Food Tests

Add few drops of idoine solution to the sample. Will turn blue black if starch is present

Add Benedicts solution (alkaline copper sulfate) to sample and heat at 80^oC in a water bath. If reducing sugar is present Benedict's solution turns from Blue to Orange-Red precipitate (copper oxide)

• If reducing sugar test is negative, boil with hydrochloric acid, cool and neutralise with sodium hydrogencarbonate solution or sodium carbonate solution; repeat Benedict’s test.
• Benedict’s solution turns from blue to Orange-red precipitate (copper oxide) – on second test.

• Add a Pbiuret reagent
• Purple colour slowly develops in the surface of the protein solution.

• Add ethanol to extract (dissolve) lipid and pour
• alcohol into water in another test tube.

The iodine bonds between the coils of the starch.

• Boiling the sample with hydrochloric acid hydrolyses any sucrose present, splitting the sucrose molecules to give glucose and fructose monosaccharides. 
• Then when Benedict’s solution is added the test will now give a positive result because the monosaccharides glucose and fructose are present.

Described as precipitate because the orange-red substance comes out of solution and forms solid particles dispersed in solution.

Lipids are soluble in alcohols, but when added to the water the lipid comes out of solution and becomes dispersed as tiny droplets in the water.

The sodium hydroxide and copper sulphate (Biuret agent) react with the peptide bonds found in protein, which results in the colour change.