The double-function dryer was used for all drying experiments
to dry the prepared longan from an initial moisture content of
approximately 84–86% w.b. to a final moisture content of 18%
w.b. as specified by the Thai industrial standard institute (TISI,
1990). The dryer was warmed up to the desired drying temperature
before transferring the prepared longan into the drying
chamber. For hot air drying, ambient air was drawn into the system
by the fan and heated up to the desired drying temperature
by PID-controlled heaters before passing through the drying
chamber. Eighty percent of exit air from the drying chamber
was recycled back into the system by adjusting the butterfly valve
No. 8 (see Fig. 1). In the case of heat pump drying, the dryer was