Animal Tissues, Organs and Systems » Required Practical: Food Tests

Testing food for biological molecules

You cannot tell what a food is made of just by looking at it. A biologist uses qualitative reagents instead. A reagent is a chemical that is added to a sample. It changes colour (or not) depending on what is in the sample. The test tells you whether a molecule is present. It does not tell you exactly how much.

In this practical you test for three groups of food molecules: carbohydrates (sugars and starch), proteins and lipids (fats and oils).

Key terms:

• Qualitative test: a test that shows whether something is present, usually by a colour change.
• Reagent: a chemical solution used to test for a substance.
• Positive result: the colour change that shows the molecule is present.
• Negative result: no colour change (or no cloudy layer in the lipid test), so the molecule is not present.
• Water bath: a beaker of hot water used to heat test tubes gently and evenly.

Step 1: Prepare the food sample

Solid food cannot react with a reagent properly. Follow these steps first.

1. Break up a small piece of the food using a mortar and pestle.
2. Transfer the food to a beaker and add distilled water.
3. Stir well with a glass rod so some of the food dissolves.
4. Filter the mixture to remove the solid lumps. The liquid that goes through is your food solution.

Always test a control alongside your food: a tube of distilled water treated in exactly the same way. It should give a negative result. This shows the reagent and equipment are clean.

Step 2: The tests

Testing for lipids

The spec names three tests: Benedict's, iodine and Biuret. Lipids are also tested for, using the ethanol emulsion test. Lipids do not dissolve in water, so you do not use a water solution here.

1. Mix a small amount of the food with ethanol in a dry test tube and shake well so any lipid dissolves.
2. Pour the ethanol into a second tube containing water.
3. Look at the water.

Positive: a cloudy white layer (an emulsion) forms. Negative: the liquid stays clear.

Another lipid test uses Sudan III stain. It turns the lipid layer red. You may be given either test.

Using a Bunsen burner and water bath safely

Benedict's test needs heat. Use a boiling water bath, not a naked flame under the test tube. A water bath heats the tube gently and stops the liquid spitting out.

• Tie back long hair and wear safety goggles.
• Set the Bunsen burner on a heatproof mat. Use the yellow safety flame when it is not heating.
• Heat a beaker of water until it boils, then stand the test tube in it using a test tube holder or rack.
• Point the open end of the tube away from everyone.
• Take care with hot water and glass. Let everything cool before you tidy up.

A worked example

Exam-style question

A student is given a white powder. It may contain starch, a sugar or protein.

(a) Describe how the student could test the powder for starch. Give the positive result. [2 marks]

(b) The student adds Benedict's reagent to a solution of the powder and heats it in a water bath. The mixture stays blue. What does this show? [1 mark]

(c) Describe a safety precaution the student should take when heating the tube. [1 mark]