Peroxide Value in Olive Oil: Understanding Freshness and Quality

1What is a peroxide?

When the double bonds of unsaturated fatty acids (especially oleic and linoleic acids) react with atmospheric oxygen, the first compounds of oxidation are formed: lipid hydroperoxides (or peroxides).

These primary oxidation products are unstable. Over time, they decompose into aldehydes, ketones, and short-chain acids. These secondary oxidation products are the molecules directly responsible for the off-flavors and smell of rancidity.

The Peroxide Value (PV) measures the concentration of these primary reactive oxygen compounds present in the oil at the time of testing, expressed in mEq O₂ per kg of oil.

2Legal limits and ideal values

According to European regulations: * Maximum legal limit for Extra Virgin Olive Oil: ≤ 20 mEq O₂/kg * An outstanding, freshly milled artisanal oil: 4–8 mEq O₂/kg * A high-quality oil stored well after 6 months: 8–12 mEq O₂/kg * Close to the limit: 15–20 mEq O₂/kg (noticeably flat and starting to degrade)

3Why peroxide values rise

Lipid oxidation is a radical-driven, autocatalytic chain reaction: once initiated, the free radicals generated accelerate the process. Factors that speed up this decay: * Light: Photo-oxidation driven by natural chlorophylls in the oil. * Heat: Every 10°C increase in temperature doubles the oxidation rate. * Oxygen: Headspace air in a half-empty container drives steady oxidation.

4The peroxide paradox

Peroxides are unstable intermediates. In a very old, completely rancid oil, the peroxide value can actually drop because almost all hydroperoxides have already degraded into the final aldehydes (rancid smell).

For a complete and reliable quality assessment, the Peroxide Value must always be read alongside UV extinction coefficients (K232 and K270), which measure both primary and secondary oxidation.