The most reliable way to check edible oil for adulteration is a fatty acid profile by gas chromatography, compared with the known range for that oil. Add specific tests for the adulterants most likely in the oil you buy.
Taste, colour and smell are not enough. A blend of cheap oil with a little strongly flavoured oil can fool an experienced buyer.
What gets added
- Cheaper oils: palm olein, cottonseed, soybean or rice bran oil mixed into groundnut, sesame or mustard oil.
- Argemone oil: from a weed seed that grows among mustard. It is toxic and linked to epidemic dropsy.
- Mineral oil: non-edible and sometimes found in cheap oil.
- Flavour or colour: used to make a refined blend look and smell like filtered or cold-pressed oil.
- Used or recovered oil: sold as fresh oil after cleaning.
Laboratory tests that work
- Fatty acid profile by GC: each oil has a typical range for acids such as palmitic, oleic and linoleic. A high palmitic level in groundnut oil, for example, suggests palm olein.
- Sterol profile: harder to fake than fatty acids and useful when blends are designed to match the profile.
- Iodine value, saponification value and refractive index: quick identity checks against typical ranges.
- Baudouin test: a colour reaction that detects sesame oil in another oil.
- Halphen test: detects cottonseed oil.
- Argemone test: a nitric acid colour test or thin layer chromatography, routine for mustard oil.
- Unsaponifiable matter: a high result can point to mineral oil.
- Bellier turbidity temperature: a classic identity check for groundnut oil.
Simple checks at the mill
None of these replace a lab, but they tell you whether a mill is likely to be honest. See whether the plant crushes seed or only buys tankers of oil. Check whether seed stock, press capacity and oil cake output match the oil they sell. A mill that sells more oil than its seed could produce is buying oil from elsewhere.
Look at the tins and drums in the store. Unlabelled drums of other oils near the filling line are a warning sign. The trader question is covered in how to tell a trader from a processor.
Build testing into the contract
The general approach to proof is in what a lab certificate should show to be valid evidence.
- State that the oil must be 100 percent of the named oil, with no blending.
- Name the tests, methods and lab, and who pays for retests.
- Draw sealed samples at loading, in front of an independent person, and keep a counter-sample.
- Test every lot, not only the first container.
How we help
We check seed intake and press capacity at the mill, draw sealed samples ourselves, and send them to an NABL-accredited lab for fatty acid and adulterant tests. You receive the original reports. Ask for a free sourcing review.