Acidulants such as citric acid have also been used extensively as PPO inhibitors. Citric acid exerts a double inhibitory effect on PPO, by lowering the pH (below that necessary for optimal PPO activity) and by chelating copper (Ibrahim, Osman, Sasari, & Abdul Rahman, 2004). Sodium chlorite was demonstrated to inhibit PPO and inactivate E. coli O15:H7 on fresh-cut apples (Luo, Lu, Zhou, & Feng, 2011). However, it caused too extensive tissue damage in fresh-cut lettuce at concentrations that were effective to prevent browning (Luo, unpublished).