Food Tests

Section 1 of 11Preparing the sample

1

Grind a solid food with a little distilled water using a pestle and mortar.

2

Filter or allow the mixture to settle to obtain a clear solution.

3

Divide the solution into separate test tubes, one for each test.

4

Run a test on distilled water as a control to show a negative result.

Section 2 of 11Starch — iodine test

Starch — iodine test

Add drops of iodine solution to the food sample.

Observe the colour straight away; no heating is needed.

A positive test shows a colour change from orange-brown to blue-black.

Section 3 of 11Glucose — Benedict's test

1

Add Benedict's solution to the sample solution in a test tube.

2

Heat in a water bath at 80 °C for 5 minutes.

3

Remove the test tube from the water bath and observe the colour.

4

A positive test shows a colour change from blue to orange or brick red.

Section 4 of 11Protein — biuret test

Protein — biuret test

Add drops of biuret solution to the food sample.

Mix and observe; no heating is needed.

A positive test shows a colour change from blue to violet or purple.

Section 5 of 11Lipids — ethanol emulsion test

Lipids — ethanol emulsion test

Mix the food sample with 2 cm³ of ethanol and shake.

Add the ethanol to an equal volume of cold water.

A positive test shows a cloudy emulsion forming.

Section 6 of 11Vitamin C — DCPIP test

1

Add 1 cm³ of DCPIP solution to a test tube.

2

Add a small amount of the food sample as a solution.

3

A positive test shows the blue colour of the dye disappearing.

4

The fewer drops needed to remove the colour, the more vitamin C the food contains.

Section 7 of 11Food tests compared

Nutrient	Reagent	Heat	Start colour	Positive result
Starch	Iodine solution	No	Orange-brown	Blue-black
Glucose (reducing sugar)	Benedict's solution	Yes, 80 °C water bath	Blue	Orange or brick red
Protein	Biuret solution	No	Blue	Violet or purple
Lipids	Ethanol then water	No	Clear	Cloudy emulsion
Vitamin C	DCPIP	No	Blue	Colourless

Section 8 of 11Safety

Eye protection

• Wear it throughout every test.

Ethanol

• It is flammable, so keep it away from flames.

Heating

• Use a water bath for the Benedict's test and point tubes away from people.

Reagents

• Biuret reagent is irritant, so wash spills off skin at once; iodine stains skin.

Section 9 of 11More from the specification

More from the specification

Nitrogenous bases and amino acids

• Nitrogenous bases and amino acids biochemical digestion separates polynucleotide bases from protein chains composed of constituent amino acids.

Carbon

• Element with four valence electrons that forms the backbone of organic molecules.

Nucleotide

• The unit of DNA made of a phosphate, a deoxyribose sugar and a base.

Organism

• A living thing, such as a plant or an animal.

Carbohydrate

• A molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.

Glycogen

• The polysaccharide in which glucose is stored in animals.

Cellulose

• A structural polysaccharide of β-glucose.

Amino acid

• The unit from which proteins are made; the sequence decides the protein's shape and function.

Double helix

• Two DNA strands wound around each other.

Section 10 of 11Key terms

Term	Meaning
Benedict's solution	Blue reagent that turns orange or brick red with glucose on heating.
Iodine solution	Orange-brown reagent that turns blue-black with starch.
Biuret solution	Blue reagent that turns violet or purple with protein.
Ethanol emulsion test	Test in which lipids dissolved in ethanol form a cloudy emulsion in water.
DCPIP	Blue dye that is decolourised by vitamin C.
Reducing sugar	A sugar such as glucose that reacts with Benedict's solution.
Control	A test on distilled water used for comparison.

Section 11 of 11Exam technique

1

Giving only the end colour — always state both the starting colour and the colour of a positive result.

2

Heating every test — only Benedict's test needs heating, in a water bath.

3

Saying a positive lipid test is a colour change — it is a cloudy emulsion.

4

Saying vitamin C turns DCPIP blue — it makes the blue dye disappear.