In the mill’s laboratory, at midnight, the technician pours a few grams of oil fresh from the centrifuge into a flask. She adds alcohol and a few drops of indicator, then lets the reagent fall from the burette, drop by drop, until the liquid changes colour. She writes down a figure: 0.12. That figure is the acidity of olive oil, one of the most quoted and least understood numbers in the whole world of olive growing. Many people think an “acidic” oil tastes strong or peppery, or that the “degrees” on a label measure intensity. Neither is true. Here is what that number really tells you, the limits the law sets, and what else to look at to know whether an extra virgin is good.
Contents
What is olive oil acidity?
Acidity is not a taste: it is the trace the olive’s life leaves in the oil. It measures the amount of free fatty acids, expressed as a percentage of oleic acid. An oil with an acidity of 0.2 contains 0.2 grams of free fatty acids per 100 grams.
In a healthy olive still hanging on the tree, those fatty acids travel bound in threes, forming the triglycerides that make up the oil’s fat. But when the fruit is bruised, pecked, falls to the ground or waits too long in a trailer, enzymes start breaking those bonds and setting them loose. The rougher the olive’s journey, the higher the acidity.
That is why this number is, above all, a record of care: low acidity speaks of sound fruit, picked at the right moment and milled within hours; high acidity gives away damaged olives, gathered from the ground or piled up for days. You cannot smell or taste it, although very acidic oils usually carry defects that you can notice in the mouth.
Legal limits on olive oil acidity by category
European rules, set out in Delegated Regulation (EU) 2022/2104, fix the maximum acidity for each category of olive oil:
| Category | Maximum acidity | What it means |
|---|---|---|
| Extra virgin olive oil | 0.8 % | Top quality, no defects on tasting and with fruitiness. |
| Virgin olive oil | 2.0 % | May have slight defects on tasting. |
| Lampante olive oil | Over 2.0 % | Not fit for direct consumption; must be refined. |
| Olive oil (refined and virgin) | 1.0 % | A blend of refined oil with virgin oil. |
| Olive pomace oil | 1.0 % | Obtained from pomace with solvents and refined. |
Good extra virgins stay far below the ceiling: 0.1 or 0.2 are common figures. But acidity is only the way in. An oil can show 0.1 and still not be extra virgin if the tasters find a defect.
The categories of olive oil, explained
On the supermarket shelf every bottle says “olive oil”, yet very different products hide behind those words:
Extra virgin olive oil
Olive juice obtained by mechanical means only, with acidity of 0.8 % or less, no defects on tasting and with fruitiness. It is the only category that keeps the fruit’s aromas and natural compounds intact.
Virgin olive oil
Also unrefined olive juice, but it allows up to 2 % acidity and some slight defects. You will rarely see it in shops.
Olive oil
A blend of refined oil with a share of virgin or extra virgin. Refining erases defects, but also the aromas and much of the polyphenols. Its maximum acidity is 1 %, and that figure says nothing about its flavour.
Olive pomace oil
Extracted with solvents from pomace, the paste left after the oil has been removed, then refined and blended with a little virgin oil. Its natural home is the restaurant deep-fryer.
To see them all together, with their uses in the kitchen, read our guide to the types of olive oil.
How a mill achieves low acidity
A 0.1 does not fall from the sky. It is the sum of many decisions taken in a hurry and in the cold, between October and December:
- Olives from the tree. Fruit that falls to the ground gets bruised, dirty and starts to ferment. Serious mills pick only from the tree and mill it separately.
- Picked at the right point. Sound fruit at the right ripeness, before it has begun to deteriorate.
- Fast milling. Ideally the same day. Every hour the olives wait in the hopper works in the enzymes’ favour.
- Controlled temperature. Malaxation below 27 °C protects the aromas and prevents unwanted reactions.
- Careful storage. Stainless steel tanks, in the dark, at a stable temperature and often under nitrogen to keep oxygen away.
All of this costs money and sleep, but you can tell in the bottle: clean, aromatic oils that stay stable for months.
How acidity is measured in the laboratory
The test that opens this article is as simple as it is precise. A small amount of oil is dissolved in a mixture of alcohol and ether and titrated with potassium hydroxide, using an indicator that changes colour at exactly the right moment. The amount of reagent used gives the percentage of free fatty acids.
Mills repeat this test on every batch as soon as it is made, and official laboratories check it in their controls. It is objective, quick and cheap, which is why for decades it was almost the only criterion for classifying oils.
What olive oil acidity does not tell you
Several legends have grown up around that number. They are worth taking apart one by one:
Acidity has nothing to do with flavour
An oil with low acidity is not necessarily mild, and one with high acidity does not taste stronger. Bitterness and pepperiness come from polyphenols and from the variety. An early-harvest Picual can make you cough and still read 0.1.
The “degrees” are not degrees of alcohol or intensity
For years, labels showed “0.4º” or “1º”, and many people read those degrees as they would on a drink: more degrees, more strength. Not so. That number only showed the acidity percentage.
“Mild” and “intense” were never quality categories
The old “mild” and “intense” oils in Spanish supermarkets were refined olive oils with a little virgin oil added. Their proper name is “olive oil – composed of refined olive oils and virgin olive oils”. Nothing to do with extra virgin.
Low acidity does not guarantee a good oil
It is a necessary condition, not a sufficient one. Other chemical parameters are needed and, above all, the verdict of a tasting panel.
Why olive oil acidity no longer appears on the label
You may have noticed that hardly any extra virgin bottle shows its acidity any more. There is a reason: Implementing Regulation (EU) 29/2012 only allows it to be stated if it comes with the peroxide value, the wax content and the ultraviolet absorbance, in lettering of the same size.
