Food Tests: Starch, Sugar, Protein and Fat

What’s actually in a slice of bread, a glass of milk or a spoonful of peanut butter? Nutrition labels tell you, but you can find out yourself with four simple chemical tests that are a standard part of school science. Each test uses a reagent that reacts with one type of biomolecule and produces an obvious colour change or cloudiness. This guide explains the purpose, the chemistry behind each test, a safe method, the results to expect, and what can go wrong.

Purpose

To identify whether starch, reducing sugars, proteins and lipids are present in a range of food samples, and to explain each result using the chemistry of the reagent.

The principle behind each test

1. Starch: iodine solution

Iodine solution (iodine dissolved in potassium iodide solution) is orange-brown. It contains triiodide ions, I₃⁻. Amylose, one of the two components of starch, coils into a helix. The I₃⁻ and longer polyiodide chains slide into the middle of this helix, and the resulting complex absorbs light strongly across much of the visible spectrum, so it looks blue-black. Sugars don’t coil into helices, so they don’t give the colour. More on this in the starch–iodine test.

2. Reducing sugars: Benedict’s reagent

Benedict’s reagent is a blue solution containing copper(II) ions (Cu²⁺) held in a complex by citrate ions, in alkaline sodium carbonate. Reducing sugars — all monosaccharides such as glucose and fructose, and some disaccharides such as maltose and lactose — have a free aldehyde or ketone group that can act as a reducing agent. When heated, they reduce blue Cu²⁺ to copper(I), which precipitates as red copper(I) oxide, Cu₂O.

The colour depends on how much precipitate forms, mixed with the remaining blue: green (trace), yellow, orange, then brick-red (a lot). That’s why Benedict’s test is described as semi-quantitative. Sucrose (table sugar) is a non-reducing sugar and gives a negative result (see the reducing sugars test).

3. Protein: biuret test

Biuret reagent is blue. It contains copper(II) sulfate in sodium hydroxide solution. In alkaline conditions, Cu²⁺ ions form a complex with nitrogen atoms in peptide bonds — the bonds that link amino acids in proteins. This complex is purple (lilac at low concentrations). The test needs at least two peptide bonds close together, so it detects proteins and peptides, not single amino acids. Details in the biuret test.

4. Lipids: emulsion test

Fats and oils are non-polar and don’t dissolve in water, but they dissolve in ethanol. If you dissolve a food sample in ethanol and then pour the solution into water, the lipid comes out of solution as tiny droplets spread through the water. These droplets scatter light, so the mixture turns cloudy white — an emulsion. Without lipid, the liquid stays clear.

Equipment

• Test tubes, test-tube rack, test-tube holder
• Pestle and mortar (for solid foods), distilled water, filter funnel and paper (optional)
• Dropping pipettes, 10 cm³ measuring cylinder
• Beaker of hot water at about 80 °C as a water bath (from a kettle), and a thermometer
• Iodine solution, Benedict’s reagent, biuret reagent (or separate dilute sodium hydroxide and dilute copper(II) sulfate solutions)
• Ethanol
• Food samples: for example, bread, potato, apple, milk, cooked egg white, glucose tablets, table sugar, vegetable oil, peanuts (check allergies first)
• Eye protection

Safety

• Wear eye protection throughout — biuret reagent contains sodium hydroxide, which can damage eyes.
• Ethanol is highly flammable. Keep it away from flames and hot plates; use a kettle, not a Bunsen burner, to supply hot water for the water bath.
• Iodine solution stains skin and clothing; Benedict’s reagent is an irritant.
• Don’t taste any food used in the lab.
• Check for food allergies (nuts, eggs, milk) before handing out samples.

Method

Preparing the samples

1. For solid foods, crush a small piece (about a pea-sized amount) with a pestle and mortar.
2. Add about 5 cm³ of distilled water and mix well. If the mixture is very cloudy, filter it.
3. Keep a separate dry, crushed sample of each food for the lipid test.
4. Label test tubes clearly with the food and the test.

