4. Food Tests

Biuret solution

• Blue (copper sulfate solution) to purple

• Cu²+ is reduced to Cu+ as a complex is formed between the nitrogen atoms

• Biuret can be run down the sides of the tube (for samples where the colour would be hard to judge) and a purple ring is formed

Benedict’s solution

• Blue (copper sulfate solution and copper) to a brick-red precipitate (green - less, yellow - some)

• A water bath at 80° is used

Non-reducing sugar test

• Acid hydrolysis - boil the substance with HCl which hydrolyses the sucrose into glucose and fructose

• Neutralise the solution with sodium hydrogen-carbonate and use the Benedict’s solution

Iodine solution

• Orange-brown to blue-black

• Colour change is due to the change of the amylose helix

• The iodine is dissolved in potassium iodide solution

Emulsion test

• Colourless to a milky-white emulsion

• Mono and disaccharides dissolves in water - no emulsion

• Starch is not soluble - visible emulsion

• Nucleic acids and proteins are insoluble in ethanol