Food tests (e.g. starch)

AS Biology AQA Unit 1, how reducing sugars, non-reducing sugars, starch, proteins and lipids can be tested for

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  • Created on: 19-10-12 16:44
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TEST FOR REDUCING SUGAR
1. Add food sample (about 2cm3 ) to a test tube (if solid grind it and add water to ensure it is in
liquid form)
2. Add an equal volume of Benedict's reagent (which is an alkaline solution of copper (II)
sulphate)
3. Heat the mixture in a gently boiling bath for 5 minutes
If a reducing sugar is present the solution will go from blue to and orange-brown precipitate of
copper (I) oxide
TEST FOR NON-REDUCING SUGARS
1. Do the test above, if solution does not change colour (stays blue) then a reducing sugar is not
present
2. Add another 2cm3 of food sample to 2cm3 of dilute hydrochloric acid in a test tube and
then place it in a gently boiling water bath for 5 minutes. The dilute hydrochloric acid will
hydrolyse any disaccharide present into its constituent monosaccharide's
3. Slowly add some sodium hydrogencarbonate solution to the test tube in order to
neutralise the acid (Benedict's reagent does not work in acidic conditions) Test with pH
paper that solution is alkaline
4. Re-test the solution with the test for a reducing sugar (see above)
5. If a non-reducing sugar was present in the original sample, the Benedict's reagent will not
turn orange-brown. This is due to the reducing sugars that were produced from the
hydrolysis of the non-reducing sugar
TEST FOR STARCH
1. Place 2cm3 of sample being tested into a tested into a test tube
2. Add two drops of iodine solution and shake or stir
3. The presence of starch is indicated by a blue-black colouration
TEST FOR PROTEINS (Biuret test which detects peptide links)
1. Place sample of solution to be tested into a test tube and add an equal volume of sodium
hydroxide solution (at room temperature)
2. Add a few drops of very dilute (0.05%) copper (II) sulphate solution and mix gently
3. A purple colouration indicates the presence of peptide bonds and hence a protein. If no
protein is present, the solution remains blue.
TEST FOR LIPIDS
1. Add food to be tested (in solution) to test tube (to a depth of about 1cm) and add about
3cm3 of ethanol
2. Shake about ten times, this will help lipids dissolve into the ethanol, wait till food has settled
3. Drain the ethanol off from above the food and pour into distilled water- presence of lipid
indicated if water goes cloudy

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