Pulsed Electric Fields (PEF) is a process, where high voltage short pulses are applied for a short duration to kill microorganisms. This technology is adopted to destroy the microbes by electroporation effect or non-thermal effect. The short pulses at a microsecond range having a voltage intensity of 10- 80 kV/cm and a treatment time of less than 1 second is generally used in Pulsed Electric Field technology. Voltage requirement varies according to the microbial species and strains. Most of the PEF researches are carried out on liquid food and buffer solution. The inactivation of microbes is mainly due to dielectric effect. Pulsed Electric Field technology is mainly used for low conducting fluid.
1. Microorganism inactivation by PEF (general principle)
Microorganisms in the food get inactivated when a high voltage passes inside them in a pulsed manner. High voltage induces transmembrane potential difference across the cell membrane, which leads to pore formation.
1.1 Electromechanical model:
This model considers membrane as a capacitor with a small value of dielectric constant. The charge gets deposited on both the phospholipid bilayer surface, when electric field is applied. This leads to increase in overall trans-membrane potential and when the overall trans-membrane potential reaches to the critical value, the microbial inactivation occurs.
Assuming the cell to be a spherical structure, the following equation can be used to calculate the induced potential.
V_i=1.5m r E cosθ (1- e^(t/t_r ) )
Source: Knorr et al., (1994)
m is membrane properties constant, r is cell radius, Eo is the strength of applied electric field, θ is angle between cell radius and electric field vectors. t is Eo application duration, tr is the relaxation time. From the above equation it can be observed that higher electric field strength is required for smaller radius cells.
1.2 Another Hypothesis about Cell Inactivation
When successive electric pulses are applied, first pulse leads to formation of reversible hydrophobic pore and the second one leads to stable hydrophilic pore. Below a pore radius of 0.3-0.5 nm, there lies a predominance of hydrophobic pores and above this, with a radius of 0.6-1.0 nm, hydrophilic pores occur. The above situation increases membrane permeability leading to dielectric breakdown of the membrane.
2. Factors affecting Inactivation of Microorganisms by PEF
Microbial inactivation due to Pulsed Electric Field (PEF) is affected by 3 parameters:
Process parameters: Electric field strength, pulse width, treatment chamber design, pulse frequency, pulse wave shape and pulse polarity.
Microbial parameters: Initial colony count of microbes, cell growth phase, cell size, microbes species.
Medium parameters: Temperature of the medium, pH of medium, medium conductivity, presence of particle, Fat content of medium.
2.1 Enzyme inactivation by Pulsed Electric Field (PEF)
When electric field is applied, enzyme gets inactivated, due to attachment and detachment of protein functional groups, charged chain movement and change in alignment of helices. The factors responsible for pulsed electric field processing are electric field strength, pulse frequency, treatment duration, pulse width and specific energy input, processing temperature and enzyme characteristics.
2.2 Effect on Bacterial spores
Bacterial spores are resistive to its vegetative body. Bacterial spores like B. polymixa and B. cereus are resistive up to electric field strength of 30kV/ cm (Barbosa- Canvosa, 1998). Electric field of 30kV/cm for 3 minutes inactivated B. subtilis by 1.3 log cycles. Electric pulses do not induce spore germination. For bacterial inactivation, first spore is treated with lysozyme to dissolve its outer covering. Subsequently, PEF is applied to inactivate the germinated spore. (Barbosa- Canvosa, 1998)
Pulsed Electric Field had a significant effect on the processing of juices. PEF inactivates microbes and enzyme with minimal nutritional and organoleptic properties changes as compared to conventional thermal technologies. The major obstacle for industrial scale up of PEF processing is its high initial cost. Therefore, the challenge lies to design a pulse generator and continuous treatment chamber for industrial requirement. Innovative developments in high-voltage pulse technology can reduce the cost of the pulse generator and make PEF processing competitive with thermal-processing methods.
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About the Author:
Assistant Professor, Department of Agricultural Engineering,
Institute of Agricultural Sciences, SOADU.
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