Food tests required practical

Aim: To use qualitative reagents to test food samples for the presence of reducing sugars, starch, protein and lipids.

Variables

• Independent: The food sample being tested.
• Dependent: The colour change seen with each reagent.
• Control: Volume of food solution tested; Volume of reagent added; Temperature and time in the water bath for the Benedict's test

Equipment

• Food samples
• Pestle and mortar
• Distilled water
• Filter funnel, filter paper and beakers
• Test tubes and rack
• Benedict's solution
• Iodine solution and spotting tile
• Biuret solution (or sodium hydroxide and copper sulfate solutions)
• Sudan III stain (or ethanol for the emulsion test)
• Water bath at about 80 °C
• Pipettes and measuring cylinder

Method

1. Grind a small amount of food with distilled water using a pestle and mortar, then filter to obtain a food solution.
2. Reducing sugars: add about 2 cm³ of food solution and an equal volume of Benedict's solution to a test tube, then heat in a hot water bath for about five minutes.
3. Record the colour: blue means no reducing sugar, while green, yellow or brick-red show increasing amounts.
4. Starch: put a few drops of food solution on a spotting tile and add iodine solution; orange-brown turning blue-black shows starch.
5. Protein: add 2 cm³ of food solution and 2 cm³ of Biuret solution to a test tube and shake; blue turning purple (lilac) shows protein.
6. Lipids: add a few drops of Sudan III to the food solution and shake; a red-stained oily layer on top shows lipid is present.
7. Carry out each test on distilled water as well, as a control for comparison.
8. Record all colour changes in a results table.

Safety

• Wear eye protection; Biuret reagent contains sodium hydroxide, which is corrosive, and copper sulfate, which is harmful.
• Heat Benedict's tests in a water bath rather than directly over a flame to avoid the liquid spitting out.
• Sudan III and ethanol are flammable, so keep them away from naked flames.
• Do not eat any food samples used in the laboratory.

Results and calculations

Record the food, each test and the colour observed in a table, stating whether each nutrient is present or absent. The Benedict's test is semi-quantitative: the further the colour moves from blue towards brick-red, the more reducing sugar was present.

Common mistakes and improvements

• Contaminating samples by reusing pipettes or a dirty pestle; use clean equipment for each food.
• Judging colours subjectively; compare against a control tube and view against a white background.
• Not heating the Benedict's test for long enough; time the heating and keep the water bath at a steady temperature.
• Using too much iodine so the colour is hard to see; add just one or two drops.

Exam tips

• Learn each test as reagent, starting colour and positive colour: Benedict's blue to brick-red (reducing sugars), iodine orange-brown to blue-black (starch), Biuret blue to purple (protein), Sudan III giving a red top layer (lipids).
• Remember that only the Benedict's test needs heating.
• Explain why the tests are qualitative and suggest how to make them more quantitative, for example using a colorimeter or comparing with colour standards.
• Link the tests to digestion questions, such as showing starch has been broken down to sugar by amylase.