Testing for biomolecules
Food tests
1) Reducing sugars
• reduce soluble blue copper sulphate containing copper (II) ions to insoluble brick–red copper oxide, containing copper (I)
• the copper oxide is seen as a brick–red ppt
• add equal volumes of Benedict’s reagent and the food sample to a test tube
• heat in a water bath at 80°C
• if reducing sugars are present, the following colour changes are observed:
2) Non–reducing sugars
• e.g., sucrose
• disaccharide is first broken down into its 2 monosaccharide constituents in a hydrolysis reaction
• this is done by adding HCl, and then neutralising the acid with an alkali such as sodium bicarbonate
• constituent monosaccharides will be reducing sugars and their presence can be tested by Benedict’s test
3) Starch
• add drops of iodine solution to the sample
• if blue–black colour is quickly produced, starch is present
• iodine solution is yellow brown
4) Lipids (emulsion test)
• sample is shaken with ethanol
• any lipids present will dissolve
• mixture of ethanol and sample is poured into a beaker containing water
• if lipids are present, a cloudy–white suspension is formed due to the lipids being unable to remain dissolved when mixed with water
• therefore, the lipid molecules form droplets throughout the liquid, this kind of mixture is called the emulsion
5) Proteins (Biuret test)
• all proteins have peptide bonds containing nitrogen atoms which form a purple complex with Cu2+ ions
• equal volumes of sample and Biuret reagent are mixed
• if proteins are present, the colour changes from blue to lilac
• instead of biuret reagent, KOH and dil. CuSO4 can be used
For extra details on performing food tests, see paper 3 notes.