Ohmic heating for thermal processing of low-acid foods containing solid particulates

Ohmic heating has potential applications for continuous sterilization processing of low-acid foods containing particulates. The main challenge is to establish a credible safety assurance protocol through experimental and modeling studies.; The electrical conductivity of food components is critical to ohmic heating. Electrical conductivities of six different fresh fruits (red apple, golden apple, peach, pear, pineapple and strawberry) and several different cuts of three types of meat (chicken, pork and beef) were determined from 25--140°C. Electrical conductivity of all products increased linearly with the temperature during ohmic heating at constant voltage gradient. Lower electrical conductivity was observed for porous materials like apples. Lean muscle cuts were much more conductive than the separable fat. There was no strong relationship between the measured fat content of the lean muscle cuts and their electrical conductivity suggesting that fat distribution or marbling might be an important factor affecting the electrical conductivity of meat.; Heating rates of solid and liquid phases during ohmic heating depend on the electrical conductivities of both phases. An ideal processing condition can be achieved when conductivities of both the phases are equal, since, under such a condition, both phases will heat at the same rate. For a chicken chowmein (low-acid food product containing particulates) it was observed that the sauce was more conductive than all the solid components (chicken, celery, mushroom, water chestnut and bean sprouts). A simple blanching method was developed to increase the electrical conductivity of solid components. Except chicken, it was possible to adjust the conductivity of all solids close to that of the sauce by blanching solids in highly conductive sauce at boiling temperature. Chicken chowmein product containing blanched solids and another product containing untreated solids were heated ohmically in a bench-scale static ohmic heater. All components of the treated product containing blanched solids heated more uniformly compared to the product containing untreated solids. Sensory test results showed that the blanched product was of good quality and had good overall acceptability. Thus, on adjusting the electrical properties of different components it may be possible to ensure more uniform heating while still maintaining product quality.; Enhancement of electrical conductivity of solid particulate foods could be achieved by salt infusion. The knowledge of diffusivity of salts in food solids would enable the determination of the pretreatment conditions necessary for ohmic heating. A simple method was developed to measure diffusivity of salt in water chestnut tissue under different levels of sodium chloride concentration and temperature. The apparent diffusion coefficient of salt in water chestnut did not change significantly with salt concentration, but as expected it increased significantly with temperature. Diffusion data were further used to solve the mass transfer problem, using Computational Fluid Dynamics (CFD) software, to predict salt concentration profile in a 3D water chestnut disc under different conditions. It was observed that after pretreatment (blanching for 90 s in salt solution at boiling temperature) salt diffused only to a certain depth in the disc and the salt concentration in the interior is essentially zero. Thus, even though it is possible to increase the overall ionic content and electrical conductivity of solids by blanching in highly conductive sauce, conductivity may not be uniform within the solids. However, even this limited diffusion is useful in improving solids heating.; Measurement of residence time distribution (RTD) is needed for determination of the fastest-moving particle, to be used for designing and biologically validating processes. Radio Frequency Identification (RFID) was used to measure residence time distribution (RTD) of particles in the ohmic heater in...