Pulsed Electric Fields In Juice Processing 2241 Introduction

Pulsed electric field (PEF) processing applies high voltage pulses to foods located between a series of electrode pairs. The electrical fields (generally at 20 to 80 kV/cm) are achieved through capacitors that store electrical energy from DC power supplies. During PEF treatment, the applied electric field increases membrane permeability of microbial cells by either forming transmembrane pores (electrical breakdown) or temporarily destabilizing the lipid bilayer and proteins of cell membranes (electroporation), causing inactivation of microorganisms [56-58].

PEF units usually consist of three major parts: the PEF generation unit, treatment chamber, and process control system (Figure 22.3). A PEF can be generated in the form of exponentially decaying, square-wave, bipolar, instant-charge-reversal, or oscillatory pulses, depending on the circuit design of the generating device. The treatment chamber in a PEF unit holds two electrodes in position with insulating materials that form a chamber. Electrochemical reactions can occur at the electrode surfaces, causing partial electrolysis of medium solution, electrode corrosion, and introduction of small particles of electrode material into the liquid medium [59]. Use of very short pulses or bipolar pulses is recommended to avoid the cumulative buildup of charges and thus minimize electrode corrosion.

High voltage pulse generator

High voltage pulse generator

Treatment chamber

Control system FIGURE 22.3 Schematic of a PEF unit.

Treatment chamber

Another major issue for the design of PEF treatment chambers is to provide a relatively uniform electric field. For example, uniform electric fields can be achieved with parallel plate electrodes if the distance between the electrodes is sufficiently smaller than the electrode surface dimension. For operation safety, pressure relief devices are necessary to avoid the destruction, or the explosion, of PEF chambers due to the possibility of the buildup of pressure, which can arise either from the expansion of dissolved air or partial vaporization promoted from local heating within the PEF chamber after a spark [60].

Critical processing factors for PEF include electric field intensity, pulse width, treatment time, temperature, and pulse wave shape. The induced potential difference across the cell membrane of a microorganism is proportional to the applied electric field (electroporation theory). A lethal effect to living cells is observed when the induced potential or transmembrane electric potential exceeds by a large margin a critical value of approximately 1V. Qin et al. [61] stated that microbial inactivation increases with an increase in the electric field intensity above the critical transmembrane potential. Pulse width influences the critical electric field and the efficiency of microbial inactivation [62]. Treatment time, defined as the product of pulse numbers and pulse duration, affects microbial inactivation when either of the two variables changes [63]. The efficiency for microbial inactivation varies with different pulse wave shapes. Oscillatory pulses are the least efficient for microbial inactivation; square wave pulses are more efficient than exponential decaying pulses; and bipolar pulses are more lethal than monopolar pulses [64,65]. Treatment temperatures can change cell membrane fluidity and permeability, thus affecting the susceptibility of cells to mechanical disruption [66]. Targeted microorganisms (type, growth stage, and initial concentration) and properties of the PEF treatment medium (pH, conductivity, and medium ionic strength) also influence the microbial inactivation efficiency. Critical processing factors of PEF treatment need to be monitored and recorded to ensure the microbiological safety of the processed food products while maintaining food quality with acceptable energy efficiency.

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