Política cambiaria y participación del BCCR en el mercado cambiario

Capítulo 2. Acciones de política del BCCR

2.4 Política cambiaria y participación del BCCR en el mercado cambiario

The effective inactivation of HIPEF on both pathogenic and spoilage microorgan-isms at pasteurization levels equivalent to those attained with conventional thermal processing has been proven in a broad variety of fruit juices (Evrendilek et al., 2000;

Yeom et al., 2000a; Elez-Martínez et al., 2004; Molinari et al., 2004; Mosqueda-Melgar et al., 2007, 2008a,c; El-Hag et al., 2008). The mechanism of microorganism inactivation by HIPEF involves the formation of local instabilities in their mem-branes by electromechanical compression and electric-field-induced tension, which causes pores to form in the membrane (electroporation) and subsequent cellular death (Coster and Zimmermann, 1975; Tsong, 1991; Ho and Mittal, 1996; Weaver and Chizmadzhev, 1996; Barbosa-Cánovas et al., 1999).

Microbial inactivation is greatly conditioned by HIPEF processing variables such as electric field strength, treatment time or number of pulses, pulse width, frequency, and pulse polarity (Martín-Belloso and Elez-Martínez, 2005). Iu et al. (2001) and Liang et al. (2002) obtained a higher inactivation of Salmonella typhimurium and Escherichia coli O157:H7 populations in orange juice and apple cider, respectively, by increasing the number of pulses and electric field strength.

Evrendilek et al. (2000), Zhong et al. (2005), and Mosqueda-Melgar (2007) reached higher microbial inactivation of E. coli O157:H7 and Salmonella enteritidis in sev-eral fruit juices when treatment time was increased. Altuntas et al. (2010) observed that inactivation of E. coli O157:H7, Staphylococcus aureus, Listeria mono-cytogenes, Erwinia carotovora, Pseudomonas syringae, Botrytis cinerea, and Penicillium expansum inoculated on sour cherry juice significantly increased with increasing electric field strength (up to 30 kV/cm) and treatment time (up to 210 μs) without adversely affecting important physical and quality parameters. Chen et al.

(2010) studied the influence of pulse rise time on the inactivation of S. aureus inoculated on apple juice. The results showed that the pulse with a shorter rise time (200 ns, 35 kV/cm) had a higher effect on the inactivation of the microorganisms and resulted in a higher transmembrane potential of S. aureus cells and higher effect on intracellular material. On the other hand, Elez-Martínez et al. (2004, 2005) observed an increase in Saccharomyces cerevisiae and Lactobacillus bre-vis inactivation in orange juice when applying treatments of low frequency. With regard to polarity, bipolar pulses are generally reported to be slightly more efficient than monopolar pulses on the inactivation of microorganisms (Qin et al., 1994;

Ho et al., 1995; Elez-Martínez et al., 2004). Liang et al. (2002) reported more than 5.0 log reductions of S. typhimurium in fresh squeezed orange juice after applying 20 pulses of 90 kV/cm and an outlet temperature of 55°C. Likewise, McDonald et al. (2000) reached up to 5.0 log reductions in the counts of E. coli in orange juice by applying a treatment of 30 kV/cm for 0.43 μs with an outlet temperature of 54°C.

Several authors have developed the optimization of treatment conditions in order to standardize the process for obtaining the maximal destruction of microorgan-isms in fruit and vegetables juices. In this way, populations of S. enteritidis, E. coli, and L. monocytogenes in melon juice were reduced by up to 3.7 log₁₀ cycles when applying 4 μs bipolar pulses of 35 kV/cm for 1709 μs at 193 Hz. On the other hand, reductions of up to 3.6 log₁₀ units of the same microorganisms were reached in water-melon juice treated at 35 kV/cm for 1682 μs (Mosqueda-Melgar et al., 2007). A treat-ment time of 1000 μs was needed for 35-kV/cm pulses applied at 100 Hz to reduce S. enteritidis in tomato juice by 4.2 log cycles (Mosqueda-Melgar et al., 2008a).

Inactivation of microorganisms by HIPEF can also be influenced by the fluid medium, the target microorganism, or the microbial characteristics (Martín-Belloso and Elez-Martínez, 2005). The pH and electrical conductivity of juices have a great influence on the safety of the product. Mosqueda-Melgar et al. (2008c) observed that E. coli O157:H7 in acidic juices exhibits high sensibility to HIPEF (Figure 5.4). In the same way, the inactivation rates of microorganisms such as L. brevis, E. coli, S. cerevisiae, Salmonella Dublin, and Listeria innocua increased with decreasing conductivity of the treatment medium (Jayaram et al., 1992; Grahl and Märkl, 1996;

Sensoy et al., 1997; Wouters et al., 2001). Juices usually have lower conductivity and pH than other liquid products, thus allowing a greater inactivation of microorgan-isms. Ferrer et al. (2007) reported that the electric field gained importance in the inactivation of E. coli in orange-carrot juice when increasing treatment intensity.

In general, Gram-negative bacteria are reported to be more susceptible to HIPEF inactivation than Gram-positive bacteria (Castro et al., 1993; Mazurek et al., 1995;

Pothakamury et al., 1996; Qin et al., 1998; Dutreux et al., 2000). However, bacterial spores are resistant to HIPEF treatments, but after germination they become HIPEF sensitive (Marquez et al., 1997; Barbosa-Cánovas et al., 1998). Moreover, microbial inactivation rates have been shown to be dependent on the growth stage and the ini-tial microbial concentration.

According to Rodrigo, Barbosa-Cánovas et al. (1998), cells in the logarithmic phase were found to be more sensitive to HIPEF treatments than those in the station-ary phase. On the contrstation-ary, yeast cells in the stationstation-ary phase are reported to be more sensitive to HIPEF treatments than those in the logarithmic phase (Molinari et al., 2004). Molinari et al. (2004) studied the inactivation of three S. cerevisiae strains by HIPEF treatment (3 pulses at 8 kV/cm or 40 pulses at 12.5 kV/cm) at different growth phases and initial inoculum sizes in orange juice. They observed that yeast inactiva-tion by HIPEF depended on the inoculum size and not on the growth phase with no significant differences among treatments.

On the other hand, the combination of HIPEF treatments with bacteriocins or other antimicrobials opens up innovative possibilities for application on low acidic products in a hurdle-type approach. Martínez-Viedma et al. (2009) observed a com-plete bacterial inactivation in freshly made orange and apple juices when combining

the addition of subinhibitory AS-48 concentrations (0.175–60 μg/mL) with HIPEF treatments (35 kV/cm at 150 Hz and 4 μs pulse duration).

Application of essential oils and their bioactive phenolic compounds in food preservation has also been proposed to enhance the antimicrobial effect of HIPEF treatments in fruit juices, but it has been limited by the strong impact that they have on the organoleptic properties (Burt, 2004). Nevertheless, the strong bacterial effects reported for these antimicrobials opens up new possibilities due to the low concentra-tion required for inhibiconcentra-tion of target bacteria (Iu et al., 2001; Raybaudi-Massilia et al., 2006). Thus, combinations of HIPEF (35 kV/cm for 1000 μs at 100 Hz and 4 μs pulse