing; run the affected
area under cold water;
seek medical assistance
Boiling water Scalding Handle boiling water Run burn under cold
with care; use tongs water; seek medical
to transfer boiling assistance
tubes; wear safety
goggles
Ethanol Highly Make sure that there Put out small fires with a
flammable are no naked flames damp cloth; evacuate the
in the room building
Method
Testing for reducing sugars
1. Add 2cm3 of test solution and Benedict’s reagent to a boiling tube.
2. Heat in a boiling water bath (80°C or higher) for five minutes
3. Remove boiling tube and observe the colour of the precipitate formed.
Testing for non-reducing sugars
Test the solution for the presence of reducing sugars. If no colour change is observed…
1. Add two drops of hydrochloric acid to 2cm3 of test solution. HCl hydrolyses the
non-reducing sugar’s glycosidic bonds, releasing reducing sugars into solution.
2. Heat the boiling tube in a water bath (80°C or higher) for two minutes.
3. Allow to cool.
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4. Add two drops of dilute sodium hydroxide solution to the boiling tube. This neutralises
the excess HCl.
5. Re-test the resulting solution with Benedict’s reagent.
6. Record the colour of the precipitate formed.
Testing for proteins
1. Add 2 cm3 of test solution and Biuret reagent to a boiling tube.
2. Shake gently to mix.
3. Record observations.
Testing for starch
1. Add 2 cm3 of test solution to a test tube.
2. Add two drops of iodine-KI reagent and gently mix.
3. Record observations.
Testing for lipids
1. Add 2 cm3 of test solution to a test tube.
2. Add 5 cm3 of ethanol and shake. Allow the mixture to settle.
3. Take a boiling tube and fill it half full with distilled water.
4. Pour the mixture into the boiling tube.
5. Record observations.
Results
Biological molecule Positive test
Reducing sugars Colour change from green to yellow to orange to brown to brick
red depending on the quantity of reducing sugar present
Non-reducing sugars Colour change from green to yellow to orange to brown to brick
red depending on the quantity of non-reducing sugar present
Protein Colour change from pale blue to purple
Starch Colour change from yellow-brown to blue-black (amylose) or
red-purple (amylopectin)
Lipids White, cloudy emulsion forms
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