When fruit and vegetables are damaged or cut, previously separated cellular contents combine. As a result, various phenolic compounds come to contact with polyphenol oxidase (PPO), and enzyme that in the response so oxygen oxidizes these substances.
The oxidation process catalyzed by PPO is the pivotal step in a series of reactions that lead to the formation of brown pigments.
When fresh blueberries are blended, a rapid surface browning takes place that has been explained by the PPO catalyzed oxidation of chlorogenic acid to chlorogenoquinone and consequent anthocyanin degradation via a coupled oxidation or condensation.
PPO is a membrane-bound enzyme that catalyses the hydroxylation of monophenols to 0-diphenols and their oxidtaion to quinines.
Chlorogenic acid and its isomers are easily degraded by PPO and it seems etherification of the carbonyl group of caffeic acid with quinic acid and tartaric leads to an increase in PPO activity in practically all cases.
Blueberry browning
