• nonlinear water diffusion;
  • numerical simulation;
  • peeling effect;
  • tomato air-drying

Abstract:  A mathematical model was formulated for the estimation, in conjunction with experimental measurements, of water diffusivity parameters during convective drying of peeled and unpeeled tomatoes. Fick's 2nd law of diffusion was solved numerically for a sphere, by explicit finite differences, considering shrinkage effect, variable diffusivity, and constant boundary conditions. Experiments were performed in a laboratory tunnel dryer. The equivalent radius of tomato decreased by 50% until the end of the process, which explains the necessity for shrinkage inclusion in the mass transfer model. The mean estimated diffusivities varied between 2.03 × 10−10 and 15.1 × 10−10 m2/s for peeled tomatoes and 0.59 × 10−10 and 15.2 × 10−10 m2/s for unpeeled tomatoes. The estimated water diffusivities and their variation with the tested drying temperatures (45, 55, and 65 °C) provide an insight of peeling effect during air-drying. Peeling was beneficial since yielded greater drying rates and shortened significantly drying times, thus saving energy during drying. In all the studied cases, good agreement was found between experimental and predicted drying curves (inline image≥ 0.99, mean relative deviation [MRD]≤ 0.12, and root mean square error [RMSE]≤ 0.03). In overall, the proposed methodology provides a reliable and easy estimation of temperature and moisture-dependent mass transfer properties and drying simulation of shrinkable food products such as tomato.

Practical Application:  Water diffusivity is a food property, difficult in estimation but essential in drying processing optimization. This property was estimated as a function of moisture content and drying temperature employing a numerical simulation procedure. The peeling effect was also studied and found beneficial for lower temperature drying (<55 °C) which is useful in the energy optimization of the drying process as well as the retention of the end-product quality.