Citric acid has been reported extensively for its inhibitory
activity on PPO and its antibrowning activity in minimally
processed fruits and vegetables (Ahvenaien, 1996).
Citric acid exerts a double inhibitory effect, by lowering
the pH (below that necessary for optimal PPO activity) and by chelating copper (Ibrahim, Osman, Saari, & Abdul-Rahman, 2004). Citric acid was found to be a non-competitive
inhibitor to lettuce PPO.