Food Tests – Starch, Reducing and Non-Reducing Sugars, Proteins and Lipids

Testing foods for biological molecules is a core practical in GCSE Biology. By applying simple chemical tests, students can identify the main nutrients present — starch, sugars, proteins, and lipids — and see how each type of food contributes to a balanced diet.

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The Core Tests

Test	Reagent / Method	Positive Result	Negative Result
Starch Test	Add a few drops of iodine solution	Blue-black colour	Yellow-brown
Reducing Sugar Test	Add Benedict’s reagent and heat in a water bath	Green → orange → brick-red (depending on sugar concentration)	Blue
Non-Reducing Sugar Test	After a negative Benedict’s test, boil sample with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, then re-test with Benedict’s	Brick-red colour	Blue
Protein Test (Biuret Test)	Add Biuret solution (sodium hydroxide + copper sulfate)	Lilac or purple colour	Blue
Lipid Test (Emulsion Test)	Mix with ethanol, then add water	Milky white emulsion	Clear

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Example Food Results

Food Sample	Starch	Reducing Sugars	Non-Reducing Sugars	Protein	Lipid
Cheese	–	–	–	+	+
Biscuits	+	+	+	–	+
Carrot	+	+ (glucose)	–	–	–
Crisps	+	–	–	–	+
Potato	+	+ (maltose)	–	–	–
Egg white	–	–	–	+	–
Egg yolk	–	–	–	+	+

(+ = positive result, – = negative result)

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The Science

Each test targets a different biological molecule:

• Iodine binds with starch helices.

• Benedict’s detects aldehyde groups in reducing sugars.

• Biuret reacts with peptide bonds in proteins.

• Ethanol–water emulsifies lipids for visibility.

These reactions demonstrate the molecular diversity of foods and how chemical testing can reveal their composition.

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Skills Highlight

• Using chemical reagents safely and accurately

• Recording qualitative results systematically

• Interpreting results in terms of macronutrient content

• Linking observations to biological function and diet

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Why It Works in Teaching

Food testing is colourful, visual, and immediately meaningful. Students connect lab results with the food they eat every day, reinforcing their understanding of macromolecules and experimental design.