Oleuropein
Oleuropein is a glycosylated seco-iridoid, a type of phenolic bitter compound found in green olive skin, flesh, seeds, and 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 (~ 13.9) in a 3 wt. % NaOH solution, most of the phenolic groups (pK ≈ 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 is also called the "California process"). The lye solution is replaced several times until the bitter taste has completely disappeared. An alternative process uses amberlite macroporous resins to trap the  oleuropein molecule 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. Green olive blackening. Green olives may be treated industrially with ferrous gluconate (0.4 wt. %) to change their color to black. Gluconate, an edible oxidation product of glucose, is used as non-toxic reactant to maintain Fe 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(II ) gluconate are also depleted in hydroxytyrosol, as iron salts are catalysts for its oxidation. Research. Oleuropein has been proposed as a proteasome activator.