The 5 Key Chemical Food Tests
FOOD TEST 1: Benedict’s test for Reducing Sugars
1. Crush the sample + mix with an equal volume of water. Add 2cm3 of this mixture to a test tube
2. Add 2cm3 of Benedict’s solution and place in a beaker of water/water bath at a minimum of
70 degrees Celsius. (Heat is required!) After 3 minutes record the colour of the solutions.
RESULT: A colour change from blue to brick red demonstrates a positive result. (Turns green, then
yellow, then orange, then brick red as concentration of reducing sugar increases)
FOOD TEST 2: Iodine solution test for Starch
1. Crush the sample + mix with an equal volume of water. Add 2cm3 of this mixture to a dimple tile
2. Add one drop of iodine solution (also called ‘iodine in potassium iodide solution’)
RESULT: A colour change from orange/brown to blue/black shows that starch is present.
FOOD TEST 3: Biuret test for Protein
1. Add 2cm3 of the test solution to a test tube (repeat step 1 as above, is sample is solid)
2. Add 2 cm3 of Biuret solution(alkaline copper sulphate)
RESULT: A colour change from blue to purple indicates a positive result
FOOD TEST 4: Emulsion test for Lipids (fats and oils)
1. Add 2cm3 of sample to a test tube.
2. Add 2cm3 of ethanol and shake carefully.
3. Add 2cm3 of water.
RESULT: A cloudy white emulsion shows the presence of lipid.
FOOD TEST 5: DCPIP test for Vitamin C
1. If the food sample is solid, you need to make an extract. Grind or crush a small amount and
put it into a test tube to a depth of about 2cm with some distilled water. Mix with a glass rod
and leave to stand for a few minutes.
2. Draw up some of the clear liquid from the mixture into a pipette. Add this one drop at a time
to a test tube containing a light blue solution of DCPIP.
RESULT: Decolourisation of DCPIP shows that a vitamin C is likely present. The greater the rate of
decolourisation, the greater the concentration of Vitamin C present.
Recording your experimental results
Task 1: Complete the following table to record results from the food tests conducted (tick/cross)
Food source to be
tested
Reducing
sugar present
Starch
present
Protein
present
Lipid
present
Vitamin C
present
1
2
3
4
5
6
Task 2: Complete the following questions, that increase in difficulty as you progressa) Circle those from the following six which are specifically classed as reducing sugars –
glucose
fructose
maltose
sucrose
lactose
galactose
b) Starch is an example of a large polysaccharide. What does the term ‘polysaccharide’ mean?
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c) Proteins have specific shapes which relate directly to their function. Excluding enzymes and
antibodies, name another type of protein and suggest how its structure relates to its function.
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d) Why must the ethanol be added before the water in the lipid emulsion test?
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e) A more scientifically accurate method of determining the concentration of Vitamin C exists, as
opposed to simply counting how many drops it takes to turn DCPIP colourless. What is this
procedure, and can you describe the method one would employ to carry it out?
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