DRYING KINETICS AND QUALITY OF LONGAN DRIED BY TWO-STAGE SUPERHEATED STEAM AND HOT AIR DRYERS

The aims of this study were to investigate a novel strategy for drying longan without stone viz. the two-stage drying using a superheated steam dryer followed by a hot air dryer (SSD/HAD), both from heat/mass transfer and product quality points of view. The experiments were performed in the two different stages of SSD/HAD using the superheated steam temperatures of 120-180°C in the first-stage and then the drying air temperatures of 60-70°C in the second-stage. The moisture content of longan at the end of the superheated steam drying stage was 200% dry basis. The effects of drying medium temperatures and the intermediate moisture content of dried longan after SSD/HAD on the drying kinetics and quality of dried longan were investigated. The quality of longan was evaluated in terms of its color characteristics and shrinkage.