Ohmic Heating of Lemon and Grapefruit Juices Under Vacuum Pressure- Comparison of Electrical Conductivity and Heating Rate

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.