Food tests

Question 1

How do you test for the presence of starch?

Answer 1

• Add 2cm3 of the test solution to a test tube or spotting tile
• Add a few drops of iodine solution and mix
• If iodine is present there will be blue/black colouration

Question 2

How do you test for the presence of protein?

Answer 2

• Place 2cm3 of the test solution in a test tube and add a small volume of sodium hydroxide
• Add a few drops of biuret solution to the test tube and mix gently
• If protein is present there will be violet colouration

Question 3

How do you test for the presence of lipids?

Answer 3

The emulsion test:
- Place 2cm3 of the test solution in a test tube and add 5cm3 of ethanol
- Place a rubber bung on the end of the test tube and invert tube several times so that the contents is thoroughly mixed
- Then allow the mixture to settle
- Pour the upper layer into 5cm3 of distilled water in a seperate test tube
- If lipids are present a cloudy/milky white emulsion forms

Question 4

How do you test for the presence of reducing sugars?

Answer 4

• Add 1cm3 of the test solution to a test tube then add 1cm3 of benedict’s reagent and agitate the tube to mix them
• Heat test tube in a water bath for 5 minutes
• If reducing sugar is present ere will be orange/red colouration

Question 5

How do you test for the presence of non-reducing sugars?

Answer 5

• Add 1cm3 of the test solution to 1cm3 of dilute hydrochloric acid (to hydrolyse any disaccharide into monosaccharides)
• Neutralise the HCl with sodium hydrogen carbonate powder (test with pH paper)
• Re-test by adding 1cm3 of benedict’s reagent and heating in a water bath for 5 minutes
• If a non-reducing sugar is present the solution will now turn orange/red (colour change is caused by donation of electrons from sugar to reagent- reduction)