PULSED ELECTRIC FIELD (PEF) TREATMENT: GENERAL CONCEPT
The PEF is a mild-technology. The food placed between two electrodes is subjected to short high voltage pulses to inactivate both spoilage and pathogen bacteria. The PEF technology has as its fundamental principle the application of short pulses (1-100 ps) of high electric fields in the order of 10-80 kV/cm (Figure 1.6). The main parameter of PEF treatment is the electric field strength (kV/cm). It is defined as a relationship between the electrical potential differences applied to the two electrodes (V) and their distance (cm). The pulses may be applied in two different forms: exponential decay or square-wave pulses, but the second one is widely applied for pasteurization of liquid food and can be used in monopolar or bipolar mode (Evrendilek and Zhang, 2005). On the contrary, the treatment time (ps) is generally measured by multiplying the pulse duration with the number of pulses. PEF treatment produces a slight temperature rise and a cooling system should be used to minimize the thermal effects. In addition, it is important to note that the presence of air bubbles or suspended particles may produce the “dielectric
FIGURE 1.6 Schematic representation of pulsed electric field technology and mechanism of action for microbial inactivation (electroporation phenomena).
break down” of the system, displayed by an electric spark (Gongora-Nieto et al., 2002). It is widely recognized that PEF is effective on food as the pasteurization process (Amiali and Ngadi, 2012; Jaeger et al., 2014). PEF can process all types of food (Raso et al., 2014).
MECHANISMS OF ACTION ON MICROBIAL INACTIVATION
The microbial inactivation is not well known. It is postulated that exposure to a high electric field causes “electroporation,” thus increasing the permeability of cell membranes. Once the critical trans-membrane voltage was exceeded for a sufficient time, non-reversible pores are formed on the cell membrane with subsequent leakage of intercellular compounds and cell lysis (Jaeger et al., 2014; Raso et al., 2014). The PEF efficacy on microorganisms depends on cell characteristics, pH, water activity, soluble solids, and electrical conductivity (Saldana et al., 2009). It is well acknowledged that gram-positive bacteria and yeasts are more resistant than gram-negative bacteria, while vegetative cells are more sensitive than spores (Saldana et al., 2009). The microbial inactivation depends on electric field intensity, treatment tune, pulse duration and pulse polarity, microbial characteristics, and medium parameters (electrical conductivity, fat content) (Amiali and Ngadi, 2012; Raso et al., 2014). PEF technology can be applied with mild-temper- ature without occurring thermal damage. Food may have various benefits in terms of better preservation of nutritional properties, flavor, color, protein functionality, and shelf life extension (Buckow et al., 2014). Furthermore, PEF treatments on dairy products retained quality and sensoiy attributes (Bendicho et al., 2002; Buckow et al., 2014). Most scientific works have been earned out on a pilot scale, but the costs for possible dairy applications are still high (Toepfl, 2011).
APPLICATION OF PULSED ELECTRIC FIELD (PEF) TREATMENT TO DAIRY SECTOR
PEF treatment is capable to inactivate E. coli, L. innocua, S. aureus, Euterobacteriaceae, and P. fluorescens (Shin et al., 2007; Sobrino-Lopez and Martin-Belloso, 2008; Shamsi et al., 2008; Guerrero-Beltran et al., 2010). To enhance microbial inactivation the main strategy is to combine temperature and electric voltage pulses. Craven et al. (2008) determined the effect of PEF treatment at different temperatures (range 15-55°C), in sterile milk inoculated with spoilage Pseudomonas, and subsequently evaluated the shelf-life prolongation. Increasing temperature also increases microbial inactivation, with a better result achieved by processing the milk at 55°C with 31 kV/cm. A similar approach was used by Guerrero-Beltran et al. (2010) that evaluated the effect of a combination of PEF (30-40 kV/ cm; 1-30 pulses) and thermal treatment (20-72°C) for less than 10 s, to inactivate L. innocuain whole milk. In this study, the authors have also confirmed that increasing the initial temperature and reducing the number of pulses, a microbial reduction of about 4.3 log cycles is obtained. It is well known that gram-negative bacteria are more sensitive to electroper- meabilization by PEF treatment than gram-positive (Dutrex et al., 2000). In a recent study conducted by Sharma et al. (2014), this observation was confirmed. PEF bacterial inactivation in whole milk was also studied by Bermudez-Aguirre et al. (2012), who demonstrate that PEF treatment did not reduce bacterial numbers, but the effectiveness increased with increasing temperature (below the detection limit fori3 aeruginosa at 50°C, while for E. coli, S. aureus and L. innocua at 55°C). As for spores, studies on the use of PEF for sterilization of milk are poor and show a limited lethal effect if they are not combined with mild heat treatments (Bermudez-Aguirre et al., 2012). To improve microbial reduction and decrease energy usage, several studies optimized the PEF treatment conditions by using high electric field values, short treatment times, and moderate temperatures (Sepulveda et al., 2009; Guerrero-Beltran et al., 2010). Physiochemical characteristics of milk (electrical conductivity and fat content) play a key role in PEF efficacy. Various studies on milk also evaluated the protective effect of fat globules on microorganisms (Picart et al., 2002; Otunola et al., 2008; Bermudez-Aguirre et al., 2011). Rodriguez-Gonzalez et al. (2011) compared the efficacy of PEF treatment in raw milk samples, inoculated with native microflora, with different fat contents (0.5% skim milk, 1.0-3.1% fluid milk) and cream (12.2%). They concluded that in samples with higher fat content lower inactivation effect was achieved (the worst result in the cream sample).