🧪 Oleuropein

Oleuropein is a bitter substance in fresh olives and olive leaves, belonging to the phenolic compounds. Chemically it is a glycoside: a glucose molecule joined to hydroxytyrosol through a special framework shared by plants of the olive family. Its formula is C25H32O13.

Young green fruit is very rich in oleuropein, up to 140 milligrams per gram of dry matter, that is, up to 14 per cent. Leaves contain 60–90 milligrams per gram. As olives ripen and turn black, oleuropein steadily declines; in some cultivars, such as Leccino, it can disappear entirely in fully black fruit.

It is oleuropein that makes fresh olives so bitter that nobody eats them straight from the tree. Lye breaks it down into non-bitter hydroxytyrosol and another compound; this is how Spanish-style and Californian-style olives are made. The Greek method needs no lye: oleuropein slowly passes into the brine and breaks down there.

In extra virgin olive oil, oleuropein derivatives give bitterness and pungency. In the EU an oil may 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 milligrams of hydroxytyrosol and its derivatives, including the oleuropein complex.