This page explains oleuropein — the polyphenol compound that dominates in the olive fruit and is the primary source of bitterness in olive oil. If you arrived here to understand why olive oil can be bitter and what that tells you about quality, this is the right place.
What is oleuropein?
Oleuropein is a naturally occurring polyphenol compound belonging to the secoiridoid glycosides. It is produced in the olive tree and occurs in high concentrations in the leaves, bark, and — above all — in the unripe olive fruit.
The compound serves a biological function in the tree: it is part of the plant's and fruit's chemical defence system against pests and micro-organisms. The intense bitterness of fresh, unprocessed olives is largely due to oleuropein.
Oleuropein in the olive fruit
In an unripe, green olive, oleuropein can account for a significant proportion of the fruit's dry weight. The content is highest early in the season — in unripe, green olives — and falls progressively as the fruit ripens and the compound's glycoside bond is hydrolysed by the fruit's own enzymes.
This is one of the reasons harvest timing is decisive for an olive oil's polyphenol composition: early-harvested olives contain more oleuropein and its derivatives; late-harvested olives considerably less.
What happens during pressing?
Intact oleuropein does not transfer efficiently into oil during pressing — it is a glycoside, meaning it has a sugar group attached to the molecule, and glycosides dissolve more readily in water than in fat.
During pressing, enzymes naturally present in the fruit flesh — primarily beta-glucosidase — are activated. These cleave the sugar group from oleuropein, releasing the oleuropein aglycone and further transformation products including hydroxytyrosol and tyrosol. These simpler polyphenol compounds dissolve in fat and carry through into the oil.
In the finished product, it is therefore not oleuropein in its original form but the oleuropein complex — a mixture of aglycones and derivatives — that is the analytically relevant category. These derivatives are what EU Regulation 432/2012 refers to as "hydroxytyrosol and its derivatives (e.g. oleuropein complex and tyrosol)" in the conditions for the authorised health claim on blood lipids and oxidative stress.
Bitterness as a quality marker
Bitterness in olive oil is a direct indicator of the presence of oleuropein derivatives. An oil without bitterness has either been made from late-harvested olives, has undergone a process that eliminated the polyphenols, or comes from a cultivar with naturally lower oleuropein content.
In professional sensory evaluation of extra virgin olive oil, bitterness is assessed positively. The IOC's sensory standard defines "amaro" (bitterness) as a positive sensory attribute in extra virgin olive oil — not a defect.
The combination of bitterness (oleuropein derivatives) and the peppery burning sensation in the throat (oleocanthal) are the sensory markers of an oil with high polyphenol content.
Oleuropein in relation to Vala Selection's oil
Our total polyphenol content for reference batch 2025-VL-MIR-001 is 1,004 mg/kg, measured by HPLC at the University of Split. Oleuropein derivatives are included in this total — they are one of the dominant compound groups in oil from the Lastovka cultivar at early harvest.
We do not publish individual compound values separately. The reason is that individual values are method-sensitive and can be misread without methodological context. The total value is traceable to a specified batch, a specified laboratory, and a specified analytical method.
What this does not mean
Oleuropein and its derivatives are food chemistry compounds in a food product — not pharmaceuticals or dietary supplements.
The only EFSA-authorised claim relevant to these compounds is: "Olive oil polyphenols contribute to the protection of blood lipids from oxidative stress." It is strictly conditional on the oil providing sufficient hydroxytyrosol and derivatives under the EU framework. See /fact/polyphenols-and-oxidative-stress/ for a full account.
Bitterness in olive oil is a sensory and analytical measure of polyphenol content. It does not imply that the oil has medicinal properties, and it does not mean a more bitter oil is automatically superior in every respect — cultivar, origin, and handling determine the full composition.
Further reading
On oleocanthal, the compound responsible for the peppery sensation: /fact/oleocanthal/. On polyphenols as a group: /fact/polyphenols/. On the EFSA claim and the role of oleuropein derivatives in it: /fact/polyphenols-and-oxidative-stress/. Product data for the current batch are on the product page.
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Sources: EU Regulation 432/2012 on authorised health claims. IOC Trade Standard COI/T.15/NC No 3/Rev. 16 (sensory assessment of extra virgin olive oil). HPLC analysis performed at the University of Split. Batch 2025-VL-MIR-001.