Effect of drying method on shrinkage and porosity

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used in order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density rho(w), the dry solid density rho(s), the dry solid bulk density rho(b0) and the volume shrinkage coefficient beta'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.