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.
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 (~ 13.9) in a 3 wt. % NaOH solution, most of the phenolic groups (pKa ≈ 10) 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 (alkaline oxidation of olives also called the California process).
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.
^ abJohnson, Rebecca; Mitchell, Alyson E. (2019). "Use of Amberlite macroporous resins to reduce bitterness in whole olives for improved processing sustainability". Journal of Agricultural and Food Chemistry. 67 (5): 1546–1553. doi:10.1021/acs.jafc.8b06014. ISSN0021-8561.