Olive Oil Acidity, Peroxide, K232, K270: Parameters Explained

A certificate of analysis lines up five or six numbers that decide an oil's grade, its shelf life and its authenticity. Most buyers know extra virgin olive oil must sit at or below 0.8% acidity — but few know what that figure actually measures, what moves it, or why it says almost nothing about taste. This article takes each parameter one at a time: the chemistry behind the number, the concrete causes of drift, and the close reading that separates a robust lot from one that is merely compliant. For how to read the document itself and the questions to put to a supplier, see our dedicated guide: how to read an olive oil COA.

Free acidity: a hydrolysis reaction, not a taste verdict

Free acidity measures the share of free fatty acids in the oil, expressed as a percentage of oleic acid. In sound fruit, fatty acids are bound to glycerol as triglycerides. When an olive's cells are damaged, an enzyme — lipase — cleaves those bonds and releases individual fatty acids. That hydrolysis is what the test quantifies. Acidity is therefore not a sharpness you taste: it is the counter of cell breakdown that has already happened before or during extraction.

What pushes it up is mechanical and biological:

• Bruised or fallen olives, whose tissue has torn.
• Olive fruit fly attack (Bactrocera oleae): the larva pierces the flesh and opens the door to enzymes and moulds.
• Delay between harvest and milling. Olives piled in crates ferment; every day of waiting costs tenths of a point.
• Storing olives in heaps, which heats and crushes the fruit at the bottom.

Hence the real gap between an oil at 0.2% and one at 0.8%: both are legally extra virgin, but the first points to sound olives milled fast, the second to raw material already pushed to the limit. At purchase, that margin is an asset: it absorbs ageing right up to the best-before date.

What acidity does not tell you: nothing about aroma, nothing about bitterness, nothing about sensory defects. An oil can read 0.3% and still carry a rancid or musty note that downgrades it at the panel test. Acidity is the entry ticket to the grade, never a certificate of overall quality.

Peroxide value: primary oxidation, a living number

The peroxide value quantifies hydroperoxides, the first products formed when oxygen attacks unsaturated fatty acids. It is expressed in milliequivalents of active oxygen per kilogram; the extra virgin limit is 20 meq O₂/kg. A fresh, well-made oil sits well below that, often under 10.

The key point, widely misunderstood: this parameter moves over time. Unlike acidity, which is roughly fixed after extraction, peroxide climbs with every exposure to oxygen, light and heat. A lot tested at 6 in February can reach 12 by August if it has been stored badly — a poorly blanketed tank, repeated transfers, a container left in the sun on a quay. That is why the analysis date matters as much as the value: a low peroxide on an early-season certificate guarantees nothing ten months later.

For the buyer, peroxide is therefore predictive. It signals how the lot will handle transport and its commercial life. Two practical rules: demand a value that leaves headroom under 20 (aim for 10-12 at loading), and plan an analysis on arrival in addition to the one at departure, since this is the figure that shifts most between the two points. The storage conditions that limit this drift are covered in our article on storing bulk olive oil.

K232, K270 and ΔK: what ultraviolet reveals

Ultraviolet spectrophotometry measures how much the oil absorbs light at specific wavelengths. Oxidation products carry conjugated double bonds that absorb at 232 and 270 nanometres; the more there are, the higher the absorbance. For extra virgin: K232 ≤ 2.50, K270 ≤ 0.22, ΔK ≤ 0.01.

• K232 captures conjugated dienes, markers of primary oxidation. It often echoes the peroxide signal and degrades early.
• K270 captures conjugated trienes and carbonyl compounds, the signature of secondary oxidation — a later stage where hydroperoxides have already broken down into the aldehydes and ketones behind rancidity. A high K270 also flags the presence of refined oil: refining generates these compounds in bulk.
• ΔK is the anti-fraud parameter. Absorbance is read at 270 nm and at 266/274 nm on either side, and a gap is calculated. In a genuine virgin oil that gap stays ≤ 0.01. Blending with deodorised or refined oil distorts the curve and pushes ΔK out of range, even when acidity and peroxide look excellent.

