Reading oil with ultraviolet: the K232 and K270 numbers
Some of the most powerful quality checks on olive oil use no taste at all — just light. When ultraviolet passes through the oil, the products of oxidation and refining absorb it at specific wavelengths, and two figures, K232 and K270, put a number on damage the eye and tongue can miss.
When fats oxidise, they do not just smell different — they change how they interact with light. The double bonds in the fatty acids rearrange into new patterns that soak up ultraviolet at particular wavelengths. Shine a UV beam through a sample and measure how much is absorbed, and you get a direct, objective read on how far the oil has travelled down the road to rancidity — or how much it has been refined. This is spectrophotometry, and it is one of the oldest and most trusted tools in the olive-oil lab.
Two windows on the damage
The test reads absorption at two wavelengths. The first, at 232 nanometres, is written K232 and reflects the earliest stage of oxidation — the first rearranged double bonds that form as an oil begins to turn. The second, at 270 nanometres, is K270 and reflects a later stage, the secondary breakdown products, as well as the tell-tale compounds that industrial refining leaves behind. Together they cover the story from first freshness slipping to full-blown refining, each with its own regulated ceiling for extra virgin.
The lines that define extra virgin
The limits are precise. To be extra virgin, an oil must keep K232 at or below about 2.50 and K270 at or below about 0.22. Cross either line and the oil falls short of the top grade. K270 does double duty: because refining creates the very compounds that absorb at 270 nanometres, an unexpectedly high K270 in something sold as virgin is a strong hint that refined oil has been slipped in. It is one of the reasons this simple UV test remains a frontline defence, alongside the chemistry and tasting panels.
ΔK: the sharpest catch of all
There is a third, subtler number, called delta-K or ΔK, worked out from absorption just above and below 270 nanometres. In a genuine virgin oil it stays vanishingly small, at or below about 0.01. But refined oil produces a characteristic little bump right at 270 nanometres, and ΔK is designed to detect exactly that bump. So a ΔK above the limit is one of the hardest-to-dodge signs that a virgin oil has been cut with refined. It is a beautiful piece of forensic chemistry: a tiny wobble in a light curve that no amount of marketing can smooth away.
| What it uses | Ultraviolet absorption (spectrophotometry) |
|---|---|
| K232 (232 nm) | Early oxidation — EVOO ≤ ~2.50 |
| K270 (270 nm) | Later oxidation & refining — EVOO ≤ ~0.22 |
| ΔK | Purity check — EVOO ≤ ~0.01 |
| Detects | Freshness loss, oxidation, hidden refining |
| Taste needed? | None — a light measurement |
| Ease of faking | ΔK in particular is very hard to fool |
What it means for you
- UV numbers are an objective freshness and purity check that needs no tasting.
- K232 catches early oxidation; K270 catches later oxidation and refining traces.
- ΔK is the sharp tool for spotting refined oil hidden in a virgin blend.
- These are lab figures — treat a producer who shares them as one worth trusting.
K232 and K270: common questions
What do K232 and K270 measure?
How much ultraviolet light the oil absorbs at 232 and 270 nanometres. Oxidation and refining create compounds that absorb at these wavelengths, so the figures reveal damage and processing.
What are the limits for extra virgin?
Broadly, K232 at or below about 2.50 and K270 at or below about 0.22, with ΔK at or below about 0.01. Crossing a limit drops the oil below extra virgin.
What is the difference between K232 and K270?
K232 reflects early oxidation; K270 reflects later oxidation and, crucially, the compounds refining leaves behind, so it also helps detect hidden refined oil.
What is ΔK?
A value worked out from absorption around 270 nanometres. It stays tiny in genuine virgin oil but rises when refined oil is present, making it a powerful fraud check.
Do I need to taste the oil for this test?
No. It is purely a light measurement, which is its strength — it puts an objective number on damage and adulteration that taste alone might miss.
Of all the lab numbers, ΔK is the one I’d trust to catch a clever cheat. Flavour can be flattered and a best-before date invented, but refined oil leaves a specific little fingerprint in the ultraviolet, and ΔK is built to see it. So when a producer volunteers a full set of UV figures — K232, K270 and ΔK — take it as a good sign: they are inviting the kind of scrutiny a fraudster avoids. These numbers need no palate and answer no marketing; they simply shine a light through the oil and report what is there.
Drawn from International Olive Council and EU UV spectrophotometric limits (K232, K270, ΔK).