Quality attributes of black olives as affected by different darkening methods

Olives can be darkened by different methods, some of which utilize chemicals and even dyes in some locations. The present study was carried out to evaluate the different darkening methods in terms of acceptance and safety of blackened olives. Olives were blackened naturally (on the tree) as well as artificially. For the latter, air oxidation was conducted on both mature and green olives using different alkalis. Moreover, blackening with nigrosine and hematin dyes was applied, since consumers prefer the appearance of such olives as well as their low price. The artificially blackened olives were compared to their naturally blackened counterparts regarding colour (subjectively and objectively), penetration reading, shrinkage percent and heavy metals content (iron, cadmium and lead). Data showed comparable colour intensity for olives blackened with 4% Na2CO3 followed by air oxidation for 120 h to those blackened with nigrosine and hematin. Notwithstanding, treatment with Na2CO3 resulted in the lowest shrinkage percent and heavy metal contents of olives, compared to those treated with other alkalis or dyes. Olives treated with Na2CO3 most closely resembled those which had been naturally blackened. Immersing Na2CO3-treated olives in sunflower oil for 1 s prior to air-oxidation significantly lowered the shrinkage percent of olives.