Test 1: starch

1. Place about 2 cm³ of food solution (or a small piece of solid food on a white tile) in a test tube.
2. Add 2–3 drops of iodine solution.
3. Record the colour. Blue-black = starch present; orange-brown = none.

Test 2: reducing sugars

1. Add 2 cm³ of food solution to a test tube.
2. Add an equal volume (2 cm³) of Benedict’s reagent.
3. Stand the tube in the water bath at about 80 °C for five minutes.
4. Record the colour against the scale: blue (none), green, yellow, orange, brick-red (most).

Test 3: protein

1. Add 2 cm³ of food solution to a test tube.
2. Add 2 cm³ of biuret reagent (or 2 cm³ of dilute sodium hydroxide followed by a few drops of dilute copper(II) sulfate).
3. Shake gently and wait a couple of minutes.
4. Record the colour. Purple/lilac = protein present; stays blue = none.

Test 4: lipids

1. Put a small amount of crushed food (or a few drops of oil) in a clean, dry test tube.
2. Add 2 cm³ of ethanol and shake well.
3. Pour the ethanol (not the solid) into a second tube containing 2 cm³ of distilled water.
4. Record whether a cloudy white emulsion forms.

Controls

Run each test with distilled water as a negative control, and with a known sample as a positive control (starch suspension, glucose solution, egg white, vegetable oil). This lets you see what “no change” and “definite change” really look like with your reagents.

Typical results

Food	Starch	Reducing sugar	Protein	Lipid
Bread	✔	small trace	✔	small
Potato	✔	trace	faint	✘
Apple	✘ (ripe)	✔	✘	✘
Milk	✘	✔ (lactose)	✔	✔
Egg white	✘	✘	✔	✘
Glucose tablet	✘	✔	✘	✘
Table sugar	✘	✘	✘	✘
Vegetable oil	✘	✘	✘	✔
Peanuts	small	✘	✔	✔

Your results may differ — unripe fruit contains more starch, and some breads contain added sugar. That variation is part of what makes the experiment interesting.

Extension: making sucrose “reducing”

Boil 2 cm³ of sucrose solution with 1 cm³ of dilute hydrochloric acid for a few minutes. This hydrolyses the glycosidic bond, splitting sucrose into glucose and fructose. Neutralise with sodium hydrogencarbonate (add until fizzing stops — Benedict’s needs alkaline conditions), then do the Benedict’s test. It now gives a positive result. This is a neat demonstration of hydrolysis of disaccharides.

Sources of error and limitations

• Colour judgement is subjective. Different students may call the same tube “yellow” or “orange”. Use a white background, compare with standards of known concentration, or use a colorimeter.
• Coloured foods (e.g. beetroot, orange juice) can mask colour changes.
• Benedict’s timing and temperature: too short or too cool a heating time underestimates the sugar.
• Contamination: a dropper used in glucose and then in another sample can give a false positive. Use a clean pipette each time.
• Wet tubes in the lipid test: water in the first tube stops the lipid dissolving in ethanol and can make the result unclear.
• Qualitative only: apart from Benedict’s, these tests show presence, not amount. A faint result may mean a little of the substance or a poor extraction.
• Biuret sensitivity: very dilute protein gives only a faint lilac that’s easy to miss.

Improving the experiment

• Prepare a set of glucose solutions of known concentration (e.g. 0.1–2 %) and heat each with Benedict’s to make a colour standard for estimating concentration.
• Filter off and dry the copper(I) oxide precipitate and weigh it, or measure the remaining blue colour with a colorimeter — both give more objective data.
• Repeat each test three times and record the most common result.

Key takeaways

• Iodine: blue-black with starch (iodine inside the amylose helix).
• Benedict’s: heated, Cu²⁺ reduced to red Cu₂O by reducing sugars; colour shows roughly how much.
• Biuret: purple with peptide bonds in proteins.
• Emulsion test: cloudy white when lipid dissolved in ethanol meets water.
• Use controls, a water bath rather than a flame, and careful colour comparison.

For the molecules being detected, see carbohydrates explained, proteins explained and lipids explained.