Heat-Assisted Pulsed Electric Field Treatment for the Inactivation of Saccharomyces cerevisiae: Effects of the Presence of Citral

Abstract

Pulsed electric field (PEF) treatment is a non-thermal technology that has shown good potential for microbial inactivation. However, in many cases, it cannot be sufficient to avoid microbial proliferation, and the combination with other stabilizing technologies is needed. In the framework of the hurdle concept, several researches have been focused on the use of PEF in combination with heat and/or antimicrobials to increase its efficacy. This study investigated the inactivation effect of PEF on a strain of Saccharomyces cerevisiae (isolated from spoiled beverages) in a model system (growth medium). The efficacy of PEF treatment was evaluated in relation to different variables, such as electric field strength (25 and 50 kV/cm), treatment time (from 1 to 5 s), initial inoculum level (4 or 6 log cfu/ml), preheating at 50°C, medium pH (4 or 6), and addition of citral at sublethal concentration (i.e., half of minimum inhibiting concentration). The data from plate counting, modeled with the Weibull equation, showed that one of the main factors affecting yeast inactivation was the preheating of the suspension at 50°C. Indeed, higher cell load reductions were obtained with heat-assisted PEF, especially in the presence of citral. The effect of initial cell load was negligible, while pH affected yeast inactivation only without preheating, with higher death kinetics at pH 6. Flow cytometry (FCM) analysis confirmed higher mortality under these conditions. However, the occurrence of injured cells, especially in samples treated at pH 4, was observed. The ability of these cells to recover from the damages induced by treatments was affected by both citral and preheating. The synergic effects of PEF, preheating, and citral were likely due to the increase of membrane permeability (especially at pH 6), as the primary target of electroporation, which favored the solubilization of citral in the cell membrane, enhancing the efficacy of the whole process. The multi-analytical approach (traditional plate counting and FCM) allowed defining parameters to increase PEF efficacy against S. cerevisiae . Moreover, FCM, able to discriminate different physiological states of the yeast population, was helpful to better clarify the action mechanism and the potential recovery of cells after treatment.

Pulsed electric field of goat milk: Impact on Escherichia coli ATCC 8739 and vitamin constituents

“…Nevertheless, 3.14 log CFU/ml reduction was achieved when PEF‐processed at 35 kV/cm for 7 μs. Microbial inactivation by PEF can be explained by the cell membranes electroporation process that takes place during PEF processing, which results in the transformation of cell membrane permeability, subsequently draining the membrane, and eventually cell damage (Montanari et al, 2019).…”

“…Moreover, the Weibull model has been reported to properly fit both thermal and nonthermal reduction of various microorganisms and can fit an extensive range of trends including linear, upward and downward curves (Bevilacqua et al, 2015). Correspondingly, the Weibull model has been effectively used to describe microbial inactivation by PEF in carrot juice (EFS: 9–12 kV/cm; time: 100–500 μs; temperature: 60°C) and sabouraud dextrose medium (EFS: 25 & 50 kV/cm; time: 1–5 s; temperature: 50°C) (Singh et al, 2017; Montanari et al, 2019). As shown in Table 1a, increased EFS tended to increase the b value of the Weibull model.…”

“…The predicted n parameter for 20–40 kV/cm was consistently <1, ranging from 0.0431–0.1208. According to Montanari et al (2019), the smaller n value shows the existence of a higher resistance subpopulation or acclimatization of certain cells to working parameters. The results also revealed that the coefficient of determination ( R 2 ) for b is 0.9710 and 0.9689 for n respectively.…”

“…Weibull model exhibits the nonlogarithmic of survival curves (Montanari et al, 2019) as the equation below: where t corresponds to the processing time, n to the shape of the survival curve, and b to the rate parameter.…”

Microbial analysis and virulence genes detection of milk preserved using heat-assisted pulsed electric field

“…1 ). In the framework of the hurdle concept, several researches have been focused on the use of PEF in combination with heat and/or antimicrobials to increase its efficacy [ 21 ]. In another trial, PEF processing of milk was combined with heat treatment up to 55–60 °C and a significant reduction was observed in the microbial load [ 9 ].…”