Tests

What is the test for reducing sugars?

Benedicts solution

• Add Benedicts reagent and heat in a water bath (80-100°C) for 2-5 minutes

• Positive result : Blue → green, yellow, orange, or brick-red precipitate (depending on concentration)

What is the test for non-reducing sugars?

• Grind sample

• Add 2 cm of food sample and add equal Benedict’s

• Boil test tube in water bath - if it does not change colour a reducing agent is not present

• add another sample to dilute hydrochloric acid and boil it in water bath- this will hydrolyse disaccharides into monsaccharides

• add sodium hydrogencarbonate to neutralise HCl - use PH paper to check it is alkaline

• re test by heating with benedicts

• should show an orange colouration to indcate the presence of monosaccharides

What is the test for starch?

• place 2cm³ of sample into test tube

• add two drops of iodine

• blue black colouration indicates starch

What is the test for lipids?

• 2cm³ sample, add 5 cm ³ ethanol

• shake to dissolve lipid

• add 5 cm³ of water

• a milky white emulsion indcates presence of a lipid

What is the test for proteins?

• place sample in test tube, add equal volume of sodium hydroxide

• add few drops of dilute copper (II) sulfate solution

• a purple colouration indicates presence of peptide bonds