Viscoelastic behaviour of dehydrated products during rehydration

Viscoelastic behaviour of dehydrated products during rehydration of apple, banana, carrot and potato was examined under uniaxial compression tests. Sa mples were dehydrated with four different drying methods: conventional, vac uum, freeze and osmotic-freeze drying and after that they were rehydrated i n an air dryer at 50 degrees C and 80% air humidity. Compression tests were performed during rehydration for various moisture contents, ranging from 0 .01 to 1.5 kg/kg dry basis. The viscoelastic behaviour of dehydrated produc ts during rehydration were examined comparing the values of the four parame ters incorporated into the stress-strain model, during rehydration with tho se of dehydrated products. The four examined parameters: maximum stress, ma ximum strain, elastic parameter and viscoelastic exponent seem to show a hy steresis phenomenon. It can be concluded that dehydrated product do not kee p their viscoelastic behaviour after rehydration due to structural damages that occur during drying. More specifically, freeze dried materials present the highest hysteresis after rehydration, losing their elasticity and beco ming more viscous. Osmotic pretreatment seems to help freeze dried material s to keep their elastic nature, probably due to solids gain. Air and vacuum dried materials showed the smallest hysteresis tendency, keeping their vis coelastic characteristics during rehydration close to those of dried materi als. (C) 1999 Elsevier Science Ltd. All rights reserved.