Food Test for Lipid (Emulsion Test)

Aim

To test a food sample for the presence of lipid (fats and oils) using the emulsion test, and to confirm the result, where useful, with the grease-spot test.

What it tests for

The emulsion test detects lipid, which includes fats and oils, in a food sample. Lipid dissolves in ethanol but not in water, so when the ethanol-lipid mixture meets water, tiny lipid droplets separate out and scatter light, forming a visible cloudy emulsion.

Variables

• Manipulated variable: the type of food sample tested, for example cooking oil, milk, groundnut, or a slice of bread.
• Responding variable: whether a cloudy white emulsion forms, which shows the presence or absence of lipid.
• Controlled variables: the volume of ethanol used, the volume of distilled water, and the amount of food sample tested each time.

Materials and apparatus

• Food samples to be tested
• Ethanol (absolute/anhydrous)
• Distilled water
• Test tubes and a test-tube rack
• A dropper or pipette
• A spatula, and a mortar and pestle for solid foods
• Filter paper for the grease-spot test

Reagent and method

1. Place a small sample of the food into a clean, dry test tube.
2. Add about 2 cm3 of ethanol to the test tube.
3. Shake the test tube thoroughly to dissolve any lipid present in the ethanol.
4. Pour the ethanol mixture slowly into a second test tube containing an equal volume of distilled water.
5. Observe the appearance of the water immediately after pouring.

Positive result

If lipid is present, a cloudy white emulsion forms in the water. If no lipid is present, the water stays clear.

An alternative method, the grease-spot test, involves rubbing the food sample onto a piece of paper; a translucent spot that remains after the paper dries indicates lipid is present, while a spot caused only by water evaporates and disappears.

Expected results

A food that contains lipid gives a cloudy white emulsion, while a food without lipid leaves the water clear. Typical observations are shown below.

Inference and conclusion

A cloudy white emulsion shows that the food sample contains lipid, because lipid dissolved in the ethanol comes out of solution as tiny droplets when it meets water and scatters light. If the water stays clear, the sample contains no lipid that the test can detect.

The emulsion test therefore gives a clear yes-or-no answer for the presence of lipid in a food.

Safety

Common mistakes

Paper 3-style questions

Question 1 (hypothesis). A student thinks fresh coconut milk contains lipid. State a hypothesis and the result that would support it.

Model answer. Hypothesis: coconut milk contains lipid. It is supported if, when the coconut milk is shaken with ethanol and the mixture is poured into water, a cloudy white emulsion forms.

Question 2 (variables and fair test). The student tests coconut milk, cooking oil and glucose solution. State the manipulated and responding variables and one variable that must be controlled.

Model answer. The manipulated variable is the type of food sample; the responding variable is whether a cloudy white emulsion forms. A controlled variable is the volume of ethanol, and of water, used for each sample, so the comparison is fair.

Question 3 (recording and inference). Coconut milk and cooking oil both turned the water cloudy, but the glucose solution stayed clear. Record these results and give the inference.

Model answer. A table with the columns 'Food sample' and 'Observation' records emulsion forms for coconut milk and cooking oil and water stays clear for glucose. The inference is that coconut milk and cooking oil contain lipid while the glucose solution does not, because only lipid produces the emulsion when the ethanol mixture meets water.

Source:SRC-DSKP-EN

Frequently asked questions