Testing for lipids
The emulsion test is used to determine whether lipids are present in a biological sample. It is a qualitative test, so it indicates the presence of lipid rather than providing an exact measurement of quantity.
The method uses the fact that lipids dissolve in an organic solvent such as ethanol but are insoluble in water.
Method
- Prepare the sample; if it is solid, grind it first. Place 2 cm³ of sample into a clean, grease-free test tube.
- Add 5 cm³ of ethanol.
- Secure the tube with a bung and shake thoroughly so that lipid present in the sample dissolves in the ethanol.
- Add 5 cm³ of water and shake gently.
- Observe the final appearance.
Interpreting the result
| Observation | Interpretation |
|---|---|
| Cloudy-white or milky emulsion forms | Lipid is present |
| Mixture remains clear | No lipid detected |
A more obvious milky appearance may occur when more lipid is present, but the emulsion test remains qualitative and does not provide a precise lipid concentration.
A negative control can be prepared by carrying out the same procedure with water in place of the biological sample. It should not produce a cloudy emulsion.
Exam Tip: For a complete emulsion-test answer, give the sequence sample + ethanol → shake → add water → shake gently and state the positive result precisely: a cloudy-white or milky emulsion indicates lipid.