Impact of high-intensity pulsed electric field variables affecting peroxidase and lipoxygenase activities of watermelon juice

The effects of pulse frequency (50-250 Hz), pulse width (1.0-7.0 μs) and polarity (monopolar or bipolar) of high-intensity pulsed electric field (HIPEF) treatments (35 kV/cm and 1000 μs total treatment time) on peroxidase (POD) and lipoxygenase (LOX) activities were evaluated using a response surface methodology. Second-order expressions were accurate enough to fit experimental results. HIPEF bipolar treatments resulted to be more effective than monopolar treatments in reducing POD and LOX activities. Watermelon juice LOX was more resistant to HIPEF than POD within the range of assayed conditions. HIPEF treatments set at 50 Hz for 1.0 μs pulse width could attain minimum residual POD activity values up to 10%, whereas the highest POD activity reduction was reached by combining high frequencies and pulse widths. Thus, POD could be totally inactivated by applying 7.0-μs bipolar pulses at frequencies higher than 114 Hz. In addition, the effect of frequency on the LOX activity was highly affected by the pulse width of the treatment. Thus, treatments conducted at 220 Hz with bipolar pulses of 1.0 μs led to the lowest residual LOX activity (50%).