Olives can be darkened by different methods, some of which utilize che
micals 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 t
he tree) as well as artificially. For the latter, air oxidation was co
nducted on both mature and green olives using different alkalis. Moreo
ver, blackening with nigrosine and hematin dyes was applied, since con
sumers prefer the appearance of such olives as well as their low price
. The artificially blackened olives were compared to their naturally b
lackened counterparts regarding colour (subjectively and objectively),
penetration reading, shrinkage percent and heavy metals content (iron
, cadmium and lead). Data showed comparable colour intensity for olive
s blackened with 4% Na2CO3 followed by air oxidation for 120 h to thos
e blackened with nigrosine and hematin. Notwithstanding, treatment wit
h Na2CO3 resulted in the lowest shrinkage percent and heavy metal cont
ents 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 pe
rcent of olives. (C) 1997 Elsevier Science Ltd.