CATALYTIC EFFECT OF METAL-CATIONS ON THE DARKENING REACTION IN RIPE OLIVE PROCESSING

The darkening step in ripe olive processing can be accelerated by usin g different metal cations, particularly cobalt, manganese and iron, in the aeration liquid. It was found that manganese and iron ions increa sed the rate of the darkening reaction as their content in the liquid increased from zero to 9 mM. Manganese cations only had a catalytic ef fect on the darkening reaction, while iron ions also fixed the dark co lour formed when olives were bottled and sterilized. However, the rate of oxidation of the orthodiphenols in the olive flesh (hydroxytyrosol and caffeic acid) was slightly modified when manganese or iron was ad ded to the aeration liquid. The better dark surface colour obtained wh en manganese or iron ions were added to the aeration liquid remained a fter bottling and sterilization of olives in a ferrous gluconate solut ion. These results suggest that manganese could be used to catalyse th e darkening reaction since this cation is generally recognized as safe .