Biological molecule tests

• To test for reducing sugars add 2cm of liquid food sample to a test tube. Add 2cm of benedicts solution and heat the mixture for 5 minutes. Original pale blue colour means no reducing sugar, light green means a trace of and a brown or red precipitate means more sugar is present.
• Reducing sugars have the ability to give electrons to other molecules. Benedicts solution contains CU2+ ions which are reduced to CU+ ions forming orange/red copper oxide precipitate (CU2O).
• To test for non-reducing sugars heat the solution with 2 drops of dilute hydrochloric acid for 3 minuted to hydrolyse the glycosidic bonds then cool. Neutralise the solution by gently adding small amounts of sodium hydrogen carbonate powder. Then test for reducing sugars.
• To test for starch add iodine solution to a sample. If starch is present the solution will change from yellow-brown-orange to blue-black.
• When dissolved in potassium iodide, the iodide forms a triiodide ion which slips in to the middle of the amylose helix and causes a colour change.
• To test for lipids add a few drops of the liquid food sample. Add 2cm of ethanol and shake thoroughly. Add 2cm of water and shake again.
• If the emulsion test is negative the solution will remain colourless. If positive a layer of cloudy white suspension forms on top of the solution (tiny globules of fat).
• Lipids are non-polar and insoluble in water. When the water is added lipids are dispersed as micelles (small droplets) throughout the solution. A layer is formed at the top as lipids are less dense than water.
• To test for proteins add biuret solution to the liquid sample and if present it will turn from blue to purple.
• The purple colour is formed by a complex between nitrogen atoms in a peptide chain and CU2+ ions.