Effect of pH Variation by Electrodialysis on the Inhibition of Enzymatic Browning in Cloudy Apple Juice

The pH of cloudy apple juice was reduced temporarily and then returned to its initial value using electrodialysis (ED) technology by water splitting with a bipolar membrane. The juice was treated in a small scale unit at temperatures between 16 and 24 °C and a constant current density of 40 mA/cm2. The pH of the juice was reduced from 3.5 to 2.0 in 30 min. However, this level of acidification required the addition of 12.3 mM K+ (KCl) every 5 min during the treatment. Exogenous K+ compensated for the K+ loss from the juice and maintained the electric charge at neutrality, which favored the accumulation of H3O+ in the juice. The reduction of juice pH to 2.0 resulted in the complete inhibition of polyphenol oxidase (PPO) activity compared with the control. Following acidification, the pH of the juice was returned to its initial value in 60 min by introducing OH- produced by water splitting. The pH adjustment of the juice slightly reactivated the PPO, but browning inhibition was complete and irreversible. The treatment enhanced the color of cloudy apple juice during storage without modifying the flavor or sugar content. However, the ED treatment slightly reduced the malic acid content and substantially reduced the mineral contents. Keywords: Electrodialysis; bipolar membranes; apple juice; enzymatic browning; polyphenol oxidase