Abstract
In this paper the resistance of bacterial foodborne pathogens to manosonication (MS), pulsed electric fields (PEFs), high hydrostatic pressure (HHP), and UV-light (UV) is reviewed and compared. The influence of different factors on the resistance of bacterial foodborne pathogens to these technologies is also compared and discussed. Only results obtained under harmonized experimental conditions have been considered. This has allowed us to establish meaningful comparisons and draw significant conclusions. Among the six microorganisms here considered, Staphyloccocus aureus is the most resistant foodborne pathogen to MS and HHP and Listeria monocytogenes to UV. The target microorganism of PEF would change depending on the treatment medium pH. Thus, L. monocytogenes is the most PEF resistant microorganism at neutral pH but Gram-negatives (Escherichia coli, Salmonella spp., Cronobacter sakazakii, Campylobacter jejuni) would display a similar or even higher resistance at acidic pH. It should be noted that, in acidic products, the baroresistance of some E. coli strains would be comparable to that of S. aureus. The factors affecting the resistance of bacterial foodborne pathogens, as well as the magnitude of the effect, varied depending on the technology considered. Inter- and intra-specific differences in microbial resistance to PEF and HHP are much greater than to MS and UV. Similarly, both the pH and aw of the treatment medium highly condition microbial resistance to PEF and HHP but no to MS or UV. Growth phase also drastically affected bacterial HHP resistance. Regarding UV, the optical properties of the medium are, by far, the most influential factor affecting its lethal efficacy. Finally, increasing treatment temperature leads to a significant increase in lethality of the four technologies, what opens the possibility of the development of combined processes including heat. The appearance of sublethally damaged cells following PEF and HHP treatments could also be exploited in order to design combined processes. Further work would be required in order to fully elucidate the mechanisms of action of these technologies and to exhaustively characterize the influence of all the factors acting before, during, and after treatment. This would be very useful in the areas of process optimization and combined process design.
Highlights
The food industry is showing growing interest in developing alternative microbial inactivation methods capable of avoiding the undesirable effects that traditional technologies such as heating or acidification cause on foods (Mañas and Pagán, 2005)
One of the most promising new technologies for microbial inactivation is pulsed electric fields (PEFs), consisting in the application of short duration (1–100 μs) high electric field pulses (10–50 kV/cm) to food placed between two electrodes (Heinz et al, 2001), PEF is capable of inactivating microorganisms while causing little changes in the sensory and nutritional quality of foodstuffs (Raso and BarbosaCánovas, 2003)
The most relevant ones are those in which increased microbial resistance is associated with the appearance of an increased proportion of sublethally injured cells, since they open up the possibility of developing combined processes capable of inactivating microorganisms under circumstances that could not be produced by the technology alone
Summary
The food industry is showing growing interest in developing alternative microbial inactivation methods capable of avoiding the undesirable effects that traditional technologies such as heating or acidification cause on foods (Mañas and Pagán, 2005). A basic pre-requisite for establishing meaningful comparisons and drawing significant conclusions is the harmonization of the experimental conditions that reigned in all studies under review In this regard, great advantage and interest can be found in the comparison of results obtained by a single research group using the same strains as well as the same protocols for obtaining suspensions and for recovering treated cells -all evaluated by the same matrices and using identical equipment-. Ultrasound consists in the use of sonic waves with frequencies exceeding 16–18 kHz, which lie above the threshold of human hearing It is one of the new microbial inactivation technologies suggested as an alternative to current heat treatments (Condón et al, 2011). In this review we will focus on manosonication (MS), a process designed and patented by our group (MTS, Spanish Patent No 9200686); MS probably represents the most promising approach to non-thermal food pasteurization involving ultrasound
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