Volatile and polyphenol composition, anti-oxidant, anti-diabetic and anti-aging properties, and drying kinetics as affected by convective and hybrid vacuum microwave drying of Rosmarinus officinalis L

Abstract

The preservation of herbs during post-harvest operations is crucial in maintaining their potency. The preservation of Malaysian rosemary (Rosmarinus officinalis L.) was investigated by determining the effect of various drying processes on active volatile components and their respective bioactivities. The drying processes included convective drying at 50, 60, and 70 °C; freeze-drying; vacuum-microwave drying (VMD) at 240, 360, and 480 W; and combination of convective pre-drying (CPD) for 30, 60, and 120 min followed by vacuum-microwave finish drying (VMFD) at 360 W. Complete moisture removal was achieved the fastest by VMD at the highest wattage. Semi-theoretical models, namely, modified Henderson–Pabis, Page, and modified Page had the best prediction accuracy of drying kinetics with the highest R2 (>0.9704) and the lowest RMSE (<0.0565). Sixty-seven compounds were identified from fresh and dried rosemary leaves with camphor (1.45 g kg−1 DW), α-pinene (1.64 g kg−1 DW), and α-terpineol (1.79 g kg−1 DW) as the dominant volatiles. The highest volatile percentage, phenolic content, and antioxidant activities were determined from convective-dried leaves at 60–70 °C, whereas the best anti-diabetic and anti-aging effects were detected from freeze-dried leaves. Non-identified compounds probably have a substantial contribution to the bioactivities of freeze-dried leaves. VMD at 480 W was the most efficient method with the least energy consumption. The sequential method of CPD-VMFD performed moderately among the investigated processes in all aspects. Future studies should further improve the drying processes and investigate the nonvolatile fraction of dried rosemary.