EFFECT OF METAL-CATIONS ON THE CHEMICAL OXIDATION OF OLIVE O-DIPHENOLS IN MODEL SYSTEMS

The catalytic effect of different metal cations on the chemical oxidat ion of caffeic acid and hydroxytyrosol in model systems and at pH 7 an d 30 degrees C was studied. The effect of the metals on the rate of co nsumption of the o-diphenols was in the order Co > Mn > Zn > Fe > Ca, Cu, Mg, Al. When the reactions were carried out in the presence of man ganese, the consumption of o-diphenols and oxygen increased and darker solutions were obtained than in the absence of the cation. An increas e in the rate of oxygen consumption by manganese in solution was also observed. The oxygen uptake after 100 h of reaction was around 2.0-2.5 mol/mol of o-diphenol when manganese was added and 1.5-1.8 mol if the cation was absent. These findings suggest that Mn could be used to ca talyze the oxidation of olive o-diphenols during ripe olive processing .