In this study, we investigated the antimicrobial effects of plasma activated fine droplet (PAD) produced from arc discharge plasma on planktonic Listeria monocytogenes and Escherichia coli O157:H7. NaCl (0.9%, w/v) was used as the feeding solution for the plasma discharge. The inactivation mechanism of the PAD treatment was also investigated. PAD mainly contains H2O2 and OCl−, which play a significant role in the inactivation process against L. monocytogenes and E. coli O157:H7. The population of L. monocytogenes and E. coli O157:H7 was significantly reduced by approximately 3 and 4 log units, respectively, within 5 min of exposure to PAD. However, the bactericidal effects of PAD against L. monocytogenes and E. coli O157:H7 showed different trends by showing 0.58 and 4.13 log reductions, respectively, after 1 min of PAD exposure time. The change of membrane integrity was evaluated using two DNA-binding fluorescence dyes, SYTO 9 and propidium iodide (PI). The breakage of the cell wall and membrane of both microorganisms was evidenced by the uptake of PI by cells after 5 min of exposure to PAD, but the effect was less in L. monocytogenes compared to E. coli O157:H7 after 1 min of PAD exposure time. The transmission electron microscopy results clearly showed morphological changes in both microorganisms, including denaturation or leakage of intracellular materials as a consequence of PAD treatment. These findings suggest that PAD-induced chemical species can eventually affect the intracellular materials of bacterial cells by passing through or attacking the cell envelope. In addition, L. monocytogenes are less susceptible to PAD compared with E. coli O157:H7.

The purpose of the present study was to evaluate the sensory and instrumental quality of thawed beef steak prepared by searing and oven cooking. Beef purchased in the local market was divided into two groups; one group was cooked in a 180℃ oven until the internal temperature reached 60℃, and the other group was oven cooked until 35℃, then cooked in a 250℃ pan until the internal temperature reached 60℃. Despite a noticeable change in appearance due to the high temperature of the searing, there was no significant difference in juiciness, water content, and cooking loss between the searingcooked and the oven-cooked steaks. However, in searing cooking, both scores of overall flavor and roast meat flavor were significantly higher than those of oven cooking. In the searing-cooked steak, the reducing sugar, which is a reactant of the Maillard reaction, was lower and Maillard-reaction products were higher than oven-cooked steak. From our results, it can be concluded that searing does not improve juiciness of the steak, but improves the flavor of beef steak due to higher levels of Maillard reaction products.