{"id":74,"date":"2026-07-01T22:51:27","date_gmt":"2026-07-01T22:51:27","guid":{"rendered":"https:\/\/olivefather.com\/?page_id=74"},"modified":"2026-09-06T16:17:52","modified_gmt":"2026-09-06T16:17:52","slug":"chemistry-of-olive-oil-quality","status":"publish","type":"page","link":"https:\/\/olivefather.com\/?page_id=74","title":{"rendered":"Free Acidity, Peroxides & Freshness: The Chemistry of Olive Oil Quality"},"content":{"rendered":"\n
\"Amber
Every lot we bottle passes through a laboratory before it passes through a tasting panel. The numbers come first.<\/figcaption><\/figure>\n\n\n\n

“Fresh” is an easy word to print. “Quality” is easier still. Neither one costs a producer anything, which is why both appear on bottles that deserve neither. What makes olive oil unusual among foods is that it does not have to be taken on trust. Its quality can be pinned to specific, repeatable chemistry \u2014 a handful of laboratory figures that a mill either meets or does not, and that no label designer can improve.<\/p>\n\n\n\n

This page is our attempt to explain those figures the way we would to a new member of the family: what each number actually measures, what it says about how the fruit was handled, and how the slow chemistry of rancidity works against every bottle from the day it is sealed. If you already know the tasting side, this is the other half of the story \u2014 the half that happens in a burette rather than a glass.<\/p>\n\n\n\n

Free acidity: the number that defines the grade<\/h2>\n\n\n\n

Inside a sound olive, the fatty acids are bound up three at a time on a glycerol backbone \u2014 triglycerides, the ordinary architecture of any fat. The moment the fruit is bruised, frost-bitten, attacked by the olive fly, left in a heap overnight or milled with dirty equipment, enzymes and microbes begin cutting those bonds and setting individual fatty acids loose in the oil. The proportion of these liberated acids is the free acidity<\/strong>, expressed by convention as grams of oleic acid per 100 grams of oil \u2014 in everyday language, a percentage.<\/p>\n\n\n\n

Two things follow that most labels quietly rely on you not knowing. First, you cannot taste free acidity. It has nothing to do with sourness or sharpness; an oil at 0.2 percent and an oil at 1.5 percent can taste equally soft. Second, because it is created by damage and delay, it is a record of everything that happened to the fruit between the branch and the separator. A very low figure is not a stylistic choice. It is proof of clean, undamaged olives, picked and milled in a hurry \u2014 the discipline described on our page on harvest and cold pressing<\/a>.<\/p>\n\n\n\n

The thresholds are set by the International Olive Council and written into European Union regulation. An oil may be called extra virgin<\/strong> only if its free acidity is no greater than 0.8 percent<\/strong>. Virgin<\/strong> olive oil is permitted up to 2.0 percent. Above that, the oil is lampante<\/em> \u2014 lamp oil \u2014 and cannot be sold for eating until it has been refined. Fruit that has been handled the way we handle it lands far below the ceiling, typically well under 0.3 percent. When a bottle boasts of “low acidity” but declines to print the figure, it is worth asking why. Our overview of what quality actually means<\/a> puts this number beside the others that make up the grade.<\/p>\n\n\n\n

Peroxide value: the first fingerprint of oxidation<\/h2>\n\n\n\n

What it measures<\/h3>\n\n\n\n

Free acidity tells you about damage before the mill. Peroxide value<\/strong> tells you about damage after it. When the unsaturated fatty acids in oil meet oxygen, the first stable products of the reaction are hydroperoxides, and the peroxide value counts them \u2014 in milliequivalents of active oxygen per kilogram of oil. For extra virgin, the legal ceiling is 20 meq O\u2082\/kg<\/strong>. A freshly milled oil from sound fruit will usually sit in single digits.<\/p>\n\n\n\n

Peroxides are a beginning, not an end. They are unstable intermediates, and as oxidation proceeds they fracture into the aldehydes, ketones and short-chain acids that the nose eventually reports as rancidity \u2014 crayon, old walnut, putty, stale peanut. So a rising peroxide value is an early warning: the oil has started to react with air even though it may still taste acceptable. Read together, a low free acidity and a low peroxide value describe an oil that was made from good fruit and<\/em> has been kept well since.<\/p>\n\n\n\n

The spectrophotometer’s second opinion<\/h3>\n\n\n\n

There is a subtlety here that the honest producer must own. Peroxides can fall as well as rise: an oil that is badly oxidised and then left long enough may show a deceptively modest peroxide value because its peroxides have already broken down into other things. That is why the grading standard also measures ultraviolet absorbance<\/strong> \u2014 the coefficients written as K232 and K270. Primary oxidation products absorb light at 232 nanometres; secondary products, and the traces left by refining, absorb at 270. For extra virgin the limits are K232 no greater than 2.50 and K270 no greater than 0.22. Together these three tests \u2014 acidity, peroxides, UV \u2014 make it very difficult for a tired or a refined oil to pass itself off as fresh.<\/p>\n\n\n\n

How an oil goes rancid<\/h2>\n\n\n\n

Rancidity in olive oil is almost entirely oxidative<\/strong>: a self-propagating chain reaction in which oxygen attacks the double bonds of unsaturated fatty acids, producing free radicals that go on to attack further molecules. Once it is properly under way it feeds itself. Four things drive it, and each one is within the control of whoever is holding the bottle.<\/p>\n\n\n\n