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 oxidation of caffeic acid and hydroxytyrosol in model systems and at pH 7 and 30 degrees C was studied. The effect of the metals on the rate of consumption 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 manganese, the consumption of o-diphenols and oxygen increased and darker solutions were obtained than in the absence of the cation. An increase 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 catalyze the oxidation of olive o-diphenols during ripe olive processing.