Characterisation of the Convective Hot-Air Drying and Vacuum Microwave Drying of Cassia alata: Antioxidant Activity, Essential Oil Volatile Composition and Quality Studies

Abstract

The preservation of active constituents in <i>Cassia alata</i> through the removal of moisture is crucial in producing a final product with high antioxidant activity. This study aims to determine the influences of various drying methods and drying conditions on the antioxidant activity, volatiles and phytosterols content of <i>C. alata</i>. The drying methods used were convective drying (CD) at 40 °C, 50 °C and 60 °C; freeze drying; vacuum microwave drying (VMD) at 6, 9 and 12 W/g; and two-stage convective pre-drying followed by vacuum microwave finish drying (CPD-VMFD) at 50 °C and 9 W/g. The drying kinetics of <i>C. alata</i> are best described by the thin-layer model (modified Page model). The highest antioxidant activity, TPC and volatile concentration were achieved with CD at 40 °C. GC-MS analysis identified the presence of 51 volatiles, which were mostly present in all samples but with quantitative variation. The dominant volatiles in fresh <i>C. alata</i> are 2-hexenal (60.28 mg 100 g^-¹ db), 1-hexanol (18.70 mg 100 g^-¹ db) and salicylic acid (15.05 mg 100 g^-¹ db). The concentration of phytosterols in fresh sample was 3647.48 mg 100 g^-¹ db, and the major phytosterols present in fresh and dried samples were β-sitosterol (1162.24 mg 100 g^-1 db). CPD-VMFD was effective in ensuring the preservation of higher phytosterol content in comparison with CD at 50 °C. The final recommendation of a suitable drying method to dehydrate <i>C. alata</i> leaves is CD at 40 °C.