Unit 4 Identification of food components
Key unit competence: Identify the composition of food nutrients in various food samples through chemical testing experiments.
Key ideas
- Food contains carbohydrates, proteins, lipids (fats and oils), vitamins, mineral salts, water and dietary fibre. Food tests use reagents that change colour when a nutrient is present.
- Starch test: add a few drops of iodine solution. Positive: blue-black. Negative: stays yellow-brown (orange). Example: cassava, potato, maize flour.
- Reducing sugar test (glucose, fructose, maltose): add an equal volume of Benedict's solution and heat in a boiling water bath for a few minutes. Positive: blue → green → yellow → orange → brick-red precipitate, depending on how much sugar is present. Negative: stays blue.
- Non-reducing sugar (sucrose): first test with Benedict's (stays blue). Then boil a new sample with dilute hydrochloric acid (hydrolyses sucrose to glucose and fructose), cool, neutralise with sodium hydrogencarbonate, then do the Benedict's test: a brick-red precipitate shows a non-reducing sugar was present.
- Protein test (Biuret): add sodium hydroxide solution then a few drops of dilute copper sulphate (or Biuret reagent) and shake; no heating. Positive: purple/violet (lilac). Negative: stays blue. Examples: beans, eggs, milk, groundnuts.
- Lipid tests: (a) Emulsion test: shake the food with ethanol, pour the liquid into water: a milky-white emulsion shows fat. (b) Grease spot test: rub food on paper: a translucent spot that lets light through after drying shows fat. (c) Sudan III: a red-stained oily layer shows fat.
- Vitamin C test: add the food solution drop by drop to DCPIP (blue). Vitamin C decolourises DCPIP (blue → colourless). The fewer drops needed, the more vitamin C the food contains. Examples: oranges, lemons, guavas, tomatoes.
- Good practice: crush and dissolve the food in water first; use clean test tubes; use a control (water instead of food) to compare colours; wear goggles when heating and handling sodium hydroxide.
- Results table: food tested, test, observation, conclusion. Write observations as colours, and conclusions as "starch present/absent".
Common misconceptions
- "Benedict's test needs no heating." No: Benedict's must be heated in a water bath; the Biuret test is the one done without heating.
- "Iodine turns blue-black with any sugar." No: iodine detects starch only; sugars need Benedict's.
- "A green colour in Benedict's test means no sugar." No: green shows a small amount of reducing sugar; only blue means none.
- "Sucrose gives a brick-red colour directly." No: sucrose is non-reducing; it must first be hydrolysed with acid.
Worked examples
Example 1. A learner tests milk: iodine stays brown; Benedict's after heating gives orange; Biuret gives purple; the emulsion test gives a milky layer. State what milk contains.
Solution
Starch absent (iodine stayed brown). Reducing sugar present (lactose: orange). Protein present (purple). Fat present (milky emulsion). So milk contains reducing sugar, protein and fat, but no starch.
Example 2. Juice A decolourised 1 cm³ of DCPIP with 4 drops, juice B with 12 drops. Which juice has more vitamin C? Explain.
Solution
Juice A. Fewer drops of A were needed to decolourise the same volume of DCPIP, so each drop of A contains more vitamin C.
Practice questions
Describe how to test a food for starch, giving the result for a positive test.
Show answer
Crush the food and mix with a little water in a test tube (or on a tile); add a few drops of iodine solution. Blue-black shows starch; yellow-brown means no starch.
Common mistake: Heating the iodine test.
Describe the Benedict's test for reducing sugars.
Show answer
Put 2 cm³ of food solution in a test tube, add an equal volume of Benedict's solution, heat in a boiling water bath for about 5 minutes. Blue → green → yellow → orange → brick-red shows increasing reducing sugar.
Common mistake: Heating directly over the flame without a water bath.
Describe how to test for protein.
Show answer
To food solution add an equal volume of sodium hydroxide solution, then a few drops of 1% copper sulphate, and shake. A purple/violet colour shows protein; blue means no protein.
Common mistake: Heating the Biuret test.
Describe the emulsion test for lipids.
Show answer
Shake the food with ethanol in a dry test tube, let it settle, pour the clear liquid into a tube of water. A milky-white emulsion shows fat.
Common mistake: Adding water before ethanol.
Describe how to test for a non-reducing sugar.
Show answer
Do Benedict's test: it stays blue. Boil a fresh sample with dilute hydrochloric acid, cool, neutralise with sodium hydrogencarbonate until fizzing stops, then do Benedict's test: brick-red shows a non-reducing sugar.
Common mistake: Forgetting to neutralise.
Explain how you would compare the vitamin C content of fresh and boiled tomato juice.
Show answer
Put 1 cm³ of DCPIP in two tubes; add each juice drop by drop with shaking until the blue colour disappears; count drops. The juice needing fewer drops has more vitamin C (fresh juice is expected to need fewer).
Common mistake: Using different volumes of DCPIP.
State the food test you would use to check each food: (a) sugar cane juice; (b) cooking oil; (c) egg white; (d) cassava.
Show answer
(a) Non-reducing sugar test (acid hydrolysis then Benedict's); (b) emulsion/grease spot/Sudan III; (c) Biuret; (d) iodine.
Common mistake: Using Benedict's alone for sugar cane.
State two safety precautions when carrying out food tests.
Show answer
Wear goggles when heating and handling sodium hydroxide/acid; use a water bath, not a direct flame, with ethanol; point test tubes away from people; keep ethanol away from flames.
Common mistake: Giving rules unrelated to the tests.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A brick-red precipitate in Benedict's test shows: a. a large amount of reducing sugar 2. A control in a food test is used to: b. iodine solution 3. The reagent used to test for starch is: c. compare the colours with no food present 4. A positive starch test gives a colour of: d. blue-black Show answer
1 → a (a large amount of reducing sugar) · 2 → c (compare the colours with no food present) · 3 → b (iodine solution) · 4 → d (blue-black)
- Green means a small amount.
- Water is tested in the same way.
- Starch turns iodine blue-black.
- The negative result is yellow-brown.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
yellow changing to brick-red · non-reducing sugar · blue changing to colourless · a non-reducing sugar such as sucrose- Sucrose is described as a .
- A food gave blue with Benedict's, but brick-red after boiling with acid and neutralising. The food contains .
- A positive result with DCPIP is .
Show answer
1: non-reducing sugar · 2: a non-reducing sugar such as sucrose · 3: blue changing to colourless
- It does not react with Benedict's directly.
- The sugar became reducing only after hydrolysis.
- Vitamin C removes the blue colour.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The Biuret test is used to detect: a. heated in a boiling water bath 2. The Biuret test is carried out: b. without heating 3. Benedict's test must be: c. sodium hydrogencarbonate 4. After boiling with acid, the sample is neutralised using: d. proteins Show answer
1 → d (proteins) · 2 → b (without heating) · 3 → a (heated in a boiling water bath) · 4 → c (sodium hydrogencarbonate)
- Proteins give a purple colour.
- It works at room temperature.
- Heat is needed for the colour change.
- Benedict's does not work in acid.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
ethanol · vitamin C · dilute hydrochloric acid · reducing sugars- DCPIP is used to test for .
- Benedict's solution is used to test for .
- To test for a non-reducing sugar, the sample is first boiled with .
Show answer
1: vitamin C · 2: reducing sugars · 3: dilute hydrochloric acid
- Vitamin C decolourises DCPIP.
- Glucose and maltose are reducing sugars.
- Acid hydrolyses sucrose into reducing sugars.