**4.3 Pulsed electric field (PEF)**

294 Salmonella – A Dangerous Foodborne Pathogen

complete loss of colony forming activity was observed for the treated cells with a formation of veins and small vesicles on the surface. TEM images showed the inner areas to be highly disrupted accompanied by a membrane deformation. In addition, shrinking and uneven dispersion of cytoplasmic materials was also observed (Figure 1). Liao et al. (2010) obtained a remarkable reduction of 5 logs for *S.* Typhimurium when carrot juice was subjected to DPCD treatment. Both temperature and pressure had a noticeable effect as the inactivation was enhanced with increasing pressure at a constant temperature or increasing temperature at a constant pressure. In contrast, inactivation of *S*. Typhimurium in PS or PS containing 10% brain–heart infusion (PS-BHI) broth was completed in 35 min in PS whereas it took 140 min in the case of PS-BHI (Erkmen, 2000). Besides, the previous study reported the presence of two phases during the destruction characterized by a slow rate of reduction in the cell number which increased sharply at the later stage. Erkmen and Karaman (2001) observed that the exposure time required to achieve the same level of *Salmonella* inactivation was drastically reduced as the pressure during the inactivation increased. Complete inactivation of *Salmonella* was reported in egg yolk, 94-98% in chicken meat strips and limited inactivation in whole egg at a pressure of 13.7 MPa at 35 °C for 2 h (Wei et al., 1991). The variation in the results clearly indicates the complex nature of food systems. A treatment of 14 MPa at 45 °C for 40 min resulted in a 34.48% and 32.74% reduction for *S.* Typhimurium in soy sauce and hot-pepper paste marinated pork products, respectively (Choi et al., 2009a). However, the technique is more suitable for liquid foods as the diffusion of CO2 into solid samples becomes a limitation due to the absence of agitation in solid foods. Also, high concentrations of CO2 can cause darkening of color of certain animal products due to the formation of metmyoglobin. Due to the complex nature of foods conflicting results are available on the effect of HPCD on sensory, chemical and physical properties of foods. In spite of the potential advantages of HPCD more research is needed to monitor and quantify sensory and chemical characteristics of foods undergoing this preservation technique.

Fig. 1. Scanning electron micrograph (upper; magnification: 20,000) and Transmission electron micrograph (lower; magnification: 50,000) images of *S*. Typhimurium cells (left: untreated; right: treated) upon application of super critical carbon dioxide at 35°C and 100

bar for 30 min (Kim et al., 2007)

Pulsed electric field (PEF) is another non-thermal technology that can be used to inactivate bacterial cells at ambient temperatures. The process involves placing the food material between two electrodes and passing pulses of high electric field (1-50 kV/cm) strengths. Since the pulses are applied for short durations (2μs to 1 ms) the negative impact on food quality due to heat processing is highly diminished (Barbosa-Cánovas et al., 2001). The technique is more suitable for liquid or semi-liquid foods which can be easily pumped. It can be used to increase the shelf life of soups, milk, whole liquid eggs and fruit juices. PEF as a non-thermal preservation method has been implemented by Genesis Juices, Oregon, USA. The application of electric field results in cellular death due to generation of pores (electroporation) in the bacterial cell membrane without having an effect on enzymes or proteins present in foods (Wouters et al., 2001). The effectiveness of the technique will strongly depend on the treatment time, electric field strength and specific energy of the pulses. For instance, Monfort et al., (2010) achieved an inactivation of 4 log for *Salmonella* Typhimurium when 45 kV/cm of electric field was applied for 30 μs. Higher number of pulses and electric field was reported to be a stronger factor for reducing the number of *S.*  Typhimurium population in orange juice (Liang et al., 2002) whereas in another study on melon and water melon juices, treatment time was found be a more important factor (Mosqueda-Melgar et al., 2007). Treatment of watermelon and melon juice with PEF resulted in a reduction of 4.27 log (at 2000 μs and 100 Hz) and 3.75 log (at 1250 μs and 175 Hz) of *S.* Enteritidis, respectively (Mosqueda-Melgar et al., 2007). In contrast, Liang et al. (2002) reported a 5 log reduction of *S.* Typhimurium in orange juice exposed to a PEF of 90 kV/cm at a temperature of 55 °C. However, the higher reduction could be a result of combination of higher acidity of orange juice in addition to relatively higher temperature and high intensity of the PEF applied. Although the technique is useful, inactivation has only been achieved in the range of 3-4 logs.
