Forced-air drying of ginseng roots: 2. Control strategy for three-stage drying process

Abstract

Previous research showed that a three-stage (38–50–38 °C) drying process produced equivalent dried root quality in a shorter time relative to drying at a constant temperature of 38 °C which is typical in commercial ginseng dryers. However, unless air temperature changes occurred at specific moisture levels, there were unacceptable changes in root colour. Digital images of roots were used to monitor volume shrinkage as an indirect measure of moisture content and to confirm critical points to adjust drying air temperature. Volume shrinkage was limited when root moisture was less than 18% (wet basis) so the drying end point at 10% moisture (wet basis) was estimated with a kinetic model. Model parameters depended on root size and shape and were estimated from thin-layer drying experiments. Quality of roots dried by a three-stage process was not significantly different from roots dried at a constant air temperature of 38 °C in terms of internal colour and ginsenoside content. Drying times were in the range of 100–150 h for the three-stage process compared to 180–200 h at 38 °C.