What Does Acidity Mean in Olive Oil?

The "0.3% acidity" printed on olive oil bottles misleads most people: they assume the oil tastes sour. In fact this figure describes not taste but the chemical state of the oil.

What is free fatty acidity?

Olive oil is made of fat molecules called triglycerides. When the fruit is bruised, left waiting, or goes mouldy, these molecules begin to break down and free fatty acids are released. The figure on the label is the percentage of those free acids, expressed as oleic acid.

So low acidity means: the olives were picked intact, processed quickly, and never given the chance to deteriorate.

The limits

• Extra virgin: max 0.8%
• Virgin: max 2.0%
• After refining: falls below 0.3%, but through processing

Most good extra virgin oils on the market sit between 0.2% and 0.5%.

What raises acidity?

• Olives collected off the ground, or bruised
• A long gap between harvest and pressing (ideally within 24 hours)
• Olive fly damage
• Storage in warm, unventilated conditions

Is acidity enough on its own?

No. Low acidity is necessary but not sufficient. Other measures matter: peroxide value (degree of oxidation), K232/K270 (ultraviolet absorbance) and the sensory panel result.

A refined oil can also be brought down to 0.2% acidity by chemical treatment — but its polyphenols and aroma are long gone. Judging by acidity alone is therefore misleading: look at the grade first (is it extra virgin?), then at the number.

How is acidity measured?

The measurement is made in a laboratory. A sample is dissolved in solvent and the free acids are neutralised with a base solution; the amount consumed gives the acidity percentage. The method is defined by international standards (IOC and Codex Alimentarius) — it cannot be judged by eye or by taste.

There is no way to measure acidity at home. The "acidity meters" sold online do not give reliable results; you have to rely on the label and the producer's analysis report.

Why doesn't it affect taste?

Free fatty acids are themselves tasteless. You would not detect a difference on the palate between an oil at 0.2% and one at 0.7%. What determines flavour is polyphenols, volatile aroma compounds and the level of oxidation.

So the logic of "low acidity means smooth" is mistaken. An early-harvest oil, high in polyphenols, bitter and pungent, usually has very low acidity too — they are separate things.

Other quality indicators

Acidity alone is not enough. An analysis report also carries:

• Peroxide value: shows how far the oil has oxidised. Extra virgin must be at most 20 meq O₂/kg; good oils sit below 10.
• K232 and K268/K270: ultraviolet absorbance values. They reveal whether the oil has been refined and its oxidation history.
• Delta-K: indicates whether blending has taken place.
• Sensory analysis: a trained tasting panel. Even with perfect chemistry, an oil with a panel-detected defect cannot be called extra virgin.

The link with harvest timing

Oil from green olives picked early (October to early November) has low acidity and high polyphenols, but yield is small. In a late harvest (December-January) the fruit ripens, yield rises, but acidity tends to climb and polyphenols fall.

Producers choose between the two: quality or yield. This is a major reason behind price differences.

Can you trust the number on the label?

Acidity reflects the value at bottling. As the oil sits through its shelf life, exposed to light and heat, that value rises. An oil labelled 0.3% that spent six months in a warm warehouse is no longer at 0.3%.

That is why the harvest date is more useful information than the acidity figure.