Drying characteristics of kiwifruit during hot air drying

Drying characteristics and l-ascorbic acid changes of a kiwifruit slice (10 mm) were investigated during hot air drying at four temperatures ranging from 40 to 70 °C. A relationship between hardening of the sample surface and the drying rate was also investigated. Using the measured data, a non-linear least squares method was applied to an exponential model for the first period of drying, and an infinite plane sheet model for the diffusion equation for the second period of drying. The empirical moisture content changes agreed well with both models. The values determined for the diffusion coefficients were 3.79 × 10−12 to 7.53 × 10−12 m2/s. An Arrhenius-type equation was used to relate the diffusion coefficient of kiwifruit to temperature, and the activation energy of kiwifruit for hot air drying was estimated. In addition, it was estimated that the drying rate of kiwifruit might increase by avoiding hardening of the sample surface. In order to examine the nutrient changes in kiwifruit during hot air drying, a first-order rate equation was applied to the changes in the decomposition of l-ascorbic acid contents during drying. An Arrhenius-type equation was applied to the estimation of the decomposition rate constant k′. Therefore, the activation energy for the decomposition during hot air drying of kiwifruit was estimated to be 38.6 kJ/mol.