Identification of Food Components
Senior 2 · Biology
Identification of Food Components
Lesson 3 of 3
20 min
Let's begin
Is there really protein in ikivuguto? Which is richer in vitamin C, orange juice or passion fruit juice? Does a groundnut paste contain oil? Three more quick tests answer these questions with a few drops of reagent.
Together with the starch and sugar tests, they let you analyse almost any food in a school laboratory.
| Colour | Meaning |
|---|---|
| stays blue | no protein |
| turns purple (violet or lilac) | protein present |
The copper ions react with the peptide bonds that join amino acids. No heating is needed. Egg white, milk and bean extract all turn purple.
(a) The emulsion test (the most reliable school test)
A cloudy white emulsion means lipid is present. The lipid cannot stay dissolved when the ethanol mixes with water, so it forms millions of tiny droplets that scatter light. If the mixture stays clear, there is no lipid. Ethanol is flammable, so keep it away from flames.
(b) The grease-spot test (a quick field test)
Rub the food on a piece of brown or white paper and let it dry. Hold it up to the light. A translucent spot that does not disappear when dry shows lipid. A spot made by water alone disappears as the water evaporates.
(c) Sudan III test
Shake the food with water and a few drops of Sudan III, a red dye that dissolves in fats. If lipid is present, a red-stained oily layer floats on top.
DCPIP (dichlorophenolindophenol) is a blue dye. Vitamin C (ascorbic acid) removes its colour.
If the blue colour disappears, vitamin C is present. The fewer drops needed, the more vitamin C the juice contains.
Question: 1 cm³ of DCPIP was decolourised by 8 drops of fresh orange juice, 20 drops of orange juice left in the sun for two days, and 5 drops of passion fruit juice. Which contained the most vitamin C? Why did the old juice need more drops?
Solution: Passion fruit juice contained the most vitamin C, because it needed the fewest drops. The old juice needed more drops because vitamin C is destroyed by heat, light and exposure to air over time. This is why fruit is best eaten fresh, and why over-cooking vegetables lowers their vitamin C.
| Test | Result for sample X |
|---|---|
| Iodine | yellow-brown |
| Benedict's | orange |
| Biuret | purple |
| Emulsion | cloudy white |
Question: Which nutrients does X contain, and what might it be?
Solution: No starch; a moderate amount of reducing sugar; protein; and lipid. Milk fits well: it contains lactose (a reducing sugar), protein and fat, but no starch.
Tip
Summary table to learn
Exam technique: in the emulsion test the ethanol goes in first, then the water. Reversing the order loses the mark.
Describe the Biuret test and its positive result.
Solution: Add sodium hydroxide solution to the food solution, then a few drops of copper(II) sulfate solution, and shake. A change from blue to purple shows that protein is present.
Why must the test tube be dry at the start of the emulsion test?
Solution: Any water in the tube would make the lipid come out of the ethanol too early, as an emulsion in the first tube. The lipid must first dissolve fully in the ethanol, and only then be added to water.
1 cm³ of DCPIP needed 12 drops of pineapple juice and 6 drops of lemon juice to lose its colour. Which juice has more vitamin C? How many times more?
Solution: Lemon juice, because it needed fewer drops: 12 ÷ 6 = 2 times as much vitamin C per drop.
A food gives: iodine blue-black; Benedict's blue; Biuret purple; emulsion clear. Name the nutrients present.
Solution: Starch and protein, with no reducing sugar and no lipid. Bean flour is a food that fits.
Summary
Sources and credits
The food-test procedures, the worked examples and the practice questions are written by Ganzaa.
