A model for simultaneous heat, water and vapour diffusion

Abstract

A model for simultaneous heat, water and vapour diffusion was developed to use for prediction of the diffusion of water inside foods during heat processing. The model is based on Fourier’s and Fick’s Laws. The diffusion of liquid water is separated from the diffusion of water vapour. The model was evaluated in a drying process. Slabs of bread crumbs, 12 × 12 × 2 cm 3 in size, were dried in a conventional oven at 210°C and the local water content and temperatures were measured during the drying in the centre, halfway to the centre and at the surface. From the measurements, the diffusion coefficient as a function of temperature and concentration and several other material parameters were estimated. The results show that the measured water content slowly increases in the centre of the sample. During the increase, the temperature remains on a plateau. When the centre starts to dry the temperature increases. Halfway to the centre the water content also increases slightly before it starts to dry out, while the surface starts drying immediately. The simulated water content levels and temperatures conform well to the experimental values and show that the evaporation and condensation model describes well the diffusion mechanisms in a porous food.