Influence of iron redox state on black ripe olive processing

The oxidation and polymerization of the phenolic compounds present in black ripe olives give rise to their black shiny colour, which is fixed by the addition of ferrous salts such as ferrous gluconate. Among the many variables that can affect the success of this fixation step, the influence of pH on this process was assessed on fruits of the Hojiblanca cultivar processed at pilot plant scale. Hence the aim of this work was to gain knowledge on the development of this fixation step, in particular on olives that were oxidized with a single alkaline treatment.

It was found that the penetration of the ferric cation into the olive flesh was an extremely slow process and it led to less-dark olives. Besides, at a pH higher than 6 the oxidation of the ferrous to the ferric cation was a very rapid reaction, and olives that were processed with initial pH adjusted to 4 units had a darker colour than those without pH correction. Ultrafiltration/nanofiltration of oxidized olive solutions spiked with ferric ions or even pure oxidized ferric solutions through membranes of 10, 3 and 1 kDa revealed that ferric ions formed high-molecular structures that inhibited the penetration of the ion into the olive flesh and therefore the fixation of the black colour.

The results presented in this study suggest that the formation of ferric ions during the colour fixation step of black ripe olives must be avoided to obtain darker olives. Consequently, the initial pH of the ferrous gluconate solution must be corrected to avoid oxidation of the ferrous to ferric cations during the fixation step. This study will contribute to the improvement of the colour fixation step of black ripe olive processing. © 2018 Society of Chemical Industry.