Development of a Continuous Pulsed Electric Field (PEF) Vortex-Flow Chamber for Improved Treatment Homogeneity Based on Hydrodynamic Optimization.

Felix Schottroff,Pauline Eppmann,Cornelia Rauh,Justus Knappert,Tobias Horneber,Anna Krottenthaler,Henry Jaeger,Christopher Mchardy

doi:10.3389/fbioe.2020.00340

Abstract

Pulsed electric fields (PEF) treatment is an effective process for preservation of liquid products in food and biotechnology at reduced temperatures, by causing electroporation. It may contribute to increase retention of heat-labile constituents with similar or enhanced levels of microbial inactivation, compared to thermal processes. However, especially continuous PEF treatments suffer from inhomogeneous treatment conditions. Typically, electric field intensities are highest at the inner wall of the chamber, where the flow velocity of the treated product is lowest. Therefore, inhomogeneities of the electric field within the treatment chamber and associated inhomogeneous temperature fields emerge. For this reason, a specific treatment chamber was designed to obtain more homogeneous flow properties inside the treatment chamber and to reduce local temperature peaks, therefore increasing treatment homogeneity. This was accomplished by a divided inlet into the chamber, consequently generating a swirling flow (vortex). The influence of inlet angles on treatment homogeneity was studied (final values: radial angle α = 61°; axial angle β = 98°), using computational fluid dynamics (CFD). For the final design, the vorticity, i.e., the intensity of the fluid rotation, was the lowest of the investigated values in the first treatment zone (1002.55 1/s), but could be maintained for the longest distance, therefore providing an increased mixing and most homogeneous treatment conditions. The new design was experimentally compared to a conventional co-linear setup, taking into account inactivation efficacy of Microbacterium lacticum as well as retention of heat-sensitive alkaline phosphatase (ALP). Results showed an increase in M. lacticum inactivation (maximum Δlog of 1.8 at pH 7 and 1.1 at pH 4) by the vortex configuration and more homogeneous treatment conditions, as visible by the simulated temperature fields. Therefore, the new setup can contribute to optimize PEF treatment conditions and to further extend PEF applications to currently challenging products.

Highlights

IntroductionFor the continuous Pulsed electric fields (PEF) treatment of liquids, the most commonly used type of treatment chamber is the so-called co-linear electrode configuration, involving ring-shaped electrodes and insulators in an alternating order
Pulsed electric fields (PEF) treatment is a process increasingly used in food and biotechnology, as the resulting electroporation can cause damage to membranes of biological cells, which enables a variety of different applications including gene transfer, enhancement of mass transfer and extraction, as well as non-thermal decontamination with a reduced thermal load (Kotnik et al, 2015).For the continuous PEF treatment of liquids, the most commonly used type of treatment chamber is the so-called co-linear electrode configuration, involving ring-shaped electrodes and insulators in an alternating order
The results show that the general design of the chamber is more important than the impact of the angle α on the overall energy input, as total specific energy input [J/kg] (Wspec) differs less than 1.6% between all configurations of the vortex treatment chamber but about 25%

Summary

Introduction

For the continuous PEF treatment of liquids, the most commonly used type of treatment chamber is the so-called co-linear electrode configuration, involving ring-shaped electrodes and insulators in an alternating order This configuration is characterized by a relatively high electrical resistance, enabling high electric field strength levels necessary for many cell disruption tasks while limiting electrical current flow, energy input and the associated temperature increase As treatments usually operate under laminar flow conditions, this corresponds to the position with the lowest flow velocity, leading to local electric field and temperature peaks, accompanied by possible negative effects on heatsensitive compounds of the product to be treated This is of special relevance for the reduction of the microbial load in bioactive products, like protein or enzyme solutions (Schottroff et al, 2019), as the occurring electric field and temperature inhomogeneities can contribute to a reduced inactivation of microorganisms, and an increased thermal destruction of valuable compounds, respectively. At neutral pH, Abbreviations: A, enzyme activity [U/L]; A0, initial enzyme activity [U/L]; ALP, alkaline phosphatase; c(t), concentration profile [mol/L]; c0, initial concentration [mol/L]; cp, specific heat capacity at constant pressure [J/(kg K)]; cchamber, conversion factor for continuous chambers [1/m]; CFD, computational fluid dynamics; CFU, colony forming units; D, decimal reduction time [min]; DTref , decimal reduction time at reference temperature [min]; E, electric field strength [V/m]; Er(t), residence time distribution function [−]; Ea, activation energy [J/mol]; Eavg , average electric field strength [V/m]; Eq, equation; F(t), cumulated residence time function [−]; f p, pulse repetition rate [Hz]; g, gravitational acceleration [m/s2]; k(T), Arrhenius rate constant function [1/s]; kT , Arrhenius rate constant for temperature T [1/s]; k0, rate constant for 1/T → 0 [1/s]; m , mass flow [kg/s]; N0, initial microbial counts [CFU/mL]; Nt, microbial counts after the treatment [CFU/mL]; p, static pressure [Pa]; pin, inlet pressure [Pa]; pout, outlet pressure [Pa]; p, pressure loss [Pa]; PEF, pulsed electric fields; POM, polyoxymethylene; R, universal gas constant [J/(mol K)]; RA, residual activity [−]; t, time [min]; t, mean residence time [min]; T, temperature [K]; [◦C]; Tref , reference temperature [K]; [◦C]; TSA, tryptic soy agar; TSB, tryptic soy broth; RA, residual activity [−]; rRA, reaction term considering inactivation [−]; U, voltage [V]; u, flow velocity vector [m/s]; uin, mean inlet flow velocity [m/s]; uout, mean outlet flow velocity [m/s]; Wspec, total specific energy input [J/kg]; z, z-value [◦C]; α, inlet angle of vortex chamber; β, inlet angle of vortex chamber; σ, electrical conductivity [S/m]; σv2ar, variance [s2]; λ, thermal conductivity [W/(m K)]; μ, dynamic viscosity [Pa s]; ρ, mass density [kg/L]; τ, stress tensor [Pa]; τp, pulse width [μs]; , electric potential [V]; ω, vorticity vector [1/s]

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