Oleuropein


Oleuropein is a glycosylated seco-iridoid, a type of phenolic bitter compound found in green olive skin, flesh and seeds, leaves, and argan oil. The term oleuropein is derived from the botanical name of the olive tree, Olea europaea.
Because of its bitter taste, oleuropein must be completely 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 consists of a molecule of elenolic acid linked to the orthodiphenol hydroxytyrosol by an ester bond, and to a molecule of glucose by a glycosidic bond. Alkaline conditions favor the elimination, or directly the decomposition, of oleuropein from the tissues of fresh green olives immersed in a lye solution. Two mechanisms occur simultaneously: first, at high pH in a 3 wt. % NaOH solution, most of the phenolic groups present in the oleuropein molecule are deprotonated and present in a dissociated state. The ionized phenolate groups significantly increase the solubility of the molecule in the tissue of the olives. The oleuropein can then more easily diffuse out of the fruits and is released into the lye solution.
Second, under alkaline conditions, the oleuropein molecule is chemically hydrolyzed into hydroxytyrosol and elenolic acid by the breakdown of the ester and glycosidic bonds. At high pH, as phenols and polyphenols, the molecule is sensitive to oxidation and can degrade faster, while olives turn black as during their normal ripening, if the solution is oxygenated by air injection.
The lye solution is replaced several times by a fresh one until the bitter taste has completely disappeared. An alternative process uses an amberlite macroporous resins to trap the oleuropein molecule directly from the solution, giving the advantage to reduce 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.

Green olive blackening

Green olives may be treated industrially with ferrous gluconate to change their color to black. Gluconate, an edible oxidation product of glucose, is used as non-toxic reactant to maintain Fe2+ in solution. When in contact with polyphenols, the ferrous ions form a black complex, giving the final color of the treated olives. Black olives treated with iron gluconate are also depleted in hydroxytyrosol, as iron salts are catalysts for its oxidation.