Drying technique
Thin-layer drying experiments under controlled conditions were conducted for Pandanus leaves in a heat pump dryer at 35 °C (27 ± 2% RH) and in hot air dryer at 45 °C (60 ± 5% RH). A convective dryer (IIC, Model TD-12) was used in this investigation. The experimental dryer consisted of a centrifugal blower, an electrical resistance air heating section, the measurement sensors and the displaying unit. A door was provided in front of side of the chamber for placing and removing the sample tray. The blower and heater of dryer were switched on for 30 min for drying air to reach a stable temperature. The air velocity was continuously measured using an anemometer (Lutron AM-4201, Vikram Scientific instruments, Kolkata, India). A heat pump-assisted batch dryer with special features of variable drying air temperatures has been fabricated (Pal and Khan 2008). The developed dryer consisted of a dehumidifier unit (evaporator, compressor, condenser and expansion device) and drying chamber. The inlet drying air passed through the drying chamber and picked up moisture from the product. In the air circuit, the moist hot air leaving the dryer is directed to pass through the evaporator, where dehumidification takes place. The air leaving the evaporator is heated in the condenser and then passed to the dryer for drying the product.