Topics 2.5 & 3.4
Proteins and lipids
Practical worksheet
Food test 3: the biuret test for
proteins and the emulsion test for
lipids
Aims
In this practical you will carry out two qualitative food tests to find out whether certain types
of food molecules occur in various food samples. For example, does protein occur in milk?
You will test materials that should give positive results and test others that should give
negative results.
You can practise manual, observational and data handling skills and then go on to make
conclusions.
There are opportunities for you to evaluate information in the discussion sections.
The biuret test for proteins, polypeptides and
peptides
Safety
 Wear eye protection
 Biuret reagent contains several ingredients. They include very dilute copper sulphate
(LOW HAZARD) as well as dilute sodium hydroxide (which is an IRRITANT BUT NOT
CORROSIVE).
 LET YOUR TUTOR KNOW IF YOU HAVE A NUT ALLERGY. You should not carry
out tests with nuts if you are allergic to them.
Equipment and materials
 Eye protection
 Biuret reagent
 Scalpel and tile and/or pestle
and mortar
 1% albumen solution
 Spatula
 Labelling equipment
 Test tubes, bungs and test tube
rack
 Distilled water in a wash bottle
 Milk
 Solid food materials, for
example, biscuit, potatoes
 Pasteur pipettes
 Paper towels
 Large plastic wastes beaker
 A 250 cm3 beaker with distilled
water to clean Pasteur pipettes
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
1
Practical worksheet
Topics 2.5 & 3.4
Proteins and lipids
Method
1. Label the test tubes with the food materials that you plan to test for protein.
2. Add 2 cm3 of the test solution to a test tube (or use a spatula to add 1 cm depth of finely
chopped up or ground material, then add 2 cm3 of distilled water, put the clean bung in
and shake it carefully at least ten times).
3. Add ten drops of biuret reagent to the test solution without the dropper pipette touching
the inside of the test tube. You may get clearer results if you do not shake the test tube.
4. Observe the test solution for a purple/violet or blue colour. Record your observations and
deductions from the biuret test in a suitable table.
Possible results
 Confirmation of proteins: a violet or purple colour. The purple/violet colour is due to the
presence of two or more peptide bonds in a compound. Proteins, polypeptides and most
peptides should give positive results.
 Confirmation of the absence of protein or a concentration that is below the level of
sensitivity of the biuret test: a blue solution.
Discussion
1. From your tests, which of the materials you tested
a) contained proteins? ……………………………………………………………………...
……………………………………………………………………………………………...
b) did not contain proteins? ………………………………………………………………...
………………………………………………………………………………………………
2. A solution of a dipeptide (made of two linked amino acids) did not give a positive result
in the biuret test. Explain why.
………………………………………………………………………………………………
………………………………………………………………………………………………
The emulsion test for lipids (fats and oils)
Safety
 Ethanol is HIGHLY FLAMMABLE and you should not use it close to naked flames. It is
HARMFUL by skin contact, inhalation or if swallowed.
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
2
Topics 2.5 & 3.4
Proteins and lipids
Practical worksheet
 LET YOUR TUTOR KNOW IF YOU HAVE A NUT ALLERGY. You should not carry
out tests with nuts if you are allergic to them.
Equipment and materials
 Scalpel and tile and/or pestle
and mortar
 A 250 cm3 beaker with distilled
water to clean Pasteur pipettes
 Labelling equipment
 Ethanol
 Spatula
 Olive oil in a dropper bottle (or
another plant oil)
 Test tubes, test tube bung, test
tube rack
 Distilled water in a wash bottle
 Dry food materials, for example,
biscuit, peanut, potato
 Pasteur pipettes
 Paper towels
 Large plastic waste beaker
Method
1. Label the test tubes suitably for the foods to be tested.
2. Chop solid food on a tile or grind with a pestle and mortar.
3. Add the food to a labelled test tube to a depth of about 1 cm. For olive oil, add two drops.
4. Add 3 cm3 of ethanol to each test tube.
5. Put a clean bung into each test tube and shake carefully at least ten times. This will help
any lipids dissolve in the ethanol.
6. Allow time for food particles to settle. When the ethanol above the food as cleared you
may proceed.
7. Use the wash bottle to dispense about 3 cm depth of distilled water into the test tubes.
8. Observe and note any changes in the test tube contents. Record your results in a suitable
table.
Possible results
 Confirmation of lipids: a white emulsion floating at or near the top of the water.
 Confirmation of the absence of lipids (or levels below the sensitivity of the test): no
white emulsion floating at or near the top of the water
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
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Topics 2.5 & 3.4
Proteins and lipids
Practical worksheet
The explanation of the emulsion test for lipids
Lipids are soluble in organic solvents such as ethanol but they are not soluble in water or in
dilute ethanol. Any dissolved lipid will precipitate in the diluted ethanol when added to
water. The lipid will form a white emulsion, which is a mixture of two immiscible liquids.
Discussion
1. From your tests, which of the materials you tested (a) contained lipids and (b) did not
contain lipids?
………………………………………………………………………………………………
………………………………………………………………………………………………
2. Why is it important to use a bung to seal the test tube before shaking in the emulsion test
rather than your thumb?
………………………………………………………………………………………………
………………………………………………………………………………………………
Testing foods for the five types of food chemicals
You may have the opportunity to test various foods for some or all of the food chemicals
using the methods noted in food tests 1, 2 and 3. You could start by including some of the
results you have already acquired from the tests carried out earlier.
Table 1 Substances present in various food materials
Food
Food tests for:
Carbohydrates
Reducing sugars
Non-reducing sugars
Proteins
Lipids
Starch
Milk
Potatoes
Biscuit
NB You can use a ‘+’ for when a substance is present, a ‘–’ for when a substance is absent,
‘+/–’ when there is a trace of substance.
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Topics 2.5 & 3.4
Proteins and lipids
Practical worksheet
Discussion
Are the five food tests limited in terms of the nutritional information they
provide?
100 cm3 of semi-skimmed milk has the following nutritional features in relation to the foods
you have been testing.
 Protein
3.6 g
 Carbohydrates
4.8 g
of which sugars
 Fat
4.8 g
1.8 g
of which saturates
1.1 g
1. Give one way in which your food tests provide more information than the nutritional
information listed above.
………………………………………………………………………………………………
2. What feature in the nutritional information for semi-skimmed milk shows that it does not
contain starch?
………………………………………………………………………………………………
3. List two different ways in which the nutritional information provides more information
than would be found with the five food tests.
………………………………………………………………………………………………
………………………………………………………………………………………………
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
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