Oleuropein
Oleuropein is a glycosylated seco-iridoid, a bitter phenolic compound found in green olive skin, flesh, seeds, and leaves. The term oleuropein is derived from the botanical name of the olive tree, Olea europaea.
Because of its bitter taste and astringency, oleuropein must be partially removed or decomposed to make olives edible. During processing of bitter and inedible green olives for consumption as table olives, oleuropein is removed from olives via a number of methods, including by immersion in lye.
Chemical treatment
Oleuropein is a derivative of elenolic acid linked to the orthodiphenol hydroxytyrosol by an ester bond and to a molecule of glucose by a glycosidic bond. When olives are immersed in a lye solution, the alkaline conditions lead to hydrolysis of the ester bond. The basic conditions also significantly increases the solubility of these derivatives, facilitating their release into the lye solution.The high pH accelerates the oxidation of the phenolics, leading to blackness, as during their normal ripening, if the solution is oxygenated by air injection.
The lye solution is replaced several times until the bitter taste has dissipated. An alternative process uses amberlite macroporous resins to trap the oleuropein directly from the solution, reducing waste water while capturing the extracted molecules.
Enzymatic hydrolysis during the maturation of olives is also an important process for the decomposition of oleuropein and elimination of its bitter taste.