Core practical 3: Food tests

Aim

To use chemical reagents to test foods for starch, reducing sugars, protein and fat.

Background

Foods contain different biological molecules: carbohydrates (starch and sugars), proteins and lipids (fats and oils). Each can be identified with a chemical reagent that gives a colour change or other visible change when the molecule is present.

Iodine solution tests for starch. Benedict's solution tests for reducing sugars such as glucose. Biuret reagent tests for protein. The ethanol emulsion test tests for fats (lipids).

A test only works if the reagent can mix with the food, so solid foods are first crushed and mixed with water (or ethanol for the fat test). A negative result is when the reagent stays its original colour, so it is important to know the start colour as well as the positive result.

A control, such as distilled water, shows what a negative result looks like and shows that the reagents are not contaminated.

Equipment

• Food samples such as potato, milk, egg white solution, glucose solution and vegetable oil
• Distilled water (for the negative control)
• Iodine solution in a dropper bottle
• Benedict's solution
• Biuret reagent
• Ethanol (about 2 cm³ per test)
• Test tubes (at least 5 per food, because the fat test uses two) and a test tube rack
• 250 cm³ beaker for a water bath, with a kettle or electric heater, and a thermometer
• Dropping pipettes
• Spotting tile (optional, for the iodine test)
• Mortar and pestle or a glass rod, and a spatula, for crushing solids
• Marker pen for labelling
• Eye protection

Risk assessment

Method

1. Put on eye protection. Half-fill a 250 cm³ beaker with hot water from a kettle and set it on a heatproof mat. Use a thermometer and top up with hot water to keep it at about 80 °C. This is the water bath.
2. Prepare the food samples. Crush solid foods (such as potato) in a mortar with a little distilled water, then use the liquid. Label one set of four tubes for each food.
3. Starch test: put about 2 cm³ of the food sample in a tube and add 3 drops of iodine solution. Record the colour. A positive result is orange-brown to blue-black.
4. Reducing sugar test: put about 2 cm³ of the food sample in a tube and add about 2 cm³ of Benedict's solution. Stand the tube in the water bath at about 80 °C for 5 minutes. Record the colour.
5. A positive result for reducing sugar is blue turning to green, yellow, orange or brick-red. The more sugar present, the further along the colour sequence the result goes.
6. Protein test: put about 2 cm³ of the food sample in a tube. Add about 2 cm³ of biuret reagent and shake gently to mix. Leave for a minute and record the colour. A positive result is blue turning to purple (lilac). (If your school uses two biuret solutions, add sodium hydroxide solution first and then a few drops of copper sulfate solution.)
7. Fat test: put about 2 cm³ of ethanol and a small amount of the food (for example 2 drops of oil, or crushed food) into a dry tube. Shake the tube well so that any fat dissolves.
8. Pour the ethanol mixture into a second tube containing about 2 cm³ of distilled water. Record whether a cloudy white emulsion forms (positive) or the liquid stays clear (negative).
9. Repeat all four tests using distilled water in place of the food sample, to show what a negative result looks like.
10. Repeat all four tests for each of the other foods, using clean tubes and clean pipettes each time.
11. Record the results in a table, writing the colour of each tube at the end of the test.
12. Use the results to decide which biological molecules are present in each food.
13. Pour the used solutions down the sink with plenty of water, or 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.