Experimental Research to Determine the Effect of Ultrasound in Drying Bo Chinh Ginseng by Ultrasound-Assisted Heat Pump Drying Method

This study focused on the experimental drying of Bo Chinh ginseng by the ultrasound-assisted heat pump drying method, in which the effects of drying air temperature (T), ultrasonic power (P), intermittency ratio of ultrasound generator (A) on the moisture effective diffusion coefficient (Deff), saponin content (Sp) and color change (dE) during the drying process were determined. The results showed that the drying time was reduced from 12.5 h at 35 °C to 8.9 h at 40 °C and greatly reduced to 5.8 h at 55 °C. The moisture diffusion coefficient Deff and color change index increased with the increase in drying temperature. Meanwhile, from 35 °C to 45 °C, the saponin content after drying tended to increase gradually, but when the temperature continued to rise from 45 °C to 55 °C, the saponin content decreased. The effect of ultrasonic power in the drying process was in particular as follows. The drying time of 8.8 h at 40 W ultrasonic power reduced to 7.8 h at 80 W ultrasonic power, and reduced to 7 h at 160 W ultrasonic power. The remaining saponin content after drying tended to increase gradually at the power level of 40–120 W (from 89.2% to 95.2%) and decrease when the power increased from 120 to 160 W (from 95.2% to 90.5%). The moisture diffusion coefficient Deff also increased with the increase in ultrasonic power. Meanwhile, the color change of dried products decreased as the ultrasonic power increased in the range of 40–120 W but increased with the ultrasonic power range from 120 to 160 W. Additionally, the experimental method and Box–Behnken design were used for optimizing the drying process with the optimal drying conditions such as drying air temperature of 45.2 °C, ultrasonic power of 127.7 W and intermittency ratio of ultrasound generator of 0.18.