Making olive oil is different from making any other cooking oil because it begins with a fruit — the olive — that is Harvested specifically to be turned into oil, not as a byproduct of something else. For a complete overview, see our Olive Oil Gastronomy: Cooking, Baking & Culinary Uses guide.For a complete overview, see our Cooking Properties guide.And unlike seed oils, which require chemical solvent extraction, olive oil production is primarily a mechanical process. The quality of the oil is determined as much by how quickly and carefully the olives are processed after harvest as it is by the fruit itself1.
Understanding how olive oil is made helps you understand why harvest timing, mill speed, and temperature control matter so much — and why fresh, recently pressed oil is meaningfully different from oil that has been sitting in a tank for months.
Olive oil quality begins with harvest timing. Olives are harvested in the autumn and early winter, but the precise timing significantly affects the oil's flavor and chemistry.
Green harvest (October–November): Olives are picked early, while still green or just starting to turn color. The oil has:
- Higher polyphenol content
- More intense bitterness and pungency
- Lower yield (less oil per kilogram of olives)
- More complex, grassy flavor
Black/ripe harvest (December–January): Olives are picked when fully ripe or overripe. The oil has:
- Lower polyphenol content
- Softer, fruitier flavor
- Higher yield
- Less bitterness
The timing that produces the best oil — and most of the oil recognized for its health benefits — is early harvest. But early harvest means lower yields, which means higher prices. The high-phenol oils that command premium prices and drive the health claims are almost always early-harvest products.
Harvest is done by hand in traditional groves or by mechanical harvesting in super-high-density (SHD) orchards. Mechanical harvesting using trunk shakers or over-the-row collectors has largely replaced hand-picking in large commercial operations, reducing labor costs substantially.
The time between harvest and milling is the most important quality variable in olive oil production. Olive fruit continues to metabolizing after picking, and the enzymes that cause fermentation and quality degradation are active from the moment the olive is separated from the tree.
The rule: mill within 24–48 hours of harvest
Any delay beyond this window causes measurable quality degradation — increased free fatty acid content, loss of fruity aromatics, and the onset of fermentation defects. The olives begin to heat and ferment, and this fermentation continues into the oil itself.
This is why the location of the mill relative to the olive groves matters: producers with their own mill on the farm can process within hours of harvest. Producers who must transport olives to a distant mill face quality compromises.
Step 1: Cleaning and washing Olives are fed through a cleaning machine that removes leaves, twigs, and other debris. They are then washed with clean water to remove surface dirt and any pesticide residue.
Step 2: Crushing Clean olives are fed into a hammer mill or blade mill that crushes them into a paste. The goal is to rupture the olive cells and release the oil, while grinding the pit (which contains compounds that can affect flavor if over-processed).
Traditional stone mills crush olives slowly with granite wheels. Modern producers use hammer mills or blade mills for faster, more consistent processing. The difference matters for flavor: stone mills generate less heat and preserve more delicate aromatic compounds. Hammer mills generate more heat but produce more consistent particle sizes.
Step 3: Malaxation The olive paste is slowly churned in a mixing tank (malaxer) for 20–45 minutes. This step allows the small oil droplets to coalesce into larger droplets that can be separated from the water and solids more efficiently.
Malaxation temperature matters: temperatures above 27°C (80°F) accelerate degradation of polyphenols and volatile aromatics. "Cold-pressed" means the entire process stayed below this temperature threshold, though some degradation occurs at lower temperatures too.
Duration matters: longer malaxation yields more oil but allows more time for enzymatic degradation of the oil's quality.
Step 4: Pressing or Centrifugation The malaxed paste is processed to separate the oil from the solids (pomace) and water.
Traditional press: The paste is spread on fiber discs (叠), stacked, and pressed at high pressure. The liquid runs off, is collected, and then spun in a vertical centrifuge to separate oil from water.
Modern decanter centrifuge: The paste is fed directly into a horizontal centrifuge (decanter) that separates oil, water, and solids in one step. This is the standard method for most commercial producers today.
Decanter centrifuges allow precise control of temperature and processing time, and produce consistent quality without the batch-to-batch variation of traditional pressing. They also allow "two-phase" or "three-phase" processing depending on water use.
The freshly pressed oil is tested chemically before it can be labeled1 :
Extra Virgin Olive Oil (the highest grade):
- Free fatty acidity ≤ 0.8% (as oleic acid)
- Peroxide value ≤ 20 meq/kg
- UV absorbency (K270) ≤ 0.22
- No sensory defects detected by a trained panel
Virgin Olive Oil (lower grade):
- Free fatty acidity 0.8–2%
- Higher peroxide values
- May have minor sensory defects
Lampante Olive Oil (not fit for consumption):
- Free fatty acidity >2%
- Significant sensory defects
- Must be refined before sale as "olive oil"
Refined Olive Oil:
- Chemically neutralized, bleached, and deodorized
- Free fatty acidity ≤ 0.3%
- No flavor, no polyphenols
- Sold as "pure olive oil" or "olive oil"
The majority of the world's olive oil production fails to meet EVOO standards and is refined into commodity oil. Only a portion — varying by harvest year — meets the chemical and sensory standards for genuine extra virgin classification.
Fresh oil is stored in stainless steel tanks under nitrogen gas (to prevent oxidation) at controlled temperature (15–18°C). Under these conditions, quality is maintained for 12–18 months. At higher temperatures or in the presence of oxygen, degradation accelerates dramatically.
Bottling seals the oil from further oxygen exposure, starting the clock on the best-by date. The bottle's journey from tank to table introduces several more degradation risks: light exposure, warm storage temperatures, and prolonged shelf time before use.
Good olive oil is:
- Harvested at the right time (early for quality, late for yield)
- Processed within 24–48 hours of harvest
- Milled carefully with temperature control
- Stored properly in stainless steel
- Bottled and shipped without excessive heat or light exposure
The difference between premium EVOO and commodity oil is not one variable — it is all of them. A single failure point in this chain can degrade the oil enough to fall from EVOO to virgin, or from virgin to lampante requiring refinement.
Once opened, olive oil degrades faster due to ongoing oxygen exposure. Stored in a cool, dark place with the cap tight, quality is maintained for 2–3 months after opening. In warm or bright conditions, the sensory qualities fade within weeks. After opening, use within 2–3 months for optimal quality.
No — color ranges from pale yellow to deep green depending on olive variety, ripeness at harvest, and processing methods. Color is not a quality indicator. A deep green oil and a pale yellow oil can both be excellent EVOO. The IOC sensory evaluation and chemical parameters are the reliable quality markers.1
Yes — light is one of the primary degradation factors for olive oil, alongside heat, oxygen, and time. Premium producers package high-quality EVOO in dark glass or tin to protect the polyphenol content and aromatic compounds from UV-induced oxidation.
1. International Olive Council. "World Olive Oil Production Standards." https://www.internationaloliveoil.org/our-products/olives/
1. USDA FoodData Central. "Oil, Olive, Extra Virgin." https://fdc.nal.usda.gov/fdc-app.html
References
- https://www.internationaloliveoil.org/our-products/olives/
- https://fdc.nal.usda.gov/fdc-app.html