Experimental and computational analysis of microbial inactivation in a solid by ohmic heating using pulsed electric fields

https://doi.org/10.1016/j.ifset.2020.102440Get rights and content

Highlights

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    PEF can be a new way to apply ohmic heating in solids, like agar cylinders, using field strengths over 1 kV/cm.

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    Reductions of 5 log10 CFU/g of Salmonella can be achieved in 60 seconds applying pulses of 3 μs of 2.5 kV/cm at 50 Hz.

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    Numerical simulation has resulted a great support to provide detailed knowledge of the temporal and spatial distribution of the agar's electric field strength and temperature in the treatment chamber difficult to measure experimentally.

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    The interphase agar-electrode was detected as the coldest point with temperatures 10 °C lower to the other points of the agar, limiting the lethal efficacy of the treatment.

Abstract

Pulsed electric field technology (PEF) has traditionally been used as a technique to inactivate microorganisms in liquid foods at temperatures below those used in heat treatments; however, application of high-intensity PEF (E>1 kV/cm) at high frequencies (>10 Hz) can allow rapid and volumetric solid food electrical heating in order to replace traditional convection/conduction heating that progresses from the heating medium to the inside of the product. This investigation is the first one to evaluate the inactivation of Salmonella Typhimurium 878 in a solid product (cylinder of technical agar used as reference solid) by applying PEF treatments (2.5 and 3.75 kV/cm, and up to 9000 microseconds) at 50 Hz. The evolution of temperature in different locations of the agar cylinder was measured by observing the variability of heating rates depending on location and PEF intensity. Microbial inactivation was determined and compared with isothermal heat treatments that predicted similar inactivation values, but did not detect additional inactivation. Computational analysis enabled us to predict temperature and microbial inactivation for any spatial and temporal distribution of the cylinder agar by detecting the coldest point in the transition zone between the high-voltage electrode, the agar, and the plastic container of the treatment chamber. In order to evaluate the variability of the temperature, computational predictions were done each 0.5-mm. The difference between the coldest and the hottest point (e.g. at the center of the cylinder) resulted in around 10 ∘C and 10 second variation in temperature and processing time, respectively. In any case, it was possible to obtain 5-log10-reductions after 60 s of PEF treatments when using 2.5 kV/cm and 50% reduction for 3.75 kV/cm. These results suggested the potential of PEF technology as a rapid heating system based on ohmic heating for microbial inactivation in solid food products.

Section snippets

Industrial relevance

Pulsed electric field technology (PEF) has traditionally been used as a technique for inactivating microorganisms in liquid foods at temperatures below those used in heat treatments. However, the ohmic heating produced when applying PEF opens this technique to be used as a new system for rapid and volumetric solid food heating, replacing the traditional convection/conduction heating. This investigation has demonstrated for the first time experimentally and by numerical simulation the

Materials and methods

The present study is two-fold. On the one hand, it was carried out a first set of PEF experiments on a solid agar cylinder in order to evaluate the thermal effect of different field strengths on heating rates at different distinct points within the solid, and on the inactivation of Salmonella Typhimurium 878. On the other hand, a numerical model (Finite Element Model-FEM) was applied in order to predict the degree of ohmic heating (OH) in the solid, thereby estimating the microbial inactivation

Results and discussion

This study was the first to evaluate microbial inactivation via PEF treatments in a solid product based on generated ohmic heating, considering Salmonella Typhimurium as reference. Moreover, inactivation in a solid (an agar cylinder) has been estimated based on the isothermal heat resistance of Salmonella Typhimurium after the application of PEF treatments and the time-temperature distribution predictions obtained by numerical simulation tools, after which the results of the actual microbial

Conclusions

This study demonstrated the potential of PEF as a system capable of rapidly achieving microbial inactivation in solid products thanks to a higher heat capacity transfer when applying field strengths over 1 kV/cm. Based on our knowledge, this is the first study on this particular aspect, and it was possible to reduce 5 or even more log10 cycles of Salmonella Typhimurium 878 in solid agar with treatment times below 1 min. These results indicated that PEF could be further investigated and

CRediT author statement

Ariza-Gracia, M.A.: Software, methodology, validation, Writing- Reviewing and Editing, Visualization

Cabello, M.P.: Investigation, Formal analysis

Cebrián, G.: Formal analysis, Funding acquisition

Calvo, B.: Conceptualization, Formal analysis, Supervision, Funding acquisition

Álvarez, I.: Conceptualization, Supervision, Data Curation, Writing- Original draft preparation.

Declaration of competing interest

There is no conflict of interest on the original manuscript “Experimental and Computational Analysis of Microbial Inactivation in a Solid by Ohmic Heating Using Pulsed Electric Fields”.

Acknowledgements

The authors thank A. Picardo for his help in designing and constructing the PEF treatment chamber.

The authors gratefully acknowledge research support from the Spanish Ministry of Economy and Competitiveness (Grant GL2017-84084-R and DPI2017-84047-R) and the Department of Industry and Innovation (Government of Aragón) through Research Group Grant T24-17R and A03-17R (cofinanciado con Feder 2014-2020: Construyendo Europa desde Aragón).

The authors also acknowledge assistance provided by CIBER-BBN,

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