Pulsed electric field Flashcards
Describe pulsed electric field
- application of pulses of high voltage to foods placed between two electrodes
- Temperature - ambient, slightly above or below ambient, typically the temperature increase is approx 3 degrees/10kV
- duration = less than one second
Describe the static systems equipment
- two stainless steel electrodes
- cylindrical nylon spacer
- electrodes polished to mirror like surfaces
- chamber area: 78 cm ^3
Describe the continuous systems equipment
- two disc shaped electrodes separated by inert spacer
- flow rates achievable: 6 to 1200ml/min
Describe pulse generation:
- DC power supplies a capacitor in series with a charging resistor
- The stored charge is released through the food when a trigger signal is applied
There are 4 types of pulse
1) Exponentially decaying - unidirectional or slow decay
2) Square or rectangular - formed using a pulse-forming network (array of capacitors and inductors) - more lethal than exponentially decaying pulses
3) Bipolar
4) Instant charge reverse pulse
Mechanism of microbial inactivation - Electrical breakdown
- Normal resting potential 10mV
- Application of voltage to microbial cells increases the potential difference leading to reduction in membrane thickness
- Membrane breakdown occurs at the critical membrane potential (Vc). Breakdown causes formation of trans-membrane pores.
- Breakdown is reversible if pores are small.
- Further increases in the electric field or its duration lead to larger pores, membrane breakdown (irreversible) and cell death.
Mechanism of microbial inactivation - Electoporation
- Temporary destabilisation of lipid bilayer and membrane proteins
- Cells become more permeable to small molecules
- Permeation causes swelling and eventual rupture of membrane
- Low doses (5-13kV/cm) used for the introduction of DNA into microbial cells.
What other mechanism of microbial inactivation
- Formation of highly reactive free radicals causing damage to intracellular components
- Inappropriate oxidation and reduction reactions disrupting metabolic pathways and reducing cellular viability
Process factors and microbial inactivation - Electric field intensity
One of the main factors influencing microbial inactivation
- typical field strengths range between 20 and 80 kV/cm
- economic preference for high field strengths
Microbial inactivation increases with an increase in electric field intensity
Process factors and microbial inactivation - Treatment time
- Defined as the product of the number of pulses and the pulse duration
- Pulse width influences microbial inactivation by affecting the critical electric field (Ec)
For pulse widths >50us the critical electric field is approx 5kV/cm
For pulse widths <2 us the critical electric field is approx 40 kV/cm
Longer pulse widths decrease Ec causing greater microbial inactivation - longer pulse widths also casue temperature elevation
Optimisation important to balance the effect of heat versus microbial inactivation
Process factors and microbial inactivation - Pulse wave shape
- Square wave pulses are more effective than exponential decay pulses
- Bipolar pulses are more lethal than monopolar pulses
Bipolar pulses cause microbial inactivation due to additional stresses placed on the microbial cells causing structural fatigue
Bipolar pulses are also more energy efficient and are associated with less deposition of solids on electrodes. - Less cleaning, less cost
Process factors and microbial inactivation - Treatment temperature
The use of moderate temperatures (50-60 degrees C) is synergistic with pulsed electric fields in the inactivation of microbial cells
The use of a constant electric field strength is associated with increased inactivation at higher temperatures
Under the same pulsed electric field - E.coli shows a 1 log reduction at 32 degrees C. E.coli shows a 6.5 log reduction at 55 degrees C
Foods with large electrical conductivities generate
smaller peak electric fields with the PEF chamber making them inappropriate for PEF treatment (studies with L.bevis have shown that if the conductivity of the medium is increased, the pulse width is reduced causing a decrease in the rate of inactivation)
Foods with an increased ionic content will have
increased conductivity and a reduced inactivation rate
What affect does reducing pH from 6.8 to 5.7 have
increases microbial inactivation from 1.45 log reductions to 2.22 log reductions under the same PEF conditions
