Effect of osmotic dewatering on rheological properties of apple subjected to convective drying

Apple v. Idared cut in cubes was dewatered by osmosis and subsequently dried by convection. Dewatering caused substantial changes in mechanical properties of apple tissue. Shrinkage due to osmotic pre-treatment was small but stiffness was half the value obtained by convective drying of apple cubes to the same final water content. In a viscous-elastic model of apple tissue, osmotic treatment increased the share of viscous element. Concentrated sugar solution in surface layers of pre-treated apple cubes modified the effect of convective drying on rheological properties of the tissue in comparison with those observed during drying of raw apple. Hardening of the surface was observed at the final stages of drying. The more water was removed the difference between osmosed and raw apple, both dried by convection, was the less evident. It is suggested that apple undergoing convective drying is composed by three compartments: surface layers, underneath tissue with lost turgor and structure defects and the core with properties similar to those of raw apple. Osmotic dewatering affects the surface compartment, hence its properties influence response of the material to stress. (C) 2000 Elsevier Science Ltd. All rights reserved.