Cambridge O Level Biology · Syllabus 5090 · Biological Molecules
Food Test
What is Food Test?
A laboratory procedure in which a reagent is added to a food sample and the resulting colour or appearance change shows whether a particular biological molecule is present. The four required tests are iodine solution for starch, Benedict's solution with heating for glucose and maltose, biuret reagents for protein, and the ethanol emulsion test for lipids. A food test is qualitative: it shows presence or absence at a detectable level, not an exact amount.
This definition is part of the Biological Molecules chapter in Cambridge O Level Biology.
Food Test in context
A food test is a laboratory procedure in which a reagent is added to a food sample and a change in colour or appearance shows whether a particular biological molecule is present. Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a hot-water bath, turns from blue through green, yellow and orange to brick-red with glucose or maltose, and a coloured precipitate forms. Biuret reagents turn from blue to lilac or purple with protein. In the ethanol emulsion test a clear mixture turns into a milky-white emulsion with lipid. Every test is qualitative: it shows presence at a detectable level, not an amount.
Questions students ask about Food Test
Why does Benedict's solution not test for starch?
Benedict's solution detects the reducing sugars glucose and maltose; iodine solution detects starch. Starch does not give a positive Benedict's result and glucose does not turn iodine solution blue-black, so a food can be positive for one test and negative for the other. Both molecules are carbohydrates, which is why students assume one reagent covers both, but each of the four food tests is specific to a different molecule.