Required practical 4: Food tests

Aim

To use qualitative reagents to test food samples for sugars, starch, protein and lipids.

Background

Foods contain carbohydrates, proteins and lipids (fats and oils). A qualitative reagent is a chemical that changes colour in the presence of a food molecule. The test tells you whether the molecule is present, but not how much (that would be quantitative).

Benedict's solution is blue and turns green, yellow, orange or brick-red when heated with sugars. Iodine solution is orange-brown and turns blue-black with starch. Biuret reagent is blue and turns purple (lilac) with protein. Sudan III stain dyes lipids red, which makes an oily layer form on top of the water.

Many foods are solid. To test them, they are first ground up with a little water so that the food molecules dissolve or mix into the liquid and can react with the reagent. A coloured food could hide a colour change, so a plain, pale food or a clear solution works best.

Equipment

• Food samples such as milk, egg white, vegetable oil, potato or bread, and glucose solution and starch solution for comparison
• Mortar and pestle (or a beaker and a glass rod), distilled water, filter funnel and filter paper
• Test tubes and a test tube rack
• Dropping pipettes and a 10 cm³ measuring cylinder (or graduated syringe)
• Benedict's solution
• Iodine solution
• Biuret reagent
• Sudan III stain solution (or ethanol for the emulsion test)
• Water bath at about 80 °C (or a beaker of hot water from a kettle on a heat-proof mat) and a test tube holder
• Thermometer and a timer
• Spotting tile (optional, for the iodine test)
• Marker pen and eye protection

Risk assessment

Method

1. Label four test tubes with the name of the food sample and the test. Do this for each food sample in turn.
2. If the food is solid, grind a small amount with a little distilled water using a mortar and pestle. Filter it, or let the solid settle, so that you can use the liquid.
3. Starch: put 2 cm³ of the food liquid into a test tube (or one drop on a spotting tile). Add a few drops of iodine solution and shake. Record the colour.
4. Sugars: put 2 cm³ of the food liquid into a test tube and add 10 drops of Benedict's solution. Place the tube in the hot water bath at about 80 °C for 5 minutes. Record the colour at the end.
5. Protein: put 2 cm³ of the food liquid into a test tube and add 2 cm³ of Biuret reagent. Mix gently and record the colour after about a minute.
6. Lipids: put 2 cm³ of the food (or the oil on its own) into a test tube with 2 cm³ of water and add a few drops of Sudan III stain solution. Shake gently and leave to settle. Look for a red-stained layer on top.
7. If your school uses the ethanol emulsion test for lipids instead: shake 2 cm³ of the food with 2 cm³ of ethanol, then pour the liquid into a tube of water. A cloudy white emulsion shows lipid is present.
8. Record the colour before and after for each test in the results table. Write 'present' or 'not present' next to each result.
9. Repeat steps 1 to 8 for each food sample. Test a sample of pure water as a negative control and a sample of the pure molecule as a positive control (for example glucose, starch, egg white and oil).
10. Write a conclusion for each food sample saying which food molecules it contains.
11. Wash out the test tubes, put the waste liquids down the sink with plenty of water as your teacher instructs, and wash your hands.

Results

Fill this table in as you go. Print the PDF for a copy to write on.