Digestion and food tests
Breaking down food
The digestive system breaks large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. This happens in two ways: mechanically, by chewing and by the muscular walls of the gut churning food, and chemically, using digestive enzymes to break down large molecules into smaller ones.
Testing for sugars and starch
(for reducing sugars): add Benedict's solution to a sample and heat in a water bath. A colour change from blue to green, yellow or brick-red shows sugar is present — the more sugar, the further the colour changes towards brick-red.
(for starch): add a few drops of iodine solution to a sample at room temperature. A colour change from browny-orange to blue-black shows starch is present.
Key terms
Testing for protein and lipids
(for protein): add Biuret solution (or sodium hydroxide followed by copper sulfate solution) to a sample. A colour change from blue to purple or lilac shows protein is present.
(for lipids): the Sudan III test gives a positive result if the stain forms a red layer at the top of the sample. The emulsion test — shaking a sample with ethanol, then adding water — gives a cloudy white emulsion if lipid is present.