Food processing by pulsed electric fields. I. Physical aspects

High voltage (10-50 kV.cm(-1)) and very short (<10 mu s) electric pulses ca n be used to pasteurize fluid foods at low or moderate temperatures apparen tly without significant sensorial quality changes. The main components of a pulsed electric field system are the high voltage-high current switches, a nd the static or continuous treatment chambers with plate or coaxial parall el electrodes. The respective energy and other characteristics of pulses wi th constant polarity (exponential decay or square wave) or pulses with reve rsing polarity are discussed. The few existing industrial or research syste ms which are commercially available are indicated. The main independent pro cess parameters are: voltage across the capacitors; number of capacitors; i nterelectrode gap; number and frequency of pulses; food resistivity; food f low rate in the treatment chamber. Variations of these parameters influence the electric field, the width, and the energy of each pulse and the rate o f 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 stru cture of the food. The electric field behavior of a food or a biological ce ll can be modeled by a resistance (conduction of charge carrying molecules) and a capacitor (polarization of dipoles) placed in parallel. Exposed to a n intense electric field, foods can undergo dielectric breakdown, in which neutral dielectric constituents or molecules become suddenly conductive. Wh en this phenomenon takes place within a biological membrane, the latter bec omes permeable to ions and to current. This may cause cell rupture. Dielect ric 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 ai r) and usually produces ozone and hydrogen peroxide. These oxidants, togeth er with localized heating arising from electrical arcing, can cause food da mage. Electrochemical reactions at the chamber electrodes may also be detri mental, but are minimal with very short duration or alternating field pulse s, and with the use of an appropriate electrode material.