Heat has been used extensively in the food industry as a preservation method, especially due to its ability to inactivate microorganisms present in foods. However, many aspects regarding the mechanisms of bacterial inactivation by heat and the factors affecting this process are still not fully understood. The purpose of this review is to offer a general overview of the most important aspects of the physiology of the inactivation or survival of microorganisms, particularly vegetative bacteria, submitted to heat treatments. This could help improve the design of current heat processes methods in order to apply milder and/or more effective treatments that could fulfill consumer requirements for fresh-like foods while maintaining the advantages of traditional heat treatments.

Ohmic heating (OH) of food has been investigated for many years as an alternative to conventional heating because it allows fast and homogeneous heating. The processing parameters that influence the most uniformity of the heating in OH are the electric field strength and the frequency. Therefore, recent trends have focused on studying the application of frequencies in the order of kHz and electric fields higher than 100 V/cm. In this regard, and considering only the applied field strength in a way to easily differentiate them, three ohmic systems could be distinguished: OH (< 100 V/cm), moderated electric fields (MEF) (100–1000 V/cm), and ohmic-pulsed electric fields (ohmic-PEF) (> 1000 V/cm). The advantages of applying higher electric fields (MEF and ohmic-PEF) over OH are, on the one hand, their much higher heating rate and, on the other hand, their capability to electroporate cells, causing the release of intracellular ionic compounds, and therefore, uniformizing the electrical conductivity of the product. This strategy is especially interesting for large solid foods where conventional heating applications lead to large temperature gradients and quality losses due to surface overtreatment. Therefore, the aim of this work is to review the state of the art of OH technologies, focusing on MEF and ohmic-PEF. The advantages and disadvantages of MEF and ohmic-PEF compared to OH and their potential for improving processes in the food industry are also discussed.

In this paper the resistance of bacterial foodborne pathogens to manosonication (MS), pulsed electric fields (PEF), 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.

The potential of the application of PEF to induce accelerate autolysis of a commercial strain of Sacharomyces cerevisiae for winemaking use was evaluated. The influence of PEF treatments of different intensity (5-25 kV/cm for 30-240 µs) on cell viability, cytoplasmic membrane permeabilization, release of mannoproteins and compounds absorbing at 260 and 280 nm has been investigated. After 8 days of incubation at 25 ºC the Abs600 of the suspension containing the control cells was kept constant while the Abs600 of the suspension containing the cells treated by PEF decreased. The measurement of the absorbance at 260 and 280 nm revealed no release of UV absorbing material from untreated cells after 8 days of incubation but the amount of UV absorbing material released drastically increased in the samples that contained cells treated by PEF after the same storage period. After 18 days of storage the amount of mannoproteins released from the untreated cell was negligible. Conversely, mannoprotein concentration increased linearly for the samples containing cells of S. cerevisiae treated by PEF. After 18 days of incubation the concentration of mannoproteins in the supernatant increased 4.2 times for the samples containing cells treated by PEF at 15 and 25 kV/cm for 45 and 150 µs. Results obtained in this study indicates that PEF could be used in winemaking to accelerate the sur lie aging or to obtain mannoproteins from yeast cultures to be used in winemaking.