Evaluation of the performance of ohmic heating for inactivation of listeria monocytogenes in milk

Research in recent years has focused on innovative technologies provide pathogen inactivation without damaging the structural properties of foods. Ohmic heating is one of the innovative technologies, providing an effective microbial inactivation with massive and rapid heating. This study aims to determine the effects of ohmic and conventional heating processes on L. monocytogenes in milk with different fat content. L. monocytogenes 4b (ATCC 13932) (0.05%) strain was inoculated into 200 mL of whole (3.1%), semi-skimmed (1.5%) and skimmed (0.14%) UHT milk. Ohmic heating was carried out by a laboratory type ohmic heater unit consisted of stainless steel electrodes, K-type thermocouple with data logger and power supply providing AC current, 0-250 V, 10 A. Conventional heating process was carried out by placing the samples in a a thermostatic water bath. Inoculated milk samples were heated up to 62 °C with ohmic and conventional heating processes, then samples were taken after each treatment at the initial (0), 2nd, 4th and 6th minutes with sterile injectors. Bacterial populations of surviving microorganisms were enumerated with spread plate method. Ohmic heating completely inactivated L. monocytogenes in 6 min and lead to 5.30 log kob/mL decrease in bacterial load in semi-skimmed and skim milk (p<0.05).However, there was only a 3.10 log kob/mL decrease in whole milk at the same process time (p<0.05). Besides, conventional heating lead to a few reduction as 0.21, 0.29 and 0.39 log kob/mL in whole, semi- skimmed and skimmed milk, respectively (p<0.05).In this study, the increased fat content lead to a decrease in the electrical conductivity and protected bacteria from thermal damage. Decreased electrical conductivity of milk leads to reduce the thermal efficiency of ohmic heating. It was concluded that fat content may have important inhibitory effects on pathogen inactivation in the ohmic heating process.