A newly designed ohmic heating cell for establishing microorganism destruction kinetics

Citation

Résumé en langage clair

Ohmic heating is a rapid and internal heating process based on the generation of heat within a food matrix due to the passage of an electrical current. A long-standing interesting research was carried out on the evaluation of the efficiency of this process to inactivate pathogens. Complicated ohmic heating systems were used in the literature to assess this inactivation. Therefore, this study was aimed to design a new laboratory scale ohmic heating system able to assess easily the destruction rate of microorganisms as in conventional thermal process. A new small cell of 3 ml active volume was designed and validated in our laboratory. The gap between the two electrodes is only 3 cm which leads to fast heat of food liquids in a continuous applied voltage. Trials with salt and sugar solutions were performed to define the coldest point of the cell as one of the main critical processing factor. Uniformity of temperature in the cell was confirmed by comparative temperature measurements at different locations inside the cell. Come-up times (CUT) for heating liquid up to 121 °C varied from 10 to 50 seconds which is comparable to those obtained using capillary tubes in conventional thermal bath. Destruction rate of selected microorganisms was assessed under defined ohmic heating conditions. Obtained rates were compared to those of conventional thermal. This novel developed ohmic cell is an appropriate and practical tool for establishing the microbial destruction rate under ohmic heating comparing to conventional thermal process.

Résumé

Ohmic heating is a rapid and volumetric internal heating process based on the generation of heat within a food matrix due to its resistance to the passage of an electrical current. A long-standing interesting research was carried out on the evaluation of the efficiency of this process to inactivate pathogens. Early literature has been inconclusive on the nonthermal effects of this process compared with conventional thermal heating. This discrepancy is the result of using different complicated ohmic heating systems and the complexity of the microorganism’s behaviour under the synergetic effect of heating and electrical field. Therefore, this study was aimed to design a new laboratory scale ohmic heating system able to assess easily the death kinetic of microorganisms as in conventional thermal process. A new small cell of 3 ml active volume was designed and validated in our laboratory. The gap between the two electrodes is only 3 cm which leads to fast heat of food liquids in a continuous applied voltage gradient mode ranging from 20 to 90 (V/cm) while maintaining the target temperature during holding time. Trials with salt and sugar solutions were performed to define the coldest spot of the cell as one of the main critical processing factor. Model solutions were heated under stirring mode at various temperatures up to 120 ºC and at different voltage gradients. Uniformity of temperature in the cell was confirmed by comparative temperature measurements at different locations inside the cell. Come-up times (CUT) for the 3 ml of liquid varied from 10 to 50 seconds which is comparable to those obtained using capillary tubes in conventional thermal bath. CUT depends on the electrical conductivity of the material, voltage gradient and proportional–integral–derivative (PID) temperature control loop. Death kinetics of selected microorganisms was tried under defined ohmic heating conditions while monitoring coldest point time-temperature profile. Obtained kinetic parameters (D and Z values) of the selected microorganisms were compared with conventional thermal bath. This novel developed ohmic cell is an appropriate and practical tool for establishing the microbial destruction rate under ohmic heating comparing to conventional thermal process.