Relationship between heat/mass transfer and ingredient degradation during drying process

Changes in ingredient content (paeoniflorin) and moisture transformation were studied during the drying process of Radix Paeoniae Alba (RPA) slice. The results show that paeoniflorin content decreases as the moisture content decreases from the initial state to 14% drying base. Paeoniflorin content in RPA slice is mainly related to drying temperature but slightly affected by gas speed. The RPA slice has the highest retention rate of paeoniflorin content (60% of initial content) at 60°C. When the drying temperature is lower than 60°C, the retention rate of paeoniflorin content decreases with the temperature decreasing because the drying process is prolonged; while the drying temperature is over 70°C, the retention rate decreases with the temperature increasing as the hydrolysis reaction of paeoniflorin is sped up. Free water is mutually transformed with bound water during drying and the degradation of paeoniflorin because of hydrolysis, diffusion, and oxidation. A model coupling heat/mass transfer and paeoniflorin degradation was presented to illustrate the results during the convective drying process. PRACTICAL APPLICATIONS: In this study, we evaluated the changes in paeoniflorin content and moisture transformation in Radix Paeoniae Alba (RPA) slice during the convective drying process. The temperature of 60°C, here, was presented for the highest preservation of paeoniflorin content. The factors affecting paeoniflorin content and moisture transformation as well as the degradation mechanism of paeoniflorin were studied. Furthermore, a model was developed to evaluate the coupling transformation of moisture, heat, and ingredient, which can be used to determine the drying parameters. Our results have given the temperature for the highest paeoniflorin preservation during drying, and our model is helpful to design the parameters for other drying processes.