Food processing by pulsed electric fields. I. Physical aspects

High voltage (10-50 kV.cm(-1)) and very short (<10 mu s) electric pulses can be used to pasteurize fluid foods at low or moderate temperatures apparently without significant sensorial quality changes. The main components of a pulsed electric field system are the high voltage-high current switches, and the static or continuous treatment chambers with plate or coaxial parallel electrodes. The respective energy and other characteristics of pulses with constant polarity (exponential decay or square wave) or pulses with reversing polarity are discussed. The few existing industrial or research systems which are commercially available are indicated. The main independent process parameters are: voltage across the capacitors; number of capacitors; interelectrode gap; number and frequency of pulses; food resistivity; food flow rate in the treatment chamber. Variations of these parameters influence the electric field, the width, and the energy of each pulse and the rate of ohmic heating of the food. The usual range of food resistivity is 0.4-100 Ohm.m and depends on temperature and on the heterogeneous/homogeneous structure of the food. The electric field behavior of a food or a biological cell can be modeled by a resistance (conduction of charge carrying molecules) and a capacitor (polarization of dipoles) placed in parallel. Exposed to an intense electric field, foods can undergo dielectric breakdown, in which neutral dielectric constituents or molecules become suddenly conductive. When this phenomenon takes place within a biological membrane, the latter becomes permeable to ions and to current. This may cause cell rupture. Dielectric breakdown may also occur within gases. Air ionization takes place when the electric field exceeds 10 kV.cm(-1) (moist air) or 30 kV.cm(-1) (dry air) and usually produces ozone and hydrogen peroxide. These oxidants, together with localized heating arising from electrical arcing, can cause food damage. Electrochemical reactions at the chamber electrodes may also be detrimental, but are minimal with very short duration or alternating field pulses, and with the use of an appropriate electrode material.