Olives can reduce their bitterness by oleuropein oxidation via an enzymatic reaction. In this study, olives of the Manzanilla and Hojiblanca varieties preserved in acidified brine were submitted to oxidation tests under overpressure of oxygen (0.3 bars) for 3 days. It was found that the oxidation of fruit preserved for more than 3–4 months showed a reduction in the concentration of oleuropein, but with great variability (28–100%). By contrast, when olives were preserved for only one month, the oxidation treatment was able to decrease more than 90% of the content of oleuropein in the fruit. These oxidised olives were stored again in the acidified medium for 6 months and no spoilage or organoleptic problems were detected. Oxidation gave rise to darker olives (L* parameter diminished from 40 to 34). Moreover, the oxidised fruit was enriched with free phenols such as hydroxytyrosol and tyrosol as a consequence of the acid hydrolysis of phenolic glucosides during the preservation step.
Debittering by oleuropein oxidation.