Free Fatty Acids: The Number No One Shows You — acidity as a freshness proxy, decoded. A more granular focus on what drives deterioration at each production stage.
The Freshness Number Hidden in Plain Sight
There is one figure that tells you more about an olive oil's condition than almost any other — and it appears on fewer than a quarter of bottles sold in Japan. Free fatty acid content, expressed as a percentage of oleic acid and commonly referred to as acidity, is the most direct chemical measure of whether an oil has been handled well or poorly from grove to shelf. It is not a flavour descriptor. It is not a stylistic choice. It is a structural indicator, and its absence from most labels is not an oversight.
Understanding what drives it — and at which point in production it rises — gives you a more useful framework for evaluating olive oil than any origin story on the back of the bottle.
What Free Fatty Acids Actually Are
In a healthy, intact olive, the oil exists inside the fruit's cells in the form of triglycerides — three fatty acid chains bonded to a glycerol backbone. That structure is stable while the cell walls remain undamaged. The moment an olive is bruised, over-ripened, attacked by fruit fly, or left in a warm pile after picking, cellular membranes begin to break down. Enzymes naturally present in the fruit — primarily lipase — cleave the fatty acid chains from the glycerol backbone, releasing them as free fatty acids.
Free fatty acids are the chemical evidence of cellular damage. The higher the percentage, the more disruption the fruit experienced before or during processing. The EU legal ceiling for extra virgin olive oil classification is 0.8%. Our current batch measures at 0.2% — not because 0.2% is a marketing figure, but because the conditions that produce it are specific, verifiable, and worth understanding.
Where Deterioration Enters at Each Stage
The acidity reading at bottling is the sum of everything that happened to the olive before it became oil. Each production stage carries its own deterioration risk, and each one is controllable.
At harvest, damage begins with contact. Mechanical harvesting equipment that beats or strips the fruit causes bruising. Bruised cells activate the lipase enzymes immediately. Hand-picking, or harvesting with equipment calibrated to minimise impact, reduces this substantially. Harvest timing also matters: overripe olives have softer cell walls that rupture more easily under any method, creating the conditions for rapid enzymatic activity before the fruit even reaches the mill.
After harvest, the clock runs faster than most consumers realise. Olives piled into collection sacks or bins begin to self-heat through the natural respiration of the fruit. Internal temperature rises accelerate enzymatic breakdown. A study published in Food Research International documented measurable acidity increases in olives left unmilled for as little as 24 hours under ambient conditions. The longer the gap between picking and pressing, the higher the starting acidity of the resulting oil — regardless of how carefully the extraction itself is managed. Our extraction window is set at four hours. That constraint exists because no milling technique can undo what the waiting has already done to the fruit.
At extraction, temperature is the remaining variable. Cold-pressing is legally defined as extraction below 27°C, but the legal definition is a ceiling, not a target. Extraction temperatures in the lower range of that window preserve both the phenolic fraction and the structural integrity of the oil more effectively than temperatures approaching the limit. Heat during milling does not directly raise free fatty acids, but it accelerates oxidation of the extracted oil before it is sealed — a separate form of deterioration that compounds the acidity problem if the fruit was already compromised.
After bottling, acidity is essentially fixed. Unlike polyphenols, which continue to degrade with exposure to light, heat, and oxygen, the free fatty acid reading does not rise significantly once the oil is bottled in dark glass and sealed. What you measure at bottling is an accurate reflection of every decision made upstream.
Why the Number Is Rarely Shown
The reason most producers do not disclose acidity is the same reason most producers do not disclose polyphenol count: because the figures are unremarkable. An oil produced from late-harvested fruit, held in bins for two days before milling, will carry an acidity reading that is technically compliant with the extra virgin standard — below 0.8% — but that tells a story the label would rather not tell. Disclosing 0.6% acidity next to a competitor disclosing 0.2% is not a conversation most producers want to invite.
Transparency on this number is not generosity. It is confidence in the process. A 0.2% reading is only achievable when the fruit is handled quickly, harvested at the right moment, and milled without delay. Those conditions are also the conditions that produce high polyphenol density and the clean, sharp flavour characteristic of early-harvest oil. The acidity figure and the taste confirm each other. When the number is low, the oil bites back. When it doesn't, there is usually a reason — and the acidity reading, if disclosed, would explain it.