From olive to oil…

An olive is about a fifth oil by weight, locked inside cells. Everything that happens at a mill is an attempt to get it out fast, cold and clean — and every shortcut shows up in the taste.

Olive oil is unique among the great cooking fats in that it is a fruit juice. No solvent, no refining, no heat is required — you crush a fruit and separate the oil from the water and solids. That simplicity is the whole appeal, and it is also why the process is so unforgiving. There is nowhere to hide a mistake, because nothing downstream will fix it. Here is what actually happens, in order, and what each step decides.

Step	What happens	What it decides	The risk
Harvest	Fruit picked, by hand or mechanical shaker	Ripeness, phenol level, bruising	Damaged fruit begins fermenting
Transport	Fruit moved in crates, not sacks	Whether the fruit arrives intact	Heat and crushing in deep piles
Cleaning	Leaves removed, fruit washed	Off-flavours, grit in the mill	Too many leaves give a grassy, bitter note
Crushing	Fruit and stone ground to a paste	Release of oil from the cells	Heat generated by the crusher
Malaxation	Paste slowly churned, 20–40 min	Yield and aroma — the key trade-off	Warm or long malaxation oxidises phenols
Extraction	Centrifugal decanter separates phases	Clean separation of oil, water, solids	Excess water use strips phenols
Filtration	Optional; removes water and solids	Shelf life versus cloudy character	Unfiltered oil ages faster
Storage	Inert tanks, cool, dark, low oxygen	Everything about how it tastes in June	Light, heat and air ruin good oil

The two decisions that matter most

Of all those steps, two dominate. The first is time between picking and crushing. Fruit begins to degrade the moment it leaves the tree: enzymes work, damaged fruit ferments, and free acidity climbs. A day is acceptable; two is a compromise; three is a defect you will taste. This is why serious producers mill the same day and why fruit travels in shallow ventilated crates rather than deep sacks, where the weight of the pile crushes and heats the bottom layer.

The second is malaxation, the slow churning of the crushed paste that lets microscopic oil droplets coalesce into larger ones the centrifuge can separate. Warm the paste and churn it longer and you get more oil per tonne — a direct, measurable gain. But you also oxidise phenols and drive off volatile aromatics, so you get more of a worse oil. This is the single most consequential trade-off in the mill, and it is exactly what cold extraction rules are designed to police: they cap the temperature so a producer cannot quietly buy yield with quality.

Presses, decanters and the word cold-pressed

Traditional mills built a stack of mats spread with paste and squeezed them under a hydraulic press. Almost nobody works that way commercially now. Modern mills use a horizontal centrifugal decanter, which spins the paste and separates oil, vegetation water and solids continuously. It is faster, cleaner and much easier to keep sanitary — old press mats are notoriously hard to keep from going rancid and tainting everything that passes over them.

This is why cold-pressed is a slightly archaic term: most oil is not pressed at all. What matters is the temperature, not the machinery. Look for a stated cold extraction and, far more usefully, a harvest date. A bottle that tells you exactly when the fruit was milled is made by someone who thinks that fact is worth being proud of — and it is the single most reliable quality signal available to a shopper, well ahead of any medal or any of the language discussed in what extra virgin really means.

What happens after the mill

• Fresh oil is settled or filtered. Unfiltered oil looks lovely and ages faster, because suspended solids and water feed spoilage.
• It is stored in stainless steel, cool and dark, often under nitrogen to exclude oxygen.
• Bottling is the point of greatest exposure to air — good mills bottle to order rather than all at once.
• At home, keep it away from the hob, in dark glass or tin, and finish it within months of opening.
• Oil does not improve with age. The harvest date, not the best-before date, tells you what you have.

How olive oil is made: common questions

How long after picking should olives be milled?

Ideally within 24 hours. Beyond that, enzymatic degradation and fermentation in damaged fruit raise acidity and produce off-flavours that cannot be undone.

What is malaxation?

The slow churning of crushed olive paste before extraction, which lets small oil droplets merge so the centrifuge can separate them. Warmer and longer raises yield but harms quality.

Does cold-pressed mean anything?

The temperature limit does; the word press mostly does not. Modern mills use centrifugal decanters rather than presses, so cold extraction is the accurate term.

Is unfiltered olive oil better?

It is fresher-tasting and often lovely, but suspended water and solids shorten its life. Filtered oil keeps better; unfiltered should be drunk young.

How should I store olive oil at home?

Cool, dark, sealed and away from the cooker, in dark glass or tin. Buy a size you will finish within a few months of opening.

Drawn from modern mill practice and the extraction standards governing virgin olive oil.