Arrhenius activation energy for water diffusion during drying of tomato leathers: The concept of characteristic product temperature

Abstract

The drying kinetics of tomato leathers was studied between 40 and 80°C and the corresponding thermal histories were recorded. As air and product temperatures were found to be considerably different during the drying runs, the original concept of "characteristic product temperature" was proposed. It is defined as the temperature corresponding to the time at which the instantaneous drying rate is equal to the mean drying rate. Then, the Arrhenius activation energies (Ea) obtained by two methods were compared: (1) correlating the diffusion coefficient (D) with the air temperature and (2) correlating D with the characteristic product temperature. Drying curves comprised a "wet zone", where moisture content fell almost linearly with time, and a subsequent "dry zone". In the first zone, the Arrhenius relationship with the air temperature yielded an Ea of 18.6kJmol-1 whereas, when utilising the characteristic product temperature the activation energy was 47.3kJmol-1, comparable to the enthalpy of water desorption. In the second zone, activation energies were 21.7 and 26.6kJmol-1, respectively. Molecular mobility theory suggests a stronger temperature dependence of diffusion-limited phenomena in high moisture content matrices and thus would forecast a higher Ea for the wet zone. Therefore, the characteristic product temperature is more meaningful than the air temperature in the Arrhenius relationship.