Microbial reduction in flour and spice using pulsed electric fields
This thesis is an investigation of pulsed electric fields (PEF) as a potential means for reducing microbial levels in flour, basil, dill, and onion powder. Electrical parameters investigated using equipment capable of applying 100 kV/cm fields included pulse widths from 1-10 [mu]s, pulse periods from 50 ms to 5 s, pulse number, and reversing polarity and simultaneously applied bipolar pulse waveforms. Pulser development included a stacked metal oxide semiconductor field effect transistor switch circuit using a distributed magnetic structure which survived pulse currents to 136 A and produced pulses with 1.6 A/ns risetime at adjustable frequency to 300 Hz. PEF environmental parameters investigated included 150 and 620 kPag overpressure, humidity, temperature, and altered gaseous atmospheres of dry air, nitrogen, argon, and carbon dioxide, plus multiple treatments at delayed intervals. Changing electrode gap and chamber volume, pulse period, adding pressure, and altering the gaseous atmosphere did not significantly affect microbicidal efficacy. Changing pulse shape and applying successive treatments improved microbicidal effect by less than 0.3 of a log-cycle. Electric field strength and pulse number had individual and strong interactive effects in achieving greater than a one-log-cycle reduction in PEF treated onion powder, yet this is less than reported from thermal, vacuum gassing, or irradiation treatment.