The effect of conventional and ohmic heating on the permeability of cell membrane in vegetables tissue

The effect of conventional and ohmic heating on the electrical behaviour and on the degree of permeabilization of cell membrane of vegetable tissue was investigated. Heating curves and electrical conductivity versus temperature profiles of cylindrical potato samples were determined in a static ohmic heating device for different field strength applied (42, 57 and 68 V/cm). The attained degree of membrane permeabilization of vegetable tissue for different final temperature (20, 30, 40, 50, 60, 70, 80 and 90°C) and field strength (0, 42 and 68 V/cm) was also evaluated by measuring complex impedance of the samples at frequency ranging from 100Hz to 10 MHz. Results showed that ohmic heating rate increased with increasing the temperature and the electric field strength applied, as a direct result of the increase of electrical conductivity. Moreover, it was also noted the existence of a transition temperature above which a sharp transition of the electrical conductivity curve was observed. The higher the field strength, the lower the transition temperature. Whatever the thermal treatment, as the final temperature increased, the damage of cell membrane of vegetable tissue also increased leading to a reduction in impedance. However, at all final temperature, the presence of an electric field yielded a higher degree permeabilization of the cell membrane than the one obtained by conventional heating. In particular, with increasing the field strength a higher permeabilization rate was observed.