Topics 2.5 & 3.4
Proteins and lipids
Teacher notes
These tests provide an opportunity for students to practise manual and observational skills safely as well as data handling skills and to go on to make conclusions. The students have the opportunity to evaluate information in the discussion sections.
Detailed instructions are given for materials, equipment and the method for the food tests.
For each food test, students should test materials that should give positive results and controls that should give negative results. You may also provide individual foods for your students to determine whether they contain the food molecules that are the subject of a particular test.
At the end of food test 3 there is a table of food test results for various types of food that you may wish your students to complete once they have covered all of the tests.
Safety

Wear eye protection

Biuret reagent contains several ingredients including very dilute copper sulfate: LOW HAZARD –
CLEAPSS Hazcard 27C; dilute sodium hydroxide: an IRRITANT BUT NOT CORROSIVE –
CLEAPSS Hazcard 91

CHECK IF ANY STUDENTS/STAFF HAVE NUT ALLERGIES. Such students/staff should not carry out tests on any foods that contain nuts or that carry a risk of nut contamination.
Teaching notes
1.
A purple/violet colour should be seen after five drops of biuret reagent with 2 cm 3 of 1% albumen and after ten drops of biuret reagent with 2 cm 3 of semi-skimmed milk.
2.
One problem with commercial biuret reagent may be that it sometimes ‘goes off’ with time and so should be checked before a practical session.
Discussion
1.
From your tests, which of the materials you tested (a) contained proteins and (b) did not contain proteins? (a) albumen and milk contained proteins ; (b) distilled water (etc.) did not contain proteins.
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
1

Topics 2.5 & 3.4
Proteins and lipids
Teacher notes
2.
A solution of a dipeptide (made of two linked amino acids) did not give a positive result in the biuret test. Explain why. Only one peptide bond is present per dipeptide molecule and at least two peptide bonds must be present for a positive biuret test result.
Safety

Industrial methylated spirits (IMS): HIGHLY FLAMMABLE and should not be used close to naked flames; HARMFUL by skin contact, inhalation or if swallowed –
CLEAPSS Hazcard 40A
 CHECK IF ANY STUDENTS/STAFF HAVE NUT ALLERGIES. Such students/staff should not carry out tests on any foods that contain nuts or that carry a risk of nut contamination.
Teaching notes
1.
Two drops of olive oil (or any alternative plant oil) are sufficient to give a positive result.
Any more and the oil is difficult to dissolve in the ethanol quickly and holds up the test.
2.
For peanuts, it is better to chop them up rather than grind them as it takes perhaps 15 to
20 minutes for the ground peanut fragments to settle in the ethanol and we need a clear solution before proceeding.
Discussion
1.
From your tests, which of the materials you tested (a) contained lipids and (b) did not contain lipids?
(a) Olive oil, biscuits, peanuts, etc.; (b) distilled water, potatoes (trace?), but the floating white material is not an emulsion, it could be fragments of potato or starch grains.
2.
Why is it important to use a bung to seal the test tube before shaking in the ethanol test rather than your thumb.
It prevents oils from the thumb contaminating the ethanol and possibly giving a false positive. It is not safe to shake the test tube without a bung.
A summary table was provided in the Practical worksheet, with several features of tabulation that may merit discussion, e.g. ‘overarching column titles’.
© 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
Teacher notes
Discussion
100 cm 3 of semi-skimmed milk has the following nutritional features with regard to foods that have been tested.

Protein
 Carbohydrates
3.6
4.8
g 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.
The carbohydrates are subdivided into reducing sugars, non-reducing sugars and starch.
2.
What feature in the nutritional information for semi-skimmed milk shows that it does not contain starch?
Starch is a non-sugar carbohydrate and all 4.8
g of carbohydrate are ‘sugars’.
3.
List two different ways in which the nutritional information provides more information than would be found with the five food tests.
Only the nutritional information is (1) quantitative; and (2) shows the amount of
saturated fats/lipids.
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
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