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

• Main Author: Jatuphong Varith
• Format: Article
• Language: English
• Published: Maejo University 2008-05-01
• Series: Maejo International Journal of Science and Technology
• Subjects: longan; drying; creep test; viscoelastic properties; indentation
• Online Access: http://www.mijst.mju.ac.th/vol2/320-330.pdf

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.