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Nutrient Testing
Year 10 Biology
Which chemical elements
do food molecules contain?
• Food molecules, like everything
else, are made up of chemical
• Carbohydrates and fats are made
of three types of element only:
carbon, hydrogen and oxygen.
• Protein molecules are made of
the same three elements, but
they also contain nitrogen.
• Some proteins have other
elements too, like sulphur.
• We carry out chemical tests to
help us identify nutrients.
What are
Starchy foods contain carbohydrates which are made of
long chains of identical small sugar molecules.
one sugar
The long chains of carbohydrates are broken down into the
smaller sugar molecules by the body.
The small molecules from carbohydrates are used by the
body to release energy and make the body work.
What are proteins?
Proteins, like carbohydrates, are made of long chains of
small molecules. In proteins, these small molecules are
not identical.
Proteins are made up of chains of small molecules called
amino acids. There are over 20 different kinds of amino acid.
Proteins are used by the body for growth and repair.
What are fats?
Fats are made up of fat molecules which contain fatty acids
and glycerol.
Fat molecules have to be broken down by the body so that
they can be used for energy storage.
Fats are also used by the body to keep heat in and to make
cell membranes.
Test for carbohydrates starch.
Orangey-brown iodine turns blue-black when it
reacts with starch.
black = starch
drop iodine solution
onto the food
Test for carbohydrates glucose (reducing sugar).
Blue Benedict’s solution turns orangey-red when
it is boiled with glucose.
up food
hot water
blue turns orange/red
= glucose
Test for Carbohydrate (glucose)
• Glucose is a small, soluble molecule.
It is a carbohydrate
• It is type of reducing sugar.
• To test food for reducing sugar we
chop a sample and dissolve in H2O, to
a height of about 2 cm in a test tube
• Add an equal volume of Benedict’s
solution (a blue coloured liquid)
• Place the test tube in a hot water
bath (60ºC) for 5 minutes.
• A predictable sequence of colour
changes will take place if reducing
sugar is present.
Reducing Sugar Test: Colour Changes
The more reducing sugar (glucose) present, the greater the colour change.
Lots of reducing sugar in food gives orange/red colour.
8A What’s in food?
Test for Protien
In the Biuret test the solution turns purple.
purple = protein
chopped up
The Biuret Test (for Protein)
• Proteins are large molecules
• To test for presence of protein in
a food sample we add a chemical
called biuret solution.
• Like the reducing sugar test, chop
up the food and dissolve in water
to a height of 2cm.
• Now add an equal volume of
buiret solution
• If protein is present (a positive
result) the solution turn
purple/violet in colour.
The Emulsion test (for fats)
• Fats (lipids) are large molecules.
• The presence of lipids can be
detected by the emulsion test.
• First collect a sample of the food
and add a few drops of ethanol.
• Cover the test tube with your
thumb, shake for a few seconds.
• Now add a few drops of water.
• Cover, shake for a few seconds.
• If lipids are present (positive
result) the solution will turn
8A What’s in food?
Fats go cloudy white when they are mixed with
ethanol and water.
grind food
with ethanol
cloudy = fat
Now see if you can use these tests to identify the
food chemicals on the next slides.

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