Peroxide Value: Measuring an Oil’s Freshness
Two lab numbers form the backbone of an olive oil’s quality certificate. Free acidity records how healthy and carefully milled the fruit was. Peroxide value records something different and just as telling — how far the oil has travelled down the road to rancid.
Every extra virgin olive oil that earns the grade has passed a battery of lab tests, and two chemistry numbers do most of the heavy lifting. Free acidity, discussed elsewhere, reflects the state of the fruit and the care of the milling. Peroxide value — PV for short — reflects the state of the oil: how much it has begun to oxidise. Think of free acidity as a birth certificate and peroxide value as a health check taken later, on the oil as it is now.
What peroxide value actually measures
Rancidity does not happen all at once; it unfolds in stages. In the first stage, oxygen attacks the oil’s fatty acids and forms compounds called peroxides — the earliest, still-tasteless chemistry of an oil going stale. Peroxide value counts them. A fresh, well-kept oil has a low count; as the oil ages or is abused by light, heat and air, the number climbs. Crucially, peroxides form before you can taste or smell the rancidity, which is what makes the measurement useful: it catches decline early.
The numbers that matter
For an oil to be legally extra virgin, its peroxide value must be no higher than 20 milliequivalents of oxygen per kilogram. That is a ceiling, not a target — the best fresh oils sit well below it, often under 10, and some quality schemes set stricter limits still. A high PV does not necessarily mean the oil was badly made; it can mean a good oil was badly kept. That is the whole point: peroxide value is the number that catches abuse in storage, not just faults at the mill.
Why it argues for dark glass and a cool shelf
You cannot run this test on your kitchen bottle, but peroxide value explains every storage habit worth having. Light, warmth and air all push peroxides up, relentlessly and irreversibly. A producer who bottles in dark glass or tin, and a shopper who keeps the bottle capped, cool and away from the window, are both fighting the same number. Free acidity and peroxide value together are a two-part fingerprint: one says the fruit was healthy and pressed with care, the other says the oil is still alive. A great oil, poorly stored, can fail a test it once passed.
| Full name | Peroxide value (PV) |
|---|---|
| What it counts | Peroxides — the first-stage products of oxidation |
| Driven by | Oxygen, light, heat and time |
| Extra virgin limit | No more than 20 meq O₂/kg |
| Fresh, well-kept oil | Often below 10 |
| Partner test | Free acidity (reflects fruit and milling) |
| Practical defence | Dark glass or tin, cool storage, keep it capped |
What it means for you
- Peroxide value is a freshness clock, not a birth certificate — it tracks the oil now.
- Oxidation is relentless and one-way; light, heat and air only push it up.
- Buy oil in dark glass or tin and keep it cool, capped and out of the light.
- A prize oil badly stored can fail a test it once passed, so freshness at home matters.
Peroxide value: common questions
What is peroxide value?
A lab measure of the peroxides formed in the first stage of oxidation — the early chemistry of an oil going stale, before you can taste or smell it.
What is the limit for extra virgin?
No more than 20 milliequivalents of oxygen per kilogram. The best fresh oils sit well below that, often under 10.
How is it different from free acidity?
Free acidity reflects how healthy and carefully milled the fruit was; peroxide value reflects how far the finished oil has oxidised since. One is about origin, the other about freshness.
Can a well-made oil still have a high peroxide value?
Yes — if it has been stored badly. Light, heat and air raise the number over time, so poor storage can spoil an oil that was excellent when bottled.
Can I test my own bottle?
No, it needs a laboratory. But the science explains why you should buy oil in dark glass or tin and keep it cool, capped and away from light.
Peroxide value is the number that keeps oil honest after it leaves the mill. Free acidity tells you the olives were healthy and handled well — but a spotless free acidity means nothing if the oil then bakes on a sunny shelf for a year. That is what PV catches, the slow oxidation you can’t yet taste. You’ll never run the test at home, but it is the scientific reason behind every boring bit of advice this site repeats : dark glass, cool cupboard, cap it tight. You’re not being fussy — you’re keeping the peroxide number down and the oil alive.
A olives101 science explainer, drawn from International Olive Council chemical standards for extra virgin olive oil.