Food test

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Food tests
Investigating the presence of proteins, carbohydrates and fats in different foods
You can test samples of foods to see whether they contain these three components.
The tests are qualitative – they only tell you if the substance is there, not how much of it.
Task 1
Here are some typical food samples – say which components you think they will contain:
Fat
Protein
Starch
Sugar
crisps
banana
chicken breast
cheddar cheese
potato
apple
white rice
chocolate
egg white
wholemeal bread
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Food tests
Practical work: food tests summary
Lipids
Translucent spot
Rub a sample on
filter paper,
warm
Non-drying grease spot
shows lipid present
Ethanol
emulsification
Stir tiny sample
thoroughly with
ethanol. Add
water
Milky if lipid present.
Haze if trace only.
Iodine test
Add a few drops
of iodine solution
to solid
Dark blue/black shows
starch present
fats/oils
Carbohydrates
starch
The next two tests each need a small sample of the liquid produced when some of the
food is stirred very thoroughly with water. Benedict’s test needs a beaker of boiling
water.
Proteins
Biuret test
Add Biuret
reagent to test
tube
Purple colour shows
protein, blue no protein
Carbohydrates
Benedict’s test
Add Benedict’s
reagent to test
tube, place in
boiling water
bath
Colour goes from clear
blue → cloudy green,
yellow, orange with
increasing glucose
sugar (glucose)
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Food tests
Results of food tests
Fill in your observations and results for each food that you tested.
Food tested
lipids
proteins
starch
glucose
Comment on your findings:
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Food tests
Teaching notes
Page 1
Use task 1 prior to carrying out the food tests, it could make a useful homework task.
Page 2
The summary sheet is a handy reference for students when carrying out the food tests.
Page 3
The final sheet can be used by students to record their observations.
N.B. See CLEAPPS Hazcards to carry out a risk assessment before doing food tests with
a class. www.cleapss.org.uk/
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