Every bottle of Merkado carries a technical sheet with four figures on it. Here is what each one actually measures, why we pay an accredited laboratory in Beja to check them, and how to use them when you are choosing between our three harvests.
Most olive oil reaches a shelf accompanied by a best-before date and a country of origin. That is the whole disclosure. The oil might be six hours old or two years old; it might have been pressed from fruit that was perfect or fruit that sat in a sack for a week in the sun. You cannot tell from the bottle, and by design you are not meant to be able to tell.
We think that is a poor way to sell something people put on their food, so we do the opposite. Our figures come from ACOS — the Associação de Agricultores do Sul, an accredited laboratory in Beja — tested against Commission Implementing Regulation (EU) 2022/2105 and the International Olive Council methods. Every value below is theirs, not ours.
Four numbers, four different questions about the oil. Read together, they tell you almost everything about how the harvest was handled.
1. Acidity — how healthy the fruit was when it was crushed
Acidity, more precisely the free fatty acid content, is the percentage of oleic acid that has broken loose from the fat molecules holding it. In an intact, healthy olive it stays locked up. When fruit is bruised, overripe, or simply left sitting too long between the branch and the mill, enzymes begin dismantling it — and acidity measures how far that dismantling had already gone on the day we pressed.
It is, in other words, a freshness measurement of the raw material rather than a property of the finished oil. Low acidity means the fruit arrived in good condition. High acidity means something in the chain was slow or careless.
The legal ceiling for extra virgin is 0.8%. Our three harvests sit at 0.20%, 0.28% and 0.31% — between a quarter and a half of the permitted maximum.
Worth correcting
Acidity is not something you can taste. You will hear it constantly — “this oil is very acidic”, usually said about an oil that is peppery. Free acidity at these concentrations is imperceptible to a human palate. What people are tasting is pungency from a compound called oleocanthal, or bitterness from polyphenols. Neither of those is acidity, and neither is measured by this test.
Acidity is a laboratory reading of how the fruit was treated, not a description of flavour. It has earned a reputation on restaurant floors that the chemistry does not support.
2. Peroxide value — how much oxygen has got in
Where acidity is about the fruit, peroxide value is about the oil. Peroxides are the first compounds that form when oil meets oxygen, and the test measures them in milliequivalents of active oxygen per kilogram of oil. The lower the figure, the less oxidation has taken place.
Extra virgin permits up to 20 meq O2/kg. Ours run between 9.6 and 12.4.
There is an honest caveat here that most producers leave out. Peroxide value is a snapshot, not a permanent condition. Peroxides are chemically unstable: they degrade into secondary compounds — the ones responsible for the waxy, stale, old-nut aromas of an exhausted oil — and as they do, the peroxide figure falls again. An oil can therefore show a modest peroxide value for two quite different reasons: because it is genuinely fresh, or because it has oxidised so far that it has moved on to the next stage.
That is precisely why the official report includes three additional ultraviolet tests alongside it. On our most recent analysis, K232 came in at 2.11 against a limit of 2.5, K270 at 0.13 against 0.22, and delta-K270 at 0.01 or under against a limit of 0.01. Those catch the secondary oxidation that peroxide value alone can miss.
The practical lesson for a buyer is that no number on any sheet protects you from what happens after the bottle leaves the producer. Peroxide value is the figure that responds fastest to heat, light and time. Store the oil dark, closed and cool, and use it within a year of harvest — and know that the figure on the sheet describes the day of testing, not the day you open it.
3. Fruity median — the only one that is a matter of taste
The first two are chemistry. This one is a panel of trained tasters working to the International Olive Council sensory method, scoring fruitiness blind and reporting the median. It is the single most misunderstood figure on a technical sheet, so it is worth being precise about what it does and does not mean.
For an oil to be classified extra virgin at all, two sensory conditions must both be met: the median for defects must be zero, and the median for fruitiness must be above zero. A pleasant but flavourless oil fails, no matter how clean its chemistry. On our latest report the defect median is 0.0 and the fruity median is 4.2 — legally extra virgin on both counts. Across the three harvests the fruity median reads 2.6, 2.7 and 4.2.
Now the part that matters commercially, because it is where buyers get led astray. The fruity median is not a score out of ten for quality. A 2.6 is not a worse oil than a 4.2. It is a different oil, picked at a different moment in the season. Fruitiness rises as the fruit ripens, so the figure is best read as a marker of ripeness and aromatic intensity, not of merit.
Why our peppery oil scores a moderate 2.6
Our Early Reserve is described as sharp and intensely peppery, and yet its fruity median is 2.6 — lower than our late-harvest oil at 4.2. This is not a contradiction. Perceived pungency comes from oleocanthal, which is a separate compound detected by a different sensation altogether. The panel is scoring fruitiness, not heat. An oil can be fiercely peppery and only moderately fruity, or gently fruity and completely mild. Those are two independent measurements.
4. FAEE — the fermentation number, and the one we care about most
FAEE stands for fatty acid ethyl esters, and it is the figure that measures fermentation. The mechanism is worth understanding, because it explains the single most common way good olives become mediocre oil.
