Changes in viscoelastic properties of longan during hot-air drying in relation to its indentation

Abstract

Changes in viscoelastic properties are related to the indentation of whole longan (Dimocarpus longan Lour.) in the drying process. The objective of this research is to determine parameters from a creep test to characterise the viscoelastic properties of on-progress dried longan. During 65C hot-air drying, the whole longan was sampled every 2 hours to perform the creep test with a constant stress of 44 kPa using a texture analyser. Viscoelastic properties, viz. retardation time (ret), instantaneous compliance (J0), retarded compliance (J1), creep compliance (J), Newtonian viscosity (0), and modulus of elasticity (E) were analysed using the four-element Burger’s model. The ret and E decreased linearly as the moisture content decreased from approximately 70% to 64-57%, then they linearly increased as the moisture content further decreased to 11%. The J and J1 increased linearly and then decreased linearly as the moisture content decreased, showing the transition moisture content of 64%. The J0 decreased as the moisture content decreased. There was no marked change in , thus it was not involved in the indentation of dried longan. The moisture content of 64-57% was found to be the critical range leading to the indentation of longan during the drying process