Influence of treatment time and pulse frequency on Salmonella Enteritidis, Escherichia coli and Listeria monocytogenes populations inoculated in melon and watermelon juices treated by pulsed electric fields Jonathan Mosqueda-Melgar, Rosa M. Raybaudi-Massilia, Olga Martín-Belloso ⁎ Department of Food Technology, University of Lleida, UTPV-CeRTA, Av. Alcalde Rovira Roure, 191, 25198 Lleida, Spain Received 16 December 2006; received in revised form 23 March 2007; accepted 13 April 2007 Abstract Consumption of unpasteurized melon and watermelon juices has caused several disease outbreaks by pathogenic microorganisms worldwide. Pulsed electric field (PEF) has been recognized as a technology that may inactivate those bacteria present in fluid food products at low temperatures. Hence, PEF treatment at 35 kV/cm, 4 μs pulse duration in bipolar mode and square shape were applied on Salmonella Enteritidis, E. coli and L. monocytogenes populations inoculated in melon and watermelon juices without exceeding 40 °C outlet temperatures. Different levels of treatment time and pulse frequency were applied to evaluate their effects on these microorganisms. Treatment time was more influential than pulse frequency (P ≤ 0.05) on the PEF microbial reduction levels for both melon and watermelon juices. Populations of S. Enteritidis, E. coli and L. monocytogenes were experimentally reduced and validated in a single process up to 3.71 ± 0.17, 3.7 ± 0.3 and 3.56 ± 0.26 log 10 units, respectively, in melon juice when 1440 μs and 217 Hz were used; whereas reductions up to 3.56 ± 0.12, 3.6 ± 0.4 and 3.41 ± 0.13 log 10 units of those microorganisms, respectively, were reached in watermelon juice treated for 1727 μs at 188 Hz. Although PEF treatment reduced the populations of the three microorganisms, L. monocytogenes was more resistant to PEF than S. Enteritidis and E. coli in both juices when treated at the same processing conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: PEF; Treatment time; Pulse frequency; Salmonella Enteritidis; E. coli; L. monocytogenes; Melon; Watermelon; Juice 1. Introduction Melon and watermelon products are regarded as potentially hazardous foods by the Food and Drug Administration (FDA) (FDA, 2001) because they may favor the growth of pathogenic microorganisms due to their low acidity (pH 5.2 to 6.7) and high water activity (0.97 to 0.99). Outbreaks of Salmonella spp. and E. coli O157:H7 have been linked with the consumption of fresh-cut as well as juice of melon and watermelon (CDC, 1991; Mohle-Boetani et al., 1999; Powell and Leudtke, 2000; CDC, 2001; Meng et al., 2001; FDA, 2001; CDC, 2002). The majority of outbreaks are linked to the presence of these pathogens on the fruit rind, presumably contaminated in the field by improperly composted fertilizer, irrigation with infected water or through infected workers (FDA, 2001). Hence, these pathogenic micro- organisms can be transferred to the edible tissues and juices when melons and watermelons are cut during preparation (Ukuku and Sapers, 2001; Sharma et al., 2005). Incidence, survival and growth of Salmonella spp., E. coli O157:H7 and L. monocytogenes on watermelon and melon slices and juices have been reported by several researchers (Fernandez Escartin et al., 1989; Golden et al.,1993; Del Rosario and Beuchat, 1995; Penteado and Leitao, 2004; Eswaranandam et al., 2004). Nowadays, fresh juices from those fruits are sold without pas- teurization and, thus, they could be potential sources of patho- genic microorganisms such as Salmonella, E. coli and Listeria which at low doses (1–100 cells ml - 1 ) may produce illness (D'Aoust et al., 2001; Meng et al., 2001; Swaminathan, 2001; Bell and Kyriakides, 2002a,b,c). These pathogenic microorganisms can be easily elimi- nated through heat, but sensorial and nutritional attributes are International Journal of Food Microbiology 117 (2007) 192 – 200 www.elsevier.com/locate/ijfoodmicro ⁎ Corresponding author. Tel.: +34 973 702 593; fax: +34 973 702 596. E-mail address: omartin@tecal.udl.es (O. Martín-Belloso). 0168-1605/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ijfoodmicro.2007.04.009