Effects of Matrix Structure on Protein Digestibility and Antioxidant Property of Different Soybean Curds During In Vitro Digestion

This study compared the three most common types of tofu (soybean curd), which were prepared by using magnesium chloride (MgCl2 tofu), calcium sulfate (CaSO4 tofu), and glucono-δ-lactone (GDL tofu) coagulants. The results showed that GDL tofu had a higher water holding capacity than MgCl2 tofu and CaSO4 tofu, which was attributed to its high surface hydrophobicity and disulfide bond content. GDL tofu possessed the lowest firmness, gumminess, and chewiness, along with a uniform network structure and a thin protein matrix. In contrast, MgCl2 tofu exhibited an inhomogeneous network structure with a thick protein matrix. Combining the results of protein hydrolysis degree, SDS-PAGE, and free amino acids during in vitro digestion, it was indicated that the degree of protein digestion in GDL tofu was the highest. After intestinal digestion, GDL tofu had the highest total phenolic content, ferric reducing antioxidant power, and DPPH value. These results demonstrated the superior protein digestibility and antioxidant property of GDL tofu during in vitro digestion due to its structural characteristics that facilitate enzyme diffusion in the matrix. The findings offer insight into the protein digestibility and antioxidant properties of different types of tofu during digestion from structural characteristic perspective and valuable reference information for consumer dietary nutrition.

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.