Food Tests

In this section, you will learn how to perform food tests in order to identify the constituents of biological specimens. 

The knowledge and the ability to conduct the test AND the accurate method of recording the observation and conclusion is essential. 

*Note: Food tests are generally qualitative in nature (ie. presence / absence)

Continue below to learn about the following food tests:

1. Test for Starch

2. Test for Reducing Sugar / Non-reducing Sugar

3. Test for Protein

4. Test for Lipid

1. Test for Starch

Procedure:

·         Carry out the test on a white tile.

·         To 2 drops of sample, add 2 drops of iodine in potassium iodide solution (i.e. iodine solution).

Observation:

·         Positive result: Blue-black colouration is observed.

           Conclusion: Starch is present.

·         Negative result: The reaction mixture / iodine solution remained yellowish-brown.

           Conclusion: Starch is absent.

2(A). Test for Reducing sugar (Benedict's Test)

Procedure:

·         Prepare a boiling water bath.

·         To 2 cm3 of unknown solution in a test tube, add an equal volume of Benedict’s reagent.  Mix by shaking. 

·         Place the test tube in a boiling water bath (i.e. 100°C) for 5 min. 

Observation:

·         Positive result: A brick-red suspension was observed. Brick-red ppt observed upon settling.

           Conclusion: Reducing sugar is present.

·         Negative result: The reaction mixture remained blue.

       Conclusion: Reducing sugar is absent. 

2(B). Test for Non-Reducing sugar 

Procedure:

·         To 2 cm3 of unknown solution in a test tube, add an equal volume of dilute hydrochloric acid.

·         Place the test tube in a boiling water bath (i.e. 100°C) for 5 min à this hydrolyses any disaccharides.

·         Cool and neutralise the mixture with excess (i.e. ³ 2 cm3) dilute sodium hydroxide. Test with red litmus paper as the Benedict’s test only works in alkaline conditions.  The mixture should turn red litmus paper blue.

·         Carry out the Benedict’s test as described above.  

Observation:

·         Positive result: A brick-red suspension was observed. Brick-red ppt observed upon settling.

           Conclusion: Non-reducing sugar is present.

·         Negative result: The reaction mixture remained blue.

       Conclusion: Non-reducing sugar is absent. 

***Note: If reducing sugar is also present, a positive result for Test for Non-Reducing Sugar means that there should be more ppt or more intense coloration for the same sample when compared to the results of a Test for Reducing Sugar. 

3. Test for Protein (Biuret Test)

Procedure:

·        To 2 cm3 of unknown solution in a test tube, add an equal volume of 5 % sodium hydroxide. Mix well by shaking.

·         Add 1 drop of 1 % copper (II) sulphate solution. Dropwise. Shake after each drop.

Observation:

·         Positive result: A violet colouration observed.

           Conclusion: Protein is present.

·         Negative result: The reaction mixture remained light-blue.

       Conclusion: Protein is absent. 

4. Test for Lipid

Procedure:

·       To the 2 cm3 unknown sample in a clean dry test tube, add 2 cm3 of absolute ethanol.  Mix by shaking vigorously. Allow the mixture to settle à any lipids will dissolve in ethanol.

·         Decant the ethanol into another clean dry test tube.

·         Add 2 cm3 of deionised water to this solution. Mix by shaking. 

Observation:

·         Positive result: Cloudy white emulsion observed. 

           Conclusion: Lipid is present.

·         Negative result: The reaction mixture remained colourless and clear.

       Conclusion: Lipid is absent.