Air drying and colour characteristics of chestnuts pre-submitted to osmotic dehydration with sodium chloride

Air drying kinetics of chestnuts (Castanea sativa Mill.) submitted previously to osmotic dehydration with sodium chloride solutions (22%, w/w, 25 °C, 8 h) were experimentally determined. Drying experiments were carried out at 45, 55 and 65 °C during 32 h. Before osmotic dehydration operation, shell and tegument tissue were carefully removed in all samples. Rough external surface was maintained (peeled samples) or also removed (cut chestnuts) in different tests in order to evaluate the effect of the presence of this layer in the mass transfer rate. A diffusional model involving shrinkage was employed to simulate the drying kinetics of the all studied systems and values of the effective coefficient of diffusion of water were obtained. These coefficients were successfully correlated with temperature by means of an exponential equation and were employed to evaluate the additional mass transfer resistances due to the presence of layers and by the acquisition of osmotic solute in cut and peeled samples. The experimental determination of local salt and moisture contents during drying showed the existence of a drying front for water that moves along from surface to the centre of the sample and that salt accumulates near surface but also achieve the centre during the osmotic dehydration. Additionally, the analyses of the surface colour of air dried samples indicated that osmotic dehydration with salt keeps the characteristics observed for non-treated cut samples and deteriorates the colour quality in the case of peeled samples.