Acidity Is Only the Beginning: Three Numbers That Reveal Olive Oil Rancidity

Why Acidity Alone Is Not Enough

The number printed most prominently on an olive oil label is usually acidity. Its formal name is free fatty acid content (FFA), and it rises when triglycerides break down under the stress of harvesting and milling, or through microbial activity in damaged fruit. The international benchmark for extra virgin grade, set by the International Olive Council (IOC), is a maximum of 0.8%; the lower the figure, the fresher the milling conditions are generally considered to have been.

The catch is that acidity records damage that has already happened — before the oil reaches the bottle. Once the oil is sealed and begins its journey through light, heat, and oxygen, the lipid oxidation that drives most of the deterioration a consumer actually experiences is not captured immediately in the FFA reading. An oil with impressively low acidity can already be deep inside an oxidative chain reaction if it has been stored carelessly. For this reason, both the IOC and EU regulations require producers to report peroxide value (PV), UV absorbance (K232 and K270), and sensory evaluation alongside acidity before a grade can be assigned.

Peroxide Value and K232·K270 — Primary and Secondary Oxidation Markers

Lipid oxidation does not happen all at once; it advances in stages. The first compounds to form are hydroperoxides. Peroxide value (PV) measures their concentration in units of meq O₂/kg. According to IOC standards, extra virgin olive oil must not exceed 20 meq O₂/kg.

Hydroperoxides are, however, unstable. Heat and time break them down into secondary oxidation products — aldehydes, ketones, and related compounds. UV absorbance readings track this progression. K232 detects conjugated dienes, the signature of primary oxidation, while K270 detects conjugated trienes and aldehydes that characterise the secondary stage. Under EU Regulation EC 2568/91 and its amendments, extra virgin olive oil must have K232 ≤ 2.50 and K270 ≤ 0.22.

Reading the two together reveals how far oxidation has progressed. A high PV combined with a low K270 points to an early stage. A low PV alongside a high K270, by contrast, signals that the primary oxidation products have already decomposed — the oil is further gone than its peroxide value suggests. K270 is also used to detect the presence of refined oil blended into supposedly extra virgin product, because the refining process leaves a characteristic secondary-oxidation fingerprint.

Hexanal — The Sharpest Language of Freshness

A marker attracting increasing attention in olive oil quality research is hexanal. It is a six-carbon aldehyde generated when linoleic acid (an omega-6 fatty acid) oxidises, and it is widely recognised as the principal compound behind the stale, cardboard-like off-notes that define rancid oil. Because of its low molecular weight, hexanal is highly volatile and can be measured at ppm or ppb concentrations by gas chromatography (GC) or GC-MS.

A study published in Food Chemistry in 2022 reported that hexanal concentrations rose significantly before trained sensory panels classified the same oils as rancid, suggesting the compound may serve as an early-warning indicator — detectable before the human nose registers the problem. Researchers also reported cases in which PV and K values remained within accepted limits while hexanal was already elevated, pointing to a potential blind spot in the conventional measurement framework.

Hexanal is not yet part of the official IOC or EU standards. Research into its use as an early freshness detection tool is ongoing at Spain's AICA (Agricultural and Food Information Centre) and Italy's CNR Institute of Food Chemistry, and a number of premium producers are reported to have begun incorporating it into their internal quality-control programmes.

Reading All Four Numbers Together

Each indicator captures oxidation at a different point on the timeline.

FFA (acidity) reflects the quality of the raw material and the hygiene of the milling process before the oil is ever bottled. Peroxide value (PV) catches the earliest stage of oxidation, when hydroperoxides are actively forming. K232 signals that the primary oxidative chain is running at pace. K270 indicates that oxidation has crossed into the secondary stage. Hexanal tracks the accumulation of the secondary volatile compounds most directly linked to sensory deterioration, at a finer resolution than any of the others.

When all four readings are low, it is evidence that the oil has been well managed across its entire life cycle. When acidity is low but K270 and hexanal are elevated, the likely interpretation is that pressing conditions were excellent but oxidation advanced during distribution or storage. Consumers rarely find all four values on a label, but it is worth knowing that a Certificate of Analysis (COA) can be requested from the producer or importer — and that it is the document where these numbers live.

Storage Changes the Numbers

Understanding the indicators leads naturally to the question of how to keep them low. Hexanal and K270 are particularly sensitive to light and heat. According to data published by the European Food Safety Authority (EFSA), olive oil stored in clear glass under direct sunlight can see its peroxide value approach the regulatory threshold within just a few days.

Dark glass or tin containers, kept in a cool, light-free environment — ideally between 14 and 18 °C — are the recommended conditions. Once opened, keeping the cap tightly sealed minimises air exposure; using the oil within 30 to 45 days of opening is a practical way to limit flavour loss. Even an oil that leaves the mill with exemplary acidity and K values will broadcast a hexanal warning if its storage environment is poor.

The quality numbers on a COA are a record of a producer's care. How the oil is stored after purchase determines whether that care reaches the last drop. Acidity alone does not come close to telling that full story.