With insufficient amounts, however, it also can be destroyed with prolonged exposure to air, light, high pH levels, and heat. In some foods, the acid may also lower pH enough to interfere with the browning enzyme. Sodium bisulfite, and other sulfites, work to block the enzymatic browning reactions by combining with highly reactive o-quinones to produce less reactive diphenols. It also works as a chelating agent to complex with copper in PPO and inhibit browning by preventing participation in browning reactions. The problem with citric acid is that it may cause off-flavors in some fruits and vegetables.
Chloride ions retard the browning reaction in such fruits as apples but is only temporary.
The concentrations needed for permanent inhibitions would make the fruit inedible. Sugars, syrups and gums may serve to inhibit browning by coating fruits and impenetrable by oxygen. Dipping in water may also inhibit browning temporarily this way. Chilling fruits or vegetables below 40C, the optimum temperature for enzymatic activity, will retard browning. But freezing won't permanently inhibit browning and once the product returns to around 40C, browning rate will increase as freezing breaks cell structure and exposes more surfaces to air.