Pigment changes during preservation of green table olive specialities treated with alkali and without fermentation: Effect of thermal treatments and storage conditions

Abstract

Large proportions of Cu-chlorophyll complexes have been quantified in certain commercial products of table olives with a strikingly bright green colour and labelled “Green olives in soda” and Campo Real-style in Spain, or Castelvetrano-style in Italy. Previous studies have shown that such compounds are not formed naturally during the processing step of these green table olive specialities. The present study aimed to discern whether the formation of Cu-chlorophyll complexes occurs spontaneously during the shelf-life of table olives or is the result of the deliberate addition of Cu (II) salts. The effect of different preservation conditions on the final pigment composition of table olives was investigated. To this end, processed fruits were packed in non-acidified or acidified brine and subjected to sterilization or pasteurization treatments, respectively. Pheophytinization of the chlorophyll compounds was the main reaction that took place during two years of storage and minor complexation of the chlorophyll structure with Cu was also observed. Up to ten different Cu-chlorophyll complexes were detected after 24 months of packing the olives, representing between 1 and 7.5% of the total chlorophyll pigments, depending on the temperature and pH values during storage. In no case was a bright green colour on the olive surface observed, and the values of total chlorophyll pigments were far from the high proportions (up to 99%) that have been quantified in alkali-processed commercial table olives without fermentation. These results demonstrated that the copper complexes were formed as a consequence of the deliberate addition of Cu (II) salts at some stage of the table olives' elaboration or preservation. The quantitative determination of copper chlorophyll complexes in commercial table olive samples with a suspiciously bright green colour is necessary to detect such bad processing practices.