The idea is that nobody judges an oil by a single number. Since fitting all those figures onto a label is a real headache, most producers have chosen to show none.
The figure still exists: mills analyse every batch and many will share the technical sheet on request. If you want to know the acidity of a specific oil in our shop, write to us and we will ask the producer.
Other quality parameters besides olive oil acidity
To earn the name extra virgin, an oil has to pass a whole list of chemical tests. These are the main ones:
- Peroxide value (maximum 20 mEq O₂/kg). Measures primary oxidation: a high value means the oil has suffered air, light or heat.
- Ultraviolet absorbance (K232 maximum 2.50; K270 maximum 0.22). Detects more advanced oxidation and the presence of refined oils.
- Ethyl esters (maximum 35 mg/kg). Reveal olives that fermented because they waited too long.
- Waxes and sterols. Expose blends with pomace oil or other vegetable oils.
Together with acidity, they make up the physico-chemical analysis. But the test that decides the category is not done with flasks: it is done with the nose.
Tasting: the definitive quality test
An oil can only be called extra virgin if an accredited tasting panel confirms that it has no defects and does have fruitiness. This is the panel test method, regulated by the International Olive Council and EU law.
Tasters look for positive attributes such as green or ripe fruitiness, bitterness and pungency, and hunt for defects such as fusty, musty, winey, rancid or earthy. If the median of the defects is zero and the median of fruitiness is above zero, the oil is extra virgin.
That is how two oils with the same acidity end up in different categories: chemistry says whether an oil meets the minimum; tasting says whether it is truly excellent. If you want to try it at home, we show you how to use an olive oil tasting glass.
And the colour of the oil? What it shows and what it does not
Another quick and misleading judgement is the one based on colour. It comes from the olive’s pigments: chlorophylls bring the greens, carotenoids the yellows and golds.
Early-harvest oils, made from green olives, tend to come out greener; those from ripe olives, more golden. Variety matters too, and some give green oils even when picked late.
But colour indicates neither quality nor acidity. A golden oil can be a magnificent extra virgin and a very green one can have defects. Precisely so that colour does not mislead, professional tasters use dark blue glasses that hide it.
Filtered or unfiltered oil
Straight from the centrifuge, oil is cloudy: it carries tiny droplets of water and particles of pulp in suspension. The mill can let it settle slowly or filter it clean.
Unfiltered oil is a temptation in its first weeks: dense to the eye and bursting with fruit. But that moisture and those fruit particles feed fermentation and oxidation, and over time they can bring defects and raise the acidity.
That is why unfiltered oil is best enjoyed within a few weeks or months, and handled with care. For long keeping, a filtered oil is safer. You will find unfiltered oils, such as Isbilya Excellence Koroneiki unfiltered, in the first months of the new harvest.
Olive oil acidity at home: storage and use
Once in your kitchen, acidity barely moves if the oil is stored well. What does slip away over time are its aromas and natural antioxidants, which cannot stand light, heat or air.
Keep it somewhere cool and dark, close it after each use and keep it away from the hob. If you buy large tins or containers, decant a little at a time into a clean oil cruet.
And in the frying pan, relax: heat does not turn a good oil “acidic”. Extra virgin is very stable thanks to its oleic acid and polyphenols, which is why it handles frying well. Just do not let it smoke, and do not reuse it too many times.
One last tip: if an oil smells rancid, vinegary or damp, say goodbye to it, even if its acidity was very low to begin with. Those smells speak of oxidation or fermentation, and no number on the label makes up for them.
How to tell if an olive oil is good
Acidity alone is not enough. These are the clues that really lead you to a good extra virgin:
- Check the category. Look for “extra virgin olive oil”. Plain “olive oil” is a blend with refined oil.
- Look at the origin. The label must state the country or region; protected designations of origin add extra controls.
- Find the harvest or packing date. The more recent, the more aroma it keeps.
- Choose packaging that shields it from light. Dark glass, tin or ceramic beat clear glass.
- Trust your nose and palate. A good extra virgin smells of fresh fruit, grass or tomato leaf. If it smells rancid, musty or vinegary, something has gone wrong.
- Buy from people who know the product. A specialist shop chooses its oils and keeps them in good condition.
To find out how long an oil lasts and how to store it, read our guide on olive oil shelf life. And if you want to understand why they taste so different, start with the olive oil varieties.
In short
Olive oil acidity tells you how the olives were treated, not how the oil tastes. In extra virgin it must be 0.8 % or less, but the real guarantee of quality is tasting.
Extra virgin olive oils selected one by one. We work with mills that look after the olive from the tree to the bottle.
Frequently asked questions about olive oil acidity
What acidity does a good extra virgin olive oil have?
By law, 0.8 % at most. The best usually sit between 0.1 and 0.3 %, although acidity alone does not guarantee quality.
Is an oil with low acidity milder?
No. Acidity cannot be perceived in the mouth. Mildness depends on the variety, the timing of the harvest and the polyphenols.
Does olive oil acidity increase over time?
It can rise slightly if the oil is stored badly, especially if it is unfiltered or exposed to heat. A filtered, well-stored oil keeps it very stable.
Why does extra virgin olive oil feel peppery?
Because of its polyphenols, not its acidity. Pungency and bitterness are positive attributes of a fresh oil rich in these compounds.
Where can I check an oil’s acidity?
On the batch’s technical sheet, which many mills provide. On the label it can only appear together with other quality parameters.
Next time you see a number next to an oil, remember that flask changing colour at midnight: it tells a story of care, but the final judge is still your palate.
