COMPARATIVE ASSESSMENT OF Lactococcus lactis VIABILITY AND SUBLETHAL INJURY UNDER OHMIC HEATING AT DIFFERENT ELECTRIC FIELD STRENGTHS AND CONVENTIONAL THERMAL TREATMENT

This study evaluates ohmic heating’s impact on Lactococcus lactis viability and sublethal injury at varying electric field strengths and holding times. Ohmic heating (OH) rapidly generates uniform internal heat, preserving food quality better than conventional methods. It inactivates microbes mainly through heat but may also cause non-thermal effects like electroporation. Factors such as electric field strength and food composition influence effectiveness. OH offers faster, potentially more effective microbial inactivation. The objective of this study was to assess the impact of ohmic heating at different electric field strengths (225 V, and 250 V) on the viability and sublethal injury of Lactococcus lactis. Treatments were conducted by heating samples to 72 °C and holding for various durations. Results were compared to conventional thermal treatment under the same temperature and time conditions. Viability and injury were determined using MRS agar and selective MRS-NaCl media to distinguish total and injured cell populations. Lactococcus lactis was cultured in MRS broth at 37 °C for 24 hours, then harvested, washed, and resuspended. Ohmic heating treatments were conducted using a lab-scale system with stainless steel electrodes at electric field strengths of 200, 225, and 250 V. Samples were heated to 72 °C and held for 30 seconds, 1 minute, or 2 minutes. Control samples underwent conventional heating at 72 °C for the same durations. Viability and sublethal injury were assessed by plating serial dilutions on MRS agar for total counts and on selective MRS-NaCl agar for injured cells. Plates were incubated at 37 °C for 24–48 hours, with injury calculated from CFU differences between media. Ohmic heating effectively reduces Lactococcus lactis viability, with bacterial counts declining more sharply as treatment duration increases, particularly above 50 °C due to protein denaturation and cellular damage. Using MRS agar supplemented with NaCl as a selective medium showed that higher salt concentrations impose osmotic stress, restricting the recovery of sub-lethally injured cells and improving the detection of fully viable bacteria. Furthermore, applying higher electric field strengths, especially at 250 V,significantly enhanced bacterial inactivation, highlighting the potent bactericidal effect of stronger electric fields during ohmic heating.Ohmic heating effectively inactivates Lactococcus lactis, with higher electric field strengths and longer holding times enhancing bacterial reduction. The method also induces sublethal injury, detectable using selective media. Compared to conventional heating, ohmic heating offers faster and more efficient microbial inactivation, highlighting its potential for improved food sterilization processes.