An olive that is heaped in a container, and especially one sealed inside plastic, exhausts the oxygen around it within hours. Anaerobic fermentation begins. Alcohols form inside the fruit, then bind with free fatty acids to produce ethyl esters — and those esters stay in the oil. The EU set a limit of 35 mg/kg for extra virgin, and it is one of the sharpest tools a buyer has, because FAEE is the fingerprint of the hours between picking and pressing. You cannot test your way out of a week-old olive.
Our three harvests read 5, 6 and 8 mg/kg.
This is also the number that quietly justifies two of our operating decisions. We do not use sealed plastic bags to collect fruit, because they suffocate it; we use ventilated crates so the olives keep breathing and stay cool from branch to mill. And we mill inside a strict 24-to-48-hour window, separating ground-fall fruit from tree fruit so the two never mix. Those choices cost us time and money every single day of harvest. FAEE is the laboratory confirming they are doing what we claim.
Reading all four together
Here are our three harvests side by side, exactly as they appear on the technical sheets.
Profile
Acidity
Peroxide
Fruity median
FAEE
The Farmer's Early Reserve
0.20%
10.4
2.6
6 mg/kg
The Miller's Mid Harvest
0.28%
9.6
2.7
5 mg/kg
The Late Lazy Picker Harvest
0.31%
12.4
4.2
8 mg/kg
Peroxide in meq O2/kg. Fruity median scored by a trained panel to IOC method COI/T.20/Doc no. 15.
Read down the fruity column and you can watch the season turn. From the Early Reserve to the Late Lazy Picker, fruitiness climbs from 2.6 to 4.2 as the fruit ripens and its aromatics open up — that is the whole reason we pick at three separate moments rather than one.
Look across the other columns and you can see what ripeness costs. Acidity drifts up from 0.20% to 0.31%, and peroxide from 10.4 to 12.4, because softer, riper fruit is marginally more vulnerable to both. The trade-off is real, and it is the reason a single “best” harvest does not exist. There is only the trade-off that suits what you intend to cook.
What matters most is the gap between our figures and the legal limits, which is substantial on every line.
Parameter
Extra virgin limit
Our range
Free acidity
0.8% maximum
0.20 – 0.31%
Peroxide value
20 meq O2/kg maximum
9.6 – 12.4
Defect median
0 required
0.0
Fruity median
Above 0 required
2.6 – 4.2
FAEE (fermentation)
35 mg/kg maximum
5 – 8 mg/kg
Limits per Commission Delegated Regulation (EU) 2022/2104 and Implementing Regulation (EU) 2022/2105.
What these sheets do not tell you
Four numbers are a floor, not a ceiling, and we would rather say so than let the sheet imply more than it can carry.
– Polyphenols and oleocanthal are not on it. These drive both the health reputation of extra virgin and the peppery catch at the back of your throat, and they are not part of the compulsory panel. A clean sheet does not by itself tell you an oil is rich in them.
– It is a test, not a certificate of the future. Every figure describes one sample on one day. Storage, light, heat and time all move peroxide value after the laboratory has signed.
– It says nothing about your palate. No number will tell you whether you prefer the Early Reserve on grilled meat or the Late pick over ice cream. Only tasting does that.
One more detail worth knowing, because it is a good test of any producer you buy from: these results carry a reference number and a date. Ours is ACOS reference 94224, issued 2 July 2026. If a label quotes remarkable figures with no laboratory, no method and no date attached, those figures are marketing rather than measurement.
How to read any testing sheet you are handed
A short checklist that works on ours and on anyone else’s.
– Is the laboratory named, accredited and the report dated? A sheet without a reference number is a brochure.
– Check the five gates for extra virgin: acidity at or below 0.8%, peroxide at or below 20, defect median of zero, fruity median above zero, FAEE at or below 35 mg/kg.
– Treat the fruity median as a style, not a grade. Higher means riper and more aromatic, not better. Match it to what you are cooking.
– Read peroxide value together with the harvest date. A low figure on old oil deserves a question.
– Use FAEE to judge the producer, not the bottle. It is the clearest evidence of how carefully fruit was handled before pressing.
The short version
Acidity — how healthy the fruit was when it was crushed. Lower is fresher. It is not a taste.
Peroxide value — how much oxygen has already got in. A snapshot of the day it was tested.
Fruity median — the only sensory figure, scored by a trained panel. A style marker, not a score out of ten.
FAEE — fermentation. The fingerprint of the hours between tree and mill, and the number that shows whether a producer was actually careful.
Taste them side by side
Numbers narrow the field; they cannot choose for you. The three harvests we mill each year are the same Galega olives, from the same valley in Colos, picked at three points in the season — which makes them an unusually clean way to learn what ripeness does to an oil.
Ask us for the technical sheets with any order, or come and taste all three at the shop in Colos and decide with your own palate. We are happy to talk through any of it, including the parts where the chemistry says something different to the folklore.
— Merkado Olive, Vale da Rosa, Colos, Alentejo
