Reagents and food testing

There are different tests which can be used to detect carbohydrates, proteins and lipids.

They involve adding a reagent to a food sample which changes colour depending on what biological molecules are present.

Sometimes it may be necessary to crush the food or add water to the food before adding the reagent.

Biuret testTesting for protein/amino acid

Food sampleReagentMethodInitial colourColour of positive result
Reducing sugar Benedict’s Add Benedict’s reagent to the food and boil in a water bath. Blue Brick red precipitate
StarchIodineAdd iodine reagent to the food.Yellow-brownBlue-black
Protein/amino acids Biuret (a mixture of sodium hydroxide and copper sulfate).Add Biuret reagent to the food.Blue Lilac/purple
Fat Ethanol Add ethanol to the food to dissolve the fat then add water.Colourless White emulsion
Emulsion testTesting for fat