Ethyl esters: the test that catches tired fruit
You cannot always taste the difference between an oil made from fresh olives and one made from fruit that sat too long. But the chemistry remembers. Fatty-acid ethyl esters are the fingerprints of fermentation, and a single regulated figure uses them to catch tired fruit — and a sneaky refining trick — that flavour alone might miss.
When olives ferment, they make alcohol — ethanol, the same alcohol as in wine. In a healthy, fast-milled oil there is very little of it about. But let the fruit sit and warm and begin to spoil, and ethanol builds up, then reacts with the oil’s free fatty acids to form a family of compounds called fatty-acid ethyl esters, or FAEEs. They are nearly tasteless in small amounts, which is exactly why they are so useful: they record a problem the palate can easily overlook.
A memory of how the fruit was treated
The logic is simple. Fresh olives, picked clean and milled within hours, barely ferment, so they yield little ethanol and few ethyl esters. Olives that were bruised, heaped, stored damp or left days before pressing ferment more, generate more ethanol, and stamp the oil with a higher FAEE figure. So the number is a chemical memory of fruit quality and mill speed — the same failings that, taken far enough, produce the fusty defect, but detectable by instrument well before your nose would catch them.
The line in the rules
Regulators put a firm ceiling on it. To be sold as extra virgin, an oil must keep its ethyl esters at or below thirty-five milligrams per kilogram — a threshold that has been tightened over the years precisely because it works. An oil above the line may be perfectly edible and even pleasant, but it does not deserve the top grade, because its chemistry shows the fruit was not in top condition. It is one of the modern quality parameters that closed a loophole tasting panels alone had left open.
The trick it also catches
There is a second, sharper reason regulators like this test. A dishonest processor can take a defective oil and gently ‘deodorise’ it — a mild, low-temperature refining that strips out the bad smells without other obvious traces, then sell it as extra virgin. It is illegal, and hard to spot by taste. But soft deodorising leaves the ethyl-ester signature largely in place while removing the smell, so an oil that tastes clean yet carries a high FAEE figure is a red flag. In that sense the test does double duty: it catches tired fruit and it catches a cover-up.
| What they are | Fatty-acid ethyl esters (FAEEs) |
|---|---|
| Made from | Ethanol from fermentation + free fatty acids |
| EVOO limit | ≤35 mg/kg |
| Raised by | Bruised, heaped, damp or long-stored olives |
| Kept low by | Clean fruit milled quickly |
| Also detects | Illegal soft deodorising of defective oil |
| Taste impact | Little at low levels — an instrument test |
What it means for you
- Ethyl esters are a chemical memory of how fresh the olives were and how fast they were milled.
- The extra-virgin limit is 35 mg/kg — above it, the fruit was not in top condition.
- A clean-tasting oil with a high FAEE figure can betray illegal deodorising.
- You cannot test this yourself — favour producers who mill fast and publish their numbers.
Ethyl esters in olive oil: common questions
What are fatty-acid ethyl esters?
Compounds formed when ethanol from fermenting olives reacts with the oil’s free fatty acids. A high level signals that the fruit fermented before milling.
Why do labs measure them?
Because they reveal fruit quality and mill speed. Fresh, quickly milled olives give low ethyl esters; bruised or long-stored fruit gives high ones.
What is the limit for extra virgin?
An extra virgin must keep ethyl esters at or below 35 mg/kg. Above that, the oil cannot carry the top grade, whatever it tastes like.
How do ethyl esters catch fraud?
Illegal soft deodorising removes bad smells but leaves the ethyl-ester signature. So an oil that tastes clean yet has a high figure suggests it was quietly refined.
Do ethyl esters affect the taste?
Very little at the levels involved, which is why the test matters — it detects a problem the palate can easily miss.
Ethyl esters are the number I always liked because they catch two sins at once. A high figure means the olives sat too long or were handled badly before milling — a memory the fruit cannot erase. And it quietly exposes the deodorising dodge, where a defective oil is gently refined to strip the smell and passed off as extra virgin: the taste is cleaned up, but the ethyl esters stay behind to tell the tale. You will never measure this at home, so lean toward producers who mill fast and are happy to show their analyses.
Drawn from the EU ethyl-ester limit for extra virgin olive oil and published quality-parameter studies.