Browning reactions in olives: Mechanism and polyphenols involved

The mechanism of the browning reaction in olives has been disclosed by following the polyphenol changes in healthy and bruised fruits and using “in vitro” models. It was later validated in independent experiments with active and denatured enzymatic extracts, treated or not with ascorbic acid to prevent oxidation. The proposed mechanism would consist of two steps. First, there is an enzymatic release of hydroxytyrosol, due to the action of the fruits’ β-glucosidases and esterases on oleuropein and hydroxytyrosol glucoside; additional hydroxytyrosol can also be produced (in a markedly lower proportion) by the chemical hydrolysis of oleuropein. In a second phase, hydroxytyrosol and verbascoside are oxidized by the fruits’ polyphenoloxidase (mainly) and by a chemical reaction, which occurs to a limited extent due to the olive flesh pH, ≈5.0.