AQA A-Level Biology: Food Tests

Test for Starch

• Add 2 cm³ of sample to be tested into a clean test tube

• Add 2 cm³ of Iodine solution

• Positive Result: Orange to Blue-Black

• Negative Result: Colour remains Orange

• The test is qualitative, not quantitative.

Test for Reducing Sugar

• Add 2cm³ of sample to be tested into a clean test tube

• Add 2cm³ of Benedict’s Solution and heat sample in a water bath

• Positive Result: Blue to Brick-Red

Test for Non-Reducing Sugar

• Add 2cm³ of sample to be tested into a clean test tube

• Add 2cm³ of Benedict’s Solution and heat sample in a water bath

• Negative Result: Colour remains Blue

• Heat the sample with Dilute HCl for hydrolysis

• Neutralise with NaOH

• Repeat Benedicts Test whilst heating solution in a water bath

• Positive Result: Blue to Brick-Red

Test for Lipids

• Add 2cm³ of sample to be tested into a clean test tube

• Add 2cm³ of Ethanol to sample and shake

• Add 2 cm³ of water to sample and shake

• Positive Result: Clear to Milky-White emulsion

Test for Protein

• Add 2cm³ of sample to be tested into a clean test tube

• Add an equal volume of NaOH and CuSO₄ at room temp

• OR add Biuret Reagent to sample and mix gently

• Negative result: Colour remains Blue

• Positive result: Blue to Purple/lilac

• Filter and dry the precipitate

• Find the mass/ weight

• A = Glucose and B = Maltose;

• Because more sugar/ precipitate after hydrolysis/ maltase action

• Quantitive OR (colour change is) subjective

• Standardises the method