1.3—Core Practical 1 - reducing sugars and starch

Benedict's reagent tests for reducing sugars: on heating, a blue solution changes through green, yellow and orange towards a brick-red precipitate as more copper(II) is reduced. Iodine solution turns blue-black when starch is present.

Use equal sample volumes, add the same reagent volume, heat Benedict's samples in a water bath for the same time, and include a water control. Compare the colour with a calibration series if estimating concentration.

A sample that stays blue in Benedict's but turns blue-black with iodine contains starch but no detectable reducing sugar under those conditions. Hydrolysing a non-reducing sugar first can reveal reducing products.

The colour is evidence of a chemical reaction, not a direct count of molecules. A semi-quantitative result is stronger when controls, repeats and a reference colour scale are used.

A negative Benedict's test means ‘not detected at this sensitivity’, not ‘no carbohydrate’. Heating, reagent volumes and subjective colour matching affect the conclusion.