Microbiological evaluation of tofu and tempeh during processing and storage

Dominant Enterobacteriaceae in tempeh were primarily originated from soybean

During tempeh production, boiling was considered as heat treatment that could significantly reduce or eliminate bacterial population in soybean before fungal inoculation. The objective of this study was to enumerate and trace Enterobacteriaceae communities in pre-boiling soybean, post-boiling soybean, and fresh tempeh designated as RTI and EMP. Standard plate count and qRT-PCR were employed to determine the culturable and non-culturable bacteria, while Enterobacterial Repetitive Intragenic Consensus PCR was conducted to determine the intraspecies genomic variations. Fresh tempeh from both RTI and EMP contained approximately 10⁷ and 10⁸ CFU/g of Enterobacteriaceae respectively. The number of bacteria in pre-boiling soybean were 10,000 times lower than in fresh tempeh. Our study showed that most Enterobacteriaceae were severely injured or quiescent during boiling process and quickly recovered up to 10⁹ CFU/g in fresh tempeh. Some Klebsiella isolates found in tempeh were genetically identical to isolates in soybean, but different from those of medical isolates. This study suggested that soybean could be the main origin of Klebsiella in fresh tempeh.

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

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 Vrms (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 Vrms, 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 Vrms 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 Vrms) 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.