🧪 Oleuropein

Oleuropein is a bitter substance in fresh olives and olive leaves, one of the phenolic compounds. Chemically it is a secoiridoid glycoside: glucose linked to hydroxytyrosol through a special skeleton shared by plants of the olive family. Its formula is C25H32O13. It is also found in other members of the olive family — ash, lilac and privet.

Young green fruit is very rich in oleuropein — up to 140 mg per gram of dry matter, that is, up to 14%; leaves contain 60–90 mg per gram. As the fruit ripens there is less and less of it, and in fully black fruit of the Leccino cultivar it may disappear. Oleuropein also plays a part in the browning of green olives where they are bruised.

It is the reason fresh olives are so bitter that nobody eats them straight from the tree. In the Spanish method, a 1.3–2.6% lye breaks oleuropein down into non-bitter hydroxytyrosol and another compound; for Californian-style black olives, treatment with 1–2% lye is repeated 2–5 times. The Greek method uses no lye: oleuropein slowly passes into the brine and breaks down there, which takes at least 3 months and sometimes more than 6. At home, olives are also soaked in water that is changed every day.

In extra virgin olive oil, oleuropein derivatives give bitterness and pungency. In the EU it is permitted to state that olive oil polyphenols contribute to the protection of blood lipids from oxidation, but only if 20 grams of the oil contain at least 5 mg of hydroxytyrosol and its derivatives, including the oleuropein complex, and if the consumer is told that the effect is obtained with 20 grams of oil a day.

Why it matters to you. Bitterness and a slight tickle in the throat from fresh extra virgin olive oil are not a defect: under the IOC tasting method they are positive attributes linked to phenolic compounds. Raw olives from the tree, on the other hand, are pointless to eat: without lye, brine, water or dry salt they are inedible.