An olive press is a machine that crushes olives and separates the oil from the fruit’s flesh, skin, and pit. The basic idea has remained the same for thousands of years: apply force to olive paste until the oil flows out. What has changed dramatically is the technology used to generate that force, from ancient stone weights to modern centrifugal systems that can process thousands of kilograms per hour.
How Olive Pressing Works, Step by Step
Turning whole olives into oil involves three distinct stages, whether you’re using a backyard press or a commercial facility.
First, the olives are crushed into a paste. Traditional mills used large granite stones rolling in a circular trough. Modern operations use stainless steel hammer mills or blade crushers that break down the fruit, pit and all, in seconds. The goal is the same: rupture the olive cells so the oil inside can escape.
Next comes a step called malaxation, which is essentially slow kneading. The olive paste is stirred in a trough for about 30 minutes at a controlled temperature. This gives tiny oil droplets time to merge into larger ones that are easier to collect. Temperature matters here: keeping the paste around 25°C (77°F) helps dissolve flavor compounds into the oil, while lower temperatures preserve certain fresh, grassy aromas. This is also where the term “cold pressed” comes from, referring to oil extracted without excessive heat.
Finally, the oil is separated from the water and solid material in the paste. This separation stage is where traditional and modern presses differ most.
Traditional Hydraulic Presses
The classic olive press that most people picture uses hydraulic pressure. Olive paste is spread onto round fiber mats (historically woven from hemp or esparto grass, now often synthetic), and these mats are stacked in layers inside a pressing chamber. A hydraulic cylinder then pushes a piston upward, compressing the stack with 40 to 60 megapascals of pressure. That’s roughly the force of a small car balanced on each square centimeter of mat.
As the stack compresses, oil and water seep through the filter cloth while the solid residue stays behind. After the pressing cycle finishes, the pressure is released, the spent pulp is removed, and the process repeats with fresh paste. The liquid that comes out is a mixture of oil and water, which then needs to settle or be spun in a small centrifuge to isolate the pure oil.
Hydraulic presses work in batches. You load, press, unload, and start again. This limits how many olives you can process in a day, but many small producers still prefer the method because it’s straightforward, relatively affordable, and produces oil with a character that some consider distinctive.
Modern Centrifugal Systems
Most commercial olive oil today is produced not by pressing at all, but by spinning. Continuous centrifugal systems replaced hydraulic presses at most large mills starting in the late 20th century. Instead of squeezing oil out, these machines use a device called a decanter, a horizontal drum that spins at high speed, generating centrifugal force 2,000 to 5,000 times stronger than gravity.
Inside the spinning drum, the three components of olive paste separate by density. Oil is lightest and collects closest to the center. Water sits in the middle layer. Solids, the heaviest material, are pushed to the outer wall. A screw-shaped conveyor inside the drum rotates at a slightly different speed than the drum itself, continuously pushing the solids toward a conical end where they’re discharged. Oil and water exit through separate outlets.
The advantages are speed and consistency. A centrifugal system runs continuously, processing olive paste as fast as the crusher and malaxer can feed it. There’s no stopping to unload mats, no manual layering of paste. The system also reduces the paste’s exposure to air, which helps preserve the oil’s freshness and antioxidant content.
How Much Oil Olives Actually Produce
Olives are not an especially oil-rich fruit, which is why olive oil costs more than most vegetable oils. On average, it takes about 10 kilograms of olives (roughly 22 pounds) to produce a single liter of oil. That number varies widely depending on the olive variety, ripeness, and growing conditions. High-yield, large-caliber olives can produce a liter from as few as 5 kilograms, while smaller or less oil-rich varieties may require up to 14 kilograms for the same amount.
This means a mature olive tree, which might produce 20 to 40 kilograms of fruit in a good year, yields only a few liters of oil. It’s one reason why genuine extra virgin olive oil from small farms commands a premium.
What Happens to the Leftovers
Pressing olives generates two types of waste: a solid residue called pomace (a mix of crushed skin, pulp, and pit fragments still containing some oil) and wastewater. Neither is trivial to deal with. A single mill can produce tons of pomace per season.
Pomace still contains enough residual oil to be worth extracting, but getting it out requires industrial methods. The pomace is dried and then washed with a chemical solvent that dissolves the remaining oil. After the solvent is evaporated off, the result is a low-grade product called pomace oil. It’s refined, blended, and sold cheaply, often for cooking at high heat. The exhausted pomace left behind can be burned as biomass fuel, composted, or used as animal feed.
How the Press Affects Oil Quality
The grade of olive oil depends on both the quality of the olives going in and how carefully the extraction is handled. The U.S. Department of Agriculture recognizes several grades, and the differences come down to chemistry and taste.
Extra virgin olive oil, the highest grade, must have zero sensory defects and a detectable fruitiness. Its free fatty acid level, a marker of how much the oil has degraded, must stay below 0.8%. Virgin olive oil allows minor flavor defects and a free fatty acid level up to 2.0%. Regular “olive oil” sold in stores is typically a blend of refined oil and some virgin oil, with a milder flavor profile.
The pressing method influences quality in several ways. Temperature is the biggest factor. Higher heat during malaxation or pressing increases oil yield but degrades delicate flavor compounds and antioxidants. Prolonged contact with air (more common in slower hydraulic presses) can accelerate oxidation. And cleanliness matters enormously: old paste residue left on fiber mats in a hydraulic press can introduce off-flavors. This is one reason centrifugal systems, with their enclosed stainless steel components, have become the industry standard for producing consistent extra virgin oil at scale.
That said, the press alone doesn’t determine quality. An expertly run hydraulic press using perfectly ripe olives picked the same day can produce oil that rivals anything from a modern mill. The key variables are freshness of the fruit, temperature control, speed from harvest to extraction, and cleanliness of the equipment.