Nutrient Testing

Year 10 Biology

Which chemical elements do food molecules contain?

• Food molecules, like everything else, are made up of chemical elements.

• 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.

Starchy foods contain carbohydrates which are made of long chains of identical small sugar molecules.

carbohydratemolecule

What are carbohydrates?

one sugar molecule

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.

Proteins, like carbohydrates, are made of long chains of small molecules. In proteins, these small molecules are not identical.

What are proteins?

protein molecule

one amino acid

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.

Fats are made up of fat molecules which contain fatty acids and glycerol.

What are fats?

fat molecule

glycerolFat 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.

fatty acids

Test for carbohydrates starch.

Orangey-brown iodine turns blue-black when it reacts with starch.

drop iodine solution onto the food

black = starch

Blue Benedict’s solution turns orangey-red when it is boiled with glucose.

blue turns orange/red= glucose

heat

chopped up food

hot water

Test for carbohydrates glucose (reducing sugar).

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?

In the Biuret test the solution turns purple.

chopped up food

potassium hydroxide

shake

coppersulphate

purple = protein

Test for Protien

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 white/cloudy.

8A What’s in food?

Fats go cloudy white when they are mixed with ethanol and water.

Now see if you can use these tests to identify the food chemicals on the next slides.

grind food with ethanol

add waterfilter

cloudy = fat

http://www.slideshare.net/mrexham/igcse-organisms-and-life-processes-revision-quiz