Food Tests in Chemistry

Aim

To test food samples for the presence of starch, reducing sugars (glucose), proteins, and fats (lipids) using standard chemical tests.

Apparatus

• Test tubes and test tube rack
• Test tube holder
• Bunsen burner or hot water bath (approximately 80 degrees C)
• Dropping pipettes
• White tile (for observing colour changes)
• Iodine solution (iodine in potassium iodide solution, I2/KI)
• Benedict’s solution (contains copper(II) sulfate in alkaline solution)
• Biuret reagent (dilute sodium hydroxide solution and dilute copper sulfate solution, or pre-mixed biuret solution)
• Ethanol
• Distilled water
• Food samples to test (e.g., bread, egg white, milk, cooking oil, sugar solution)

Method

Test 1: Starch (iodine test)

1. Place a small sample of food in a test tube (or on a white tile if testing a solid directly).
2. Add 2-3 drops of iodine solution to the food sample.
3. Observe the colour change.

Test 2: Reducing sugars (Benedict’s test)

1. Dissolve a small amount of food in water (if not already a liquid) in a test tube.
2. Add approximately 2 cm3 of Benedict’s solution to the food solution.
3. Heat the mixture in a hot water bath at approximately 80 degrees C for 2-3 minutes.
4. Observe the colour change.

Test 3: Protein (biuret test)

1. Dissolve a small amount of food in water in a test tube.
2. Add approximately 2 cm3 of dilute sodium hydroxide solution.
3. Add 2-3 drops of dilute copper sulfate solution and mix gently (do not shake vigorously).
4. Observe the colour change.

Test 4: Fats/lipids (ethanol emulsion test)

1. Place a small amount of food in a test tube.
2. Add approximately 2 cm3 of ethanol and shake well to dissolve any fat.
3. Pour the ethanol solution into a test tube containing approximately 2 cm3 of cold distilled water.
4. Observe the appearance.

Expected results

Test	Reagent	Positive result	Negative result
Starch	Iodine solution	Colour changes from brown to blue-black	Remains brown/yellow
Reducing sugar	Benedict’s solution (heated)	Colour changes from blue to green, then yellow, then orange, then brick-red/orange-red (depending on sugar concentration)	Remains blue
Protein	Biuret reagent (NaOH + CuSO4)	Colour changes from blue to purple/violet/lilac	Remains blue
Fat/lipid	Ethanol + water	White/milky emulsion forms	Solution remains clear

Analysis

Starch test: Iodine molecules fit inside the helical structure of the starch molecule (amylose), forming a blue-black complex. This is a physical trapping rather than a chemical reaction, which is why the colour disappears on heating (the helix unwinds) and returns on cooling.

Benedict’s test: Reducing sugars (glucose, maltose, fructose) reduce the Cu2+ ions in Benedict’s solution to Cu+ ions, which form a precipitate of copper(I) oxide (Cu2O), which is orange-red. The intensity of the colour change indicates the concentration of reducing sugar: low concentration gives green, medium gives yellow-orange, high gives brick-red.

Biuret test: The copper(II) ions in alkaline solution form a violet-coloured complex with the peptide bonds (-CONH-) in proteins. At least two peptide bonds must be present for the test to work.

Ethanol emulsion test: Fats dissolve in ethanol but not in water. When the ethanol solution is added to water, the fat comes out of solution as tiny droplets, forming a white, cloudy emulsion.

Sources of error

• Temperature for Benedict’s test: The solution must be heated sufficiently (at least 70-80 degrees C). Insufficient heating gives a false negative. Use a water bath rather than a direct flame for even heating.
• Concentration effects: Very dilute solutions may give weak positive results that are difficult to interpret. Use a positive control (known glucose solution) and a negative control (distilled water) alongside the unknown.
• Cross-contamination: Use clean test tubes and pipettes for each test to avoid carrying reagents between samples.
• Fat solubility: If the food sample does not dissolve in ethanol, no fat will be transferred and the emulsion test gives a false negative. Crush or grind solid foods before testing.

Exam tips

Examiners frequently present the results in a table and ask you to identify what nutrients a food sample contains. Memorise each test as a triplet: reagent, positive result, and what it tests for. The Benedict’s test must include heating — stating Benedict’s without heating loses a mark.

For higher-level responses, explain why each test works rather than simply stating the result. For the starch test, mention the iodine-starch complex. For Benedict’s, mention the reduction of Cu2+ to Cu2O. These mechanistic details earn supplement-level marks.

Control experiments are important: a positive control (known sample that should give a positive result) confirms the reagent is working, and a negative control (distilled water) confirms that the reagent alone does not give a false positive.