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In this study, the effects of ohmic pre-drying technique on moisture ratio, drying rate, drying time, specific energy consumption, drying efficiency, and effective moisture diffusivity of tomato paste were investigated. Pre-drying experiments were carried out in an ohmic laboratory dryer at voltage gradient levels of 6, 8, 10, 12, 14, and 16 V/cm and oven at 105  and 1.0 m/s air velocity (control sample). Pre-drying was accomplished till the moisture content of the tomato paste reduced from initial moisture content of 90% (w.b.) to a safer level of 70% (w.b.). The ohmic pre-drying reduced the drying time of tomato paste by 80-97%, compared with the hot air drying. Pre-drying took place mainly in warming up, constant rate, and falling rate periods. Six available moisture-ratio models were fitted to the pre-drying data. The results showed that the Midilli et al. model is the most appropriate model for pre-drying behavior of tomato paste. The effective moisture diffusivity varied from 5.39 × 10(-8) to 3.91 × 10(-7)m(2)/s with an activation energy of 2.082 (V/g.cm). Both specific energy consumption and drying efficiency were considerably enhanced by increasing voltage gradient. It was found that the specific energy consumption and drying efficiency varied from 3.72 to 2.29 MJ/kg water and 67.8 to 83.8%, respectively.

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Ohmic heating fundamentally depends on electrical conductivity. In this study, grapefruit and lemon juices were ohmically heated under vacuum conditions. The electrical conductivity was measured at voltage gradients (10, 20, and 30 V/cm) and vacuum pressure (0 [atmospheric pressure], −30, and −60 kPa) for four temperature ranges (30, 40, 50, and 60 °C), meanwhile the heating rate was also reported at the same level of pressure and voltage gradient. The electrical conductivity and heating rate considerably vary by voltage gradient relative to pressure. Grapefruit had considerably lower electrical conductivity (about 20%) relative to lemon juice for the same pressure–voltage gradient treatment, while the percent reduction of heating rate (grapefruit relative to lemon) varied from 19 to 32%. The multivariate linear regression of electrical conductivity, including temperature and voltage, was found to be a more suitable model. pH assessments showed that pressure significantly affected the pH of grapefruit and lemon juices (P < 0.01). The combination of different treatments, which created a shorter residence time, caused a greater decrease in pH.