Inactivation Kinetics and Membrane Potential of Pathogens in Soybean Curd Subjected to Pulsed Ohmic Heating Depending on Applied Voltage and Duty Ratio

Abstract

The aim of this research was to investigate the efficacy of the duty ratio and applied voltage in the inactivation of pathogens in soybean curd by pulsed ohmic heating (POH). The heating rate of soybean curd increased rapidly as the applied voltage increased, although the duty ratio did not affect the temperature profile. We supported this result by verifying that electrical conductivity increased with the applied voltage. Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in soybean curd were significantly (P < 0.05) inactivated by more than 1 log unit at 80 V_rms (root mean square voltage). To elucidate the mechanism underlying these results, the membrane potential of the pathogens was examined using DiBAC₄(3) [bis-(1,3-dibutylbarbituric acid)trimethine oxonol] on the basis of a previous study showing that the electric field generated by ohmic heating affected the membrane potential of cells. The values of DiBAC₄(3) accumulation increased under increasing applied voltage, and they were significantly (P < 0.05) higher at 80 V_rms, while the duty ratio had no effect. In addition, morphological analysis via transmission electron microscopy showed that electroporation and expulsion of intracellular materials were predominant at 80 V_rms Moreover, electrode corrosion was overcome by the POH technique, and the textural and color properties of soybean curd were preserved. These results substantiate the idea that the applied voltage has a profound effect on the microbial inactivation of POH as a consequence of not only the thermal effect, but also the nonthermal effect, of the electric field, whereas the duty ratio does not have such an effect.IMPORTANCE High-water-activity food products, such as soybean curd, are vulnerable to microbial contamination, which causes fatal foodborne diseases and food spoilage. Inactivating microorganisms inside food is difficult because the transfer of thermal energy is slower inside than it is outside the food. POH is an adequate sterilization technique because of its rapid and uniform heating without causing electrode corrosion. To elucidate the electrical factors associated with POH performance in the inactivation of pathogens, the effects of the applied voltage and duty ratio on POH were investigated. In this study, we verified that a high applied voltage (80 V_rms) at a duty ratio of 0.1 caused thermal and nonthermal effects on pathogens that led to an approximately 4-log-unit reduction in a significantly short time. Therefore, the results of this research corroborate database predictions of the inactivation efficiency of POH based on pathogen control strategy modeling.

Keywords: Applied voltage; duty ratio; electrode corrosion; membrane potential; pulsed ohmic heating; soybean curd.

FIG 1

Pulsed ohmic heating system used in this research (A) and inoculation (×) and temperature measurement (circles) points (B).

FIG 2

Temperature profile depending on the duty ratio at 48 V_rms (A) and applied voltage (V_rms) at 0.1 duty ratio (B) during POH treatment. The error bars represent standard deviations.

FIG 3

Electric current (in amperes) depending on the duty ratio at 48 V_rms (A) and applied voltage (V_rms) at 0.1 duty ratio (B) during POH treatment. The error bars represent standard deviations.

FIG 4

Electrical conductivity (in siemens per meter) depending on the duty ratio at 48 V_rms (A) and applied voltage (V_rms) at 0.1 duty ratio (B) during POH treatment. The error bars represent standard deviations.

FIG 5

Reduction (log CFU per milliliter) of E. coli O157:H7 (A), S. Typhimurium (B), and L. monocytogenes (C) inoculated on samples after 48-V_rms POH treatment at duty ratios of 0.05 and 0.1. The error bars represent standard deviations. Different uppercase letters for the same treatment temperature indicate a significant difference (P < 0.05).

FIG 6

Reduction (log CFU per milliliter) of E. coli O157:H7 (A), S. Typhimurium (B), and L. monocytogenes (C) inoculated on samples after POH treatment at different applied voltages at 0.1 duty ratio. The error bars represent standard deviations. Different uppercase letters for the same treatment temperature indicate a significant difference (P < 0.05).

FIG 7

TEM microphotograph of pathogen cells that were not subjected (A) or subjected (B to F) to the POH process. The cells were treated at duty ratios of 0.05 (B) and 0.1 (C) under the same applied voltage (48 V_rms) and at 60 V_rms (D), 70 V_rms (E), and 80 V_rms (F) under the same duty ratio (0.1). Treatment times were 490, 310, 200, and 160 s at 48, 60, 70, and 80 V_rms, respectively.