Typical drift patterns: a K232 rising on its own points to incipient oxidation from storage; an abnormal K270 with a normal ΔK suggests an oil already deeply oxidised; a ΔK out of range, by contrast, cannot be explained by ageing — it points to a blend. An oil can show perfect acidity and peroxide and still be caught out by this UV trio alone.

Purity and freshness parameters

Beyond the classic four, several analyses complete the picture — above all to detect fraud and manufacturing faults.

• Fatty acid ethyl esters (FAEE). These form when fermented olives release ethanol, which esterifies with free fatty acids. It is a direct marker of olive quality and freshness: olives held too long, heated, or a paste over-warmed during malaxation. EU rules cap FAEE at 35 mg/kg for extra virgin. A high figure, even with compliant acidity, reveals raw material that has fermented.
• Stigmastadienes. These hydrocarbons arise during refining; they are all but absent from a virgin oil. The extra virgin limit is 0.05 mg/kg. It is one of the most reliable tests for detecting the addition of cheap refined oil.
• Waxes. Wax content (the sum of C42+C44+C46 esters) separates a pressed oil from a solvent-extracted pomace oil, which is richer in waxes. Extra virgin limit: ≤ 150 mg/kg.
• Panel test. Sensory analysis by a trained panel remains the judge of grade: median of defects = 0 and median of fruitiness > 0 to stay extra virgin. Chemistry and nose complement each other; we cover defects and their origins in our article on the sensory panel test and defects.

Summary table

Parameter	What it measures	Extra virgin limit (EU/IOC)	What degrades it	Recommended buying margin
Free acidity	Free fatty acids (hydrolysis)	≤ 0.8%	Damaged olives, fruit fly, harvest-to-mill delay	≤ 0.4%
Peroxide value	Primary oxidation (hydroperoxides)	≤ 20 meq O₂/kg	Oxygen, light, heat, storage	≤ 10-12 at loading
K232	Conjugated dienes (primary oxidation)	≤ 2.50	Incipient oxidation, storage	≤ 2.20
K270	Trienes/carbonyls, refined oil	≤ 0.22	Advanced oxidation, blending	≤ 0.18
ΔK	Blending with refined oil	≤ 0.01	Fraudulent blend	strict compliance
FAEE	Freshness, olive fermentation	≤ 35 mg/kg	Fermented olives, warm malaxation	≤ 20
Stigmastadienes	Presence of refined oil	≤ 0.05 mg/kg	Blending with refined oil	strict compliance
Waxes (C42+C44+C46)	Pomace vs pressed oil	≤ 150 mg/kg	Solvent extraction	strict compliance

Why you must always read the parameters together

No single number is enough. Perfect acidity can sit next to a drifting peroxide; a low peroxide can mask a fraudulent ΔK; a spotless UV profile can hide a high FAEE that betrays fermented olives. Reading is combined.

Profile of a fresh, well-made oil: acidity 0.2-0.3%, peroxide 6-10, K232 under 2.2, K270 under 0.15, ΔK nil, low FAEE. Profile of a borderline oil: acidity 0.7%, peroxide 18, K232 at 2.45, K270 at 0.21 — technically extra virgin, but with no reserve to carry through six months of trade. The price can be identical; the real value is not.

Contracting with margin

The practical consequence: never align your purchase specifications with the legal thresholds. Setting acidity ≤ 0.4% and peroxide ≤ 12 at loading gives the oil room to age to its best-before date without dropping out of grade. Three clauses are worth writing into the contract:

• Numeric specifications per parameter, with margin below the regulatory limits.
• Analysis at loading AND on arrival, with the procedure and the (ISO 17025-accredited) laboratory agreed in advance.
• Handling of deviations: what happens if peroxide at destination exceeds the contractual threshold? Discount, rejection, contradictory re-analysis.

That is the framework we apply: every lot ships with its COA, tied to its mill of origin, and an independent SGS-type counter-analysis is available at loading. See the detail of our quality approach.

From parameters to the actual lot

Understanding the chemistry means you can negotiate specifications that hold. The next step is concrete: compare real certificates. Explore our bulk Tunisian olive oil, then request a quote on the lot whose parameters match your specification — COA in hand.