Effect of relative humidity on drying characteristic and quality of Panacis Quinquefolii Radix / 中草药

ABSTRACT

Objective: Taking Panacis Quinquefolii Radix (PQR) as study object, the drying characteristic and quality was investigated under constant relative humidity (RH) and step-down RH drying method in order to provide foundation for improving drying efficient and quality of PQR. Methods: At drying temperature 55 ℃, the effect of constant RH (20%, 30%, and 40%), step-down RH, when RH 40% was kept for 1, 5, and 9 h and then decreased to 20%, and continuously dehumidification drying conditions on drying characteristic, moisture effective diffusion coefficient, rehydration ratio, shrinkage ratio, total ginsenoside content and microstructure were investigated. Results: With constant RH drying condition, the lower the RH was, the higher the drying rate was. When RH was 20%, the drying time was shortened by 6.8% compared with RH of 40%. With step-down RH drying condition, when RH 40% was held for 5 h and then decreased to 20%, the drying time was shortened by 3.4% compared with dehumidification drying method. Also, a transient increasing drying rate phase was appeared. Moisture effective diffusion coefficient ranged from 1.49 × 10-10 to 2.50 × 10-10 m2/s. Rehydration ratio mainly depended on the damage degree of the PQR cellular structure and the moisture content before rehydrating. Additionally, the rehydration ratio and shrinkage ratio increased with the increase of RH. High RH was benefit for reserving and transferring of ginsenoside content. The microstructure results showed that under continuous dehumidification drying process, the PQR surface was crusted so that the drying time was prolonged and rehydration ratio was decreased. On the other hand, step-down RH drying method was benefit for porous structure formation, which was helpful for shortening drying time and improving rehydration ratio. When RH 40% was kept for 5 h and then decreased to 20%, the comprehensive score of this drying condition achieved its maximum value as (0.61 ± 0.01). Such drying condition was regarded as the best drying process with the rehydration ratio, shrinkage ratio and total ginsenoside content of 2.23 ± 0.12, 0.26 ± 0.06, and (5.01 ± 0.04)%, respectively. Conclusion: Step-down RH drying method can improve PQR drying efficient and quality and such conclusion provided theoretical foundation and technical support for how to adjust RH during hot air drying of PQR.