Effect of pulsed electric field treatment on polyphenol oxidase, total phenolic compounds, and microbial growth of apple juice

A pulsed electric field (PEF) process was applied to unclarified apple juices at 30 and 40 kV cm^-1 for 50, 100, 150, and 200 pulses after preheating to 40 °C. Afterwards, polyphenol oxidase (PPO) enzyme levels, loss of total phenolic (TP) compounds, and alterations of the physiochemical parameters of the apple juice were determined. Total yeast/mold and mesophilic bacteria loads were periodically evaluated during storage at 25 °C for a 3-month period and the cloud quality of the apple juice was analyzed after storage. The results of the present study have indicated that total yeast/mold and mesophilic bacterial activity could be completely inactivated under all PEF processing conditions. Moreover, no microbial activity occurred in PEF-treated apple juices during the 3-month storage period, whereas the control had more than a 6 log cfu mL^-1 microbial load at the end of storage. PPO activity became completely inactive at 100 or more pulses at 40 kV cm^-1, while residual activity decreased by up to 32.1%, 7.0%, 2.9%, and 1.6% at 30 kV cm^-1 at 50, 100, 150, and 200 pulses, respectively. Kinetic data for PPO activity and TP content were described using a first-order kinetics model with a high R² value, ranging from 0.906 to 0.983. However, there was a negative correlation ranging from -0.907 to -0.938 between TP compounds and PPO inactivation. In addition, only minor changes were observed in physical appearance and color, pH, conductivity, and Brix levels of apple juices after the PEF processing.