The Olive Mill: Crushing, Malaxation, the Centrifuge and the Myth of the First Cold Press
Extra virgin olive oil is the only major cooking fat that is simply the juice of a fruit, squeezed out and left alone. No refining, no solvent, no heat beyond what the fruit brings with it. That is also why it is so easy to ruin. An olive is a fruit that begins to spoil the moment it is picked, and the oil inside it is decided by three things the mill controls: how ripe the fruit was, how many hours passed between tree and crusher, and how gently the oil was coaxed out. This is what happens inside a good mill in the few weeks of the harvest, and what the phrase on the label used to mean before the machines changed. Our buying guide tells you how to choose the bottle; our history of olive oil tells you how long people have been doing this.
The harvest decision
Olives ripen from green through purple to black over the autumn, and the mill's first choice is when to pick. Green, early olives give less oil, and that oil is intensely green, bitter, peppery and high in the polyphenols that make a great oil taste alive and keep it fresh for a year. Ripe black olives give more oil, softer and buttery, with less bitterness and a shorter life. Most serious producers pick early, in October and November in the Mediterranean, when the fruit is just turning, accepting a lower yield for a better oil, which is why the finest bottles cost more per liter than the yield alone would justify. The harvest itself is done by hand with rakes and nets under the trees, or with mechanical shakers on larger estates, and the fruit goes into ventilated crates rather than sacks, because bruised or heated olives start to ferment and the oil turns.
The race to the mill
The number that matters most and appears on no label is the hours between picking and crushing. A good mill processes within a day, and the best within a few hours. Olives left in a pile overnight warm up and begin to ferment and mold, and the result is the musty, winey, vinegary defects that keep an oil from qualifying as extra virgin at all. Producers who own their own mill, or who deliver to one nearby, control this; producers who ship fruit to a distant cooperative mill do not, and it shows.
Crushing
The olives are washed, stripped of leaves and crushed whole, pit and all, into a paste. The traditional tool was a pair of granite millstones turning in a trough, and a few mills still use them for the gentle, slow crush they give. Most now use a stainless hammer or disc mill that does the job in seconds. The choice affects flavor: stone milling is gentler and cooler and tends to give a softer, rounder oil, while hammer milling breaks the fruit more completely and produces more of the bitter, peppery compounds that many modern producers want. Neither is wrong. The paste at this stage is a dark green mash of flesh, skin, pit fragments, water and microscopic droplets of oil.
Malaxation: the slow stir
This is the step that decides the character of the oil, and almost nobody outside the trade has heard of it. The paste goes into a horizontal steel trough, the malaxer, where slow blades turn it gently for twenty to forty minutes at a controlled temperature. The stirring lets the tiny oil droplets find one another and merge into larger drops that can be separated, and it develops the aromas as enzymes work on the paste. Longer and warmer gives more oil and rounder flavor; shorter and cooler gives less oil, more intensity and more of the fresh, grassy, bitter notes that define a high-end oil. Modern malaxers are sealed and often flushed with nitrogen to keep oxygen from dulling the aroma, and the temperature is held below about 27°C, which is where the phrase "cold" on a label now lives. The producer's choices here are the equivalent of a winemaker's fermentation decisions, and two mills can take the same fruit and make two different oils in this trough.
Separation: the centrifuge
The paste is pumped into a horizontal centrifuge, the decanter, which spins it at high speed and separates it by density into oil, water and solid pomace. A second, vertical centrifuge cleans the last traces of water and fine solids from the oil. This is where "pressing" ended. Until the 1960s and 1970s, the paste was spread on woven mats stacked in a tower and squeezed under a hydraulic press; the oil and water ran out together and were separated by settling. Presses were slow, exposed the paste to air, and left mats that could carry the taste of yesterday's batch into today's. Nearly every quality producer replaced them with centrifuges, which are faster, cleaner and cooler, and the result is that almost no fine oil on the market today is pressed at all.
What "first cold pressed" actually means
The phrase is a fossil. "First" distinguished the initial pressing from later ones, in which the mats were re-pressed, sometimes with hot water, to squeeze out lower-grade oil; a centrifuge extracts once, so there is no second pressing to be first before. "Cold" meant no hot water was added to the mats; it now refers, loosely, to the malaxer temperature. "Pressed" describes a machine most producers no longer own. The regulated terms that matter are "extra virgin," which is defined by chemistry and by a blind taste panel that must find no defects, and, in Europe, "cold extraction," which sets a temperature limit. A bottle can say "first cold pressed" and be industrial; another can say nothing of the kind and be superb. The harvest date and the producer's name tell you more.
Storage and bottling
Fresh oil is cloudy with fruit particles and often filtered, though some producers bottle it unfiltered for the first weeks of the season as a fleeting specialty. It is stored in stainless tanks under nitrogen, away from light and heat, its two enemies, and bottled in dark glass or tins. From the day it is made, it begins a slow decline: the polyphenols that give a green oil its bite fade over the year, and by the next harvest a good oil is pleasant but no longer what it was. This is why a harvest date on the label matters more than a best-by date, why our buying guide tells you to buy small and buy this year's, and why our Denver kitchens, with their dry air and strong sun, should keep the bottle in a cupboard rather than on the counter.
What the mill puts in the bottle
Green, bitter and peppery: an early harvest, a hard crush, a short cool malaxation, a fast centrifuge. Golden, buttery and mild: a later harvest, a longer warmer stir. Musty, flat or winey: fruit that waited too long for the mill, whatever the label says. The great regional styles are these choices made consistently over generations: the fierce green Coratina of Puglia, the mild Arbequina of Catalonia, the peppery Tuscan blends, the buttery oils of Liguria. The bottles on our olive oil shelf are chosen by producers who own their mills and pick early, and they are at 263 Josephine Street in Cherry Creek, Denver, and ship nationwide from theambrosianpantry.com. Taste a spoonful of a new-harvest oil in November and you will taste the malaxer.
