Effect of pulsed electric field processing parameters on Salmonella enteritidis inactivation

Abstract

Pilot scale continuous pulsed electric field treatment of liquid products was tested on the effects of energy input (0 < Q < 300 kJ kg⁻¹), electric field strength (25 < E < 70 kV cm⁻¹), square wave pulse width (0.05, 0.1, 0.25, 0.5, 1, 2 and 3 μs) and initial product temperature (4 < T_INIT < 20 °C) on Salmonella enteritidis inactivation in a model solution composed of 28 mM sodium sulfate and 28 mM glucose.

For Q = 0–100 kJ kg⁻¹, the decimal reduction number [DRN = log(N₀/N)] can be considered as linearly related to Q with the decimal reduction energy [Q_D] varying between 44 ± 3.2 kJ kg⁻¹ for 0.05 μs, 37 ± 2.5 kJ kg⁻¹ for 0.1 μs and 32 ± 1.4 kJ kg⁻¹ for 0.25–3 μs pulse width. For Q = 0–300 kJ kg⁻¹, the relation between Q and log(N₀/N) was of power law type with the threshold energy level Q₀ = 9 ± 2.6 kJ kg⁻¹ and the power coefficient 3.17 ± 0.21. For Q = 65 kJ kg⁻¹ the increase of T_INIT by 6.6(±0.7) °C raises the DRN by one unit. The same effect increased the products’ electrical resistance by 16(±1.4) Ω. For an overall treatment time of 1 μs, the DRN is linearly related to E, with threshold (E₀) and decimal reduction (E_D) electric field strength: E₀ = 19 ± 1.8 and E_D = 29.7 ± 1.2 kV cm⁻¹, respectively.

Introduction

Interest in non-thermal processes for food preservation has been growing for more than three decades with increasing consumer demand for microbiologically safe and minimally processed food products. Among these processes, pulsed electric field process (PEF) represents one of the more promising technologies for the replacement of traditional thermal pasteurization (Barsotti and Cheftel, 1999, De Jong and Van Heesch, 1998, Heinz et al., 2002, Jeyamkondan et al., 1999, Sale and Hamilton, 1967, Van Loey et al., 2002, Vega-Mercado et al., 1995, Wouters et al., 2001). The PEF process consists of the application of short time (2 μs to 1 ms) high voltage pulses (5–50 kV cm⁻¹) to liquid food placed between two electrodes, in order to inactivate microorganisms by mechanical effects on cell membrane with minimized ohmic heating. Microbial inactivation ranging between 2 and 6 decimal reductions was obtained at non-lethal temperatures for yeasts and many bacteria, according to microorganism type and growth stage (Castro et al., 1993, Grahl and Märkl, 1996, Márquez et al., 1997, Qin et al., 1996, Wouters et al., 2001, Zhang et al., 1994), physical properties of the treatment media (Dunn and Pearlman, 1987, Grahl et al., 1992, Ho et al., 1995, Qin et al., 1995, Vega-Mercado et al., 1997, Zhang et al., 1994), PEF treatment conditions of electric field strength, treatment time, pulse wave shape, temperature (Bliska et al., 2000, Castro et al., 1993, De Jong and Van Heesch, 1998, Grahl and Märkl, 1996, Heinz et al., 2002, Hülsheger et al., 1983, Jayaram et al., 1992, Jeantet et al., 1999, Jeantet et al., 2004, Knorr et al., 1994, Márquez et al., 1997, Peleg, 1995, Qin et al., 1996, Sale and Hamilton, 1967) and treatment chamber design (Qin et al., 1995, Zhang et al., 1995, Lubicki and Jayaram, 1997, Jeyamkondan et al., 1999).

However, the scale up of PEF processing from these results remains often difficult, because of the heterogeneity of the procedures (batch or continuous mode), the incomplete characterization of the equipment and the absence of direct measurement of the key process factors (voltage, current, pulse width, temperature reached after treatment, etc.).

Considering this, a new concept of pulsed continuous electric field equipment has been developed. The spark gap switching technology used is designed to deliver square wave pulses with direct measurement and a great range of pulse duration, frequency and electric field strength (Jeantet et al., 2003, Jeantet et al., 2004). The aim of the present work is to study, with this equipment, the effect of PEF processing parameters (energy input, pulse width, initial product temperature, electrical resistance and electric field strength) on Salmonella enteritidis inactivation in a model solution.