How Is Olive Oil Made? Pressing, Grades and a Very Large Fraud Problem

Harvest timing sets the character of the oil. Picked green and early, the fruit yields less oil but one that is peppery, bitter and high in the polyphenols that give it both its flavour and its keeping quality; picked ripe and black, it gives more oil, milder and sweeter, that will not last as long. Traditional harvesting beats the branches so the fruit falls onto nets, which bruises it; careful producers pick by hand or use vibrating machines that shake the trunk. The critical rule afterwards is speed, because as soon as the skin is broken, enzymes and microbes begin producing free fatty acids, and fruit left in a heap for days will ferment. Good mills process within hours of picking, and the sacks or crates the fruit arrives in matter, since fruit piled deep in a sealed bag heats and spoils from the inside.

The mechanics have changed since antiquity in speed rather than principle. Leaves are blown off and the fruit is washed, then crushed whole, stones included, by a hammer or disc mill in a modern plant or by a stone wheel in a traditional one, producing a paste. The paste is then stirred slowly in a trough for twenty to forty minutes, a step called malaxation, which lets the microscopic droplets of oil coalesce into larger ones that can be separated; this is where temperature control matters, since warmth increases yield and drives off the aromatic compounds, and the term cold extracted means this step was held below 27 degrees. Separation is then done by centrifuge in almost all modern mills, spinning the paste so that oil, water and solids stratify by density. The older method of spreading paste on fibre mats and stacking them under a hydraulic press is now rare, which makes the words cold pressed on a label largely decorative.

The categories are legally defined and describe chemistry and tasting, not marketing:

• •Extra virgin: obtained by mechanical means only, with free acidity no higher than 0.8 percent, plus a peroxide limit and other chemical criteria, and it must pass a sensory panel with zero defects and some detectable fruitiness
• •Virgin: the same mechanical production, acidity up to 2 percent, and a small level of sensory defect permitted
• •Lampante: virgin oil that fails the standard, historically burned in lamps, and not sellable for eating until refined
• •Refined: lampante oil treated with heat, chemicals and deodorising, which removes the defects and also the flavour, colour and most of the polyphenols
• •Olive oil, sometimes labelled pure: refined oil blended with a little virgin oil to give it back some taste and colour
• •Olive pomace oil: extracted with solvents from the leftover paste, and a different product entirely

The gap between what extra virgin sells for and what refined or seed oil costs makes adulteration extremely profitable, and the methods range from mild to criminal. The commonest is simply mislabelling, selling virgin or refined oil as extra virgin, which is hard for a consumer to detect and, in the case of deodorised oil, hard for standard chemistry to detect too, since the refining removes the markers that would give it away. Others include blending in cheaper seed oils, passing off oil from one country as another's, and mixing old oil with new. Academic testing has repeatedly found large proportions of supermarket samples failing the extra virgin standard, and the Italian authorities have run several major prosecutions, including the 2017 case against a network shipping Syrian and Turkish oil relabelled as Italian. Regulators have responded with isotope and DNA testing and stricter traceability, and the practical advice for a buyer is a harvest date on the bottle, dark glass, a stated single origin rather than a blend of European Union oils, and an expectation that a genuinely good oil will cost several times what the cheapest does.

Oil deteriorates through oxidation, and light, heat, air and time all accelerate it, which is why the clear bottle on a shelf above the cooker is the worst possible arrangement. A good oil is at its best within about a year of harvest and should be stored dark, cool and tightly closed. The peppery catch at the back of the throat that beginners mistake for a fault is the compound oleocanthal, an anti-inflammatory that researchers have compared to ibuprofen in its mechanism, and it is a marker of freshness and quality rather than a defect. The frequently repeated advice not to cook with extra virgin is largely wrong: its smoke point is around 190 to 207 degrees, above most domestic frying, and its high antioxidant content makes it more stable under heat than several refined oils with higher smoke points, a result confirmed in a 2018 Australian study that heated a range of oils and measured the degradation products. What heating does destroy is the delicate aroma, which is the real argument for keeping the best bottle for finishing rather than frying.

Olives are crushed whole into a paste, stirred slowly so the oil droplets merge, and separated by centrifuge, with quality decided by how soon after picking the fruit is milled and by keeping the process below 27 degrees. Extra virgin is a legal standard requiring mechanical extraction, acidity under 0.8 percent and a clean sensory panel result, while refined oil has been stripped of flavour and polyphenols. Adulteration and mislabelling are widespread, and the useful signals on a bottle are a harvest date, dark glass and a single stated origin.