Effect of amino acids on the chemical oxidation of olive o-diphenols in model systems

Addition of certain amino acids (l-tryptophan, l-histidine, l-cysteine and l-cystine) to model solutions of caffeic acid and hydroxytyrosol (the two most characteristic o-diphenols of ripe olives) improved the dark colour obtained after oxidation. A detailed study carried out with l-cysteine and caffeic acid showed that, after an initial inhibitory effect, increasing concentrations of amino acid led to darker solutions. The same effect was also found in hydroxytyrosol solutions with increasing amounts (2–8 mm) of cysteine without lag phase, except for 10 mm level. Cysteine also produced darker colour with oxidised storage brine of ripe olives. These results open the door to a possible use of this amino acid as additive or aid in processing to produce darker and homogeneous ripe olives.