Intensified inactivation efficacy of pulsed ohmic heating for pathogens in soybean milk due to sodium lactate

Pulsed ohmic heating (POH) is a promising alternative to conventional heat processing for microbial safety. It has been reported that the efficacy of POH depends on the concentration of electrolytes. This research investigated the effect of sodium lactate as an electrolyte on the efficacy of POH with respect to the heating rate and inactivation of pathogens in PBS and soybean milk. When PBS and soybean milk with 0–3% sodium lactate were subjected to POH, as the sodium lactate content increased, the electrical conductivity increased, thereby accelerating the heating rate of POH. For instance, the heating rate of POH was 0.3 and 1.3°C/s for 0 and 3% of sodium lactate in soybean milk. Reductions in pathogens increased as the sodium lactate content increased. When 3% of sodium lactate was added to PBS or soybean milk, the bacterial reduction was more than 1 log unit compared to 0%. We hypothesized that sodium lactate intensified the damage to bacterial membrane by accelerating the heating rate of POH and amplifying the electric current; thus, the extent of bacterial cell membrane damage was determined using propidium iodide (PI) and transmission electron microscopy (TEM). The PI uptake values of pathogens significantly (P < 0.05) increased as the sodium lactate content increased, indicating that the membrane damage increased in proportion to the sodium lactate content. Furthermore, the analysis of TEM images of Escherichia coli O157:H7 and Listeria monocytogenes showed fatal morphological cell rupture at 3% sodium lactate. After soybean milk with 0–3% sodium lactate was treated with POH, no electrode corrosion was observed, and pH and color of samples were not changed. Thus, this research corroborated that sodium lactate intensifies the efficacy of POH without electrode corrosion and quality degradation of soybean milk.