Practical- Food tests

Describe the test for starch

• Add 5 drops of iodine to starch solution

• In the presence of starch iodine will turn blue-black

Describe the test for reducing sugars (glucose)?

• Add 1ml of Benedict’s reagent to the sample

• Heat in a water bath for 3 minutes at 95 degrees celsius

• Brick red precipitate forms

Describe the test for non-reducing sugars (sucrose)

• Carry out Benedict’s test and acquire a negative results

• Boil with two ml of 1 M dilute hydrochloric acid

• Neutralise the acid with NaHCO₃

  • Add the powder with a spatula until it stops fizzing

• Boil with benedicts

• Blue before hydrolysis, brick red after hydrolysis

Describe the test for lipids

• Dissolve sample in 3ml of ethanol

• Add 3ml of water

• White emulsion forms

Describe the test for peptide bonds in proteins

• Add 2ml of Biuret reagent to 2ml of the sample

• Changes from pale blue to lilac

What is the science behind Benedict’s test for reducing sugars?

Cu²⁺ ions (blue) in the Benedict’s reagent are reduced to Cu⁺ (brown)

What is the science behind the test for non-reducing sugars?

The acid hydrolyses the glycosidic bond between glucose and fructose and then the glucose and fructose react with the Benedict’s reagent

What is the science behind the emulsion test for lipids?

The ethanol dissolves the lipid which then forms suspended droplets when shaken with water

What is the science behind the test for peptide bonds in proteins?

A complex forms between the nitrogen atoms in the peptide and the Cu²⁺ ions

What is hydrolysis?

A chemical reaction in which a molecule of water is added to a substance which causes it to break down