Evaluation of the effectiveness of ohmic heating applied with different electric field on the inactivation of listeria monocytogenes in protein added milks

Milk, which has a very important place in human nutrition, can harm human health if it is not produced, stored and processed under appropriate conditions. Milk and dairy products constitute an important source for the transmission of Listeria monocytogenes to humans. The aim of this study was to determine the effects of ohmic heating at different electric field intensities on L. monocytogenes inactivation in milk with protein powder addition. In this context; 2.5%, 5% and 7.5% protein powder was added to 200 ml milk samples with 1.5% fat content. The samples were inoculated with L. monocytogenes 4b (ATCC 13932) strain. Ohmic heating (OH) was carried out in a laboratory-type pilot unit using stainless steel electrodes, K-type thermocouple, datalogger and a power supply providing 0-250 V, 10 amps, AC current. The pathogen-added milk samples were heated to 63 °C by applying an electric field strength of 10 and 20V/cm using OH treatment. Then, L. monocytogenes were counted and pH, color (L*, a*, b*) and hydroxymethylfurfurol levels were determined. The results were evaluated by analysis of variance. Ohmic heating applied with an electric field intensity of 10 V/cm caused the L. monocytogenes level in the milk group containing % 2.5 protein to decrease below the detection limit (<1 log) with an average 5 log reduction at 9 min (p<0.05). Similarly, the application of an electric field intensity of 20 V/cm in milk with % 2.5 and % 5protein addition caused the L. monocytogenes level to decrease below the detection limit (<1 log) in 2.5 min (p<0.05). As a result of the OH, no change was observed in the L* (brightness) values of the samples, but a slight increase was observed in pH, a* (redness) and b* (yellowness) values compared to the control group. As a result, inactivation of L. monocytogenes by ohmic heating was found to be dependent on the duration of the OH process, the protein concentration in the milk and the voltage gradient applied.