EFFECTS OF HYDROPHILIC COATINGS ON OSMOTIC DEHYDRATION AND HOT AIR DRYING OF FOOD MATERIALS

Abstract

The feasibility of coupling hydrophilic coatings and dehydration processes for producing high quality products has been investigated in this research. Osmotic dehydration (OD) is a useful process to remove a moderate amount of water from food materials before further drying. In this thesis, the effect of the coatings such as sodium alginate (SA) and low methoxyl pectinate (LMP) on mass transfer during OD and hot air drying has been studied. Mass diffusivities for water and solute during OD by using different osmotic agents have been determined. Analytical solution of Fick¿s second law of diffusion was applied to estimate the values of mass diffusivities. The influences of OD process temperature and concentration of osmotic solution on the mass diffusivities in the coated and non-coated foods have been studied systematically through both experiments and computer simulation. The model has been validated with experimental data, and simulation results have shown how the operating conditions of OD affect the mass diffusivities of coated and non-coated foods. Larger dehydration index (DEI) was observed in the coated foods during OD at high temperature of 55 °C. The moisture transfer behaviour in the coated and non-coated foods when subjected to hot air drying after the OD at 55 °C by using sucrose was also investigated. Analytical solution of Fick¿s second law diffusion models was again applied for the evaluation of moisture diffusion using experimental moisture loss profile. The evaluation was carried out repeatedly under the condition of either without a consideration of shrinkage during the drying or with a consideration of thickness and volume variation respectively. The effect of coatings on diffusion rate during the hot air drying was analyzed. In addition, the model was validated with experimental data and simulation results demonstrated how the consideration of variation in volume and thickness/radius would affect the mass diffusivity values of water. The application of coatings prior to OD and hot air drying provided a higher rate of water diffusion. Finally, microstructural characteristics and viscoelastic properties of the coated and non-coated foods after the OD and hot air drying were investigated as those characteristics were strongly related with shrinkage of the cells during the OD and hot air drying, which affected the water removal rate significantly.