The effect of various drying strategies on the greenness, chlorophyll, bioactive compounds, antioxidant activity, and anti-tyrosinase of dried Acanthus ilicifolius L. leaves

Abstract

The most essential process of Acanthus ilicifolius L. tea production is drying. The current convective drying method (e.g., hot-air drying) of producing dried A. ilicifolius is inadequate due to its high energy consumption and long drying time, thus leading to undesirable final product quality. The objective of the present work was therefore to evaluate how the different standard drying methods and conditions namely hot-air drying (HAD), microwave drying (MWD), microwave-vacuum drying (MWVD), and freeze drying (FD) affect the greenness, chlorophyll content, antioxidant content, antioxidant activity, and anti-tyrosinase activity of A. ilicifolius leaves. MWD increased the drying rate of A. ilicifolius leaves by 6.7 times as compared to HAD. The logarithmic model best described moisture evolution during MWD and MWVD. The coefficient of diffusivity for MWD and MWVD was 10 times higher than that of HAD. The greenness of samples dried using HAD, MWD, and MWVD was 58.6 - 78.9, 51.7 - 73.9, and 56.4 - 68.8% lower than that of FD, respectively. As compared to HAD at 80°C, the greenness of the sample dried using MWD and MWVD at 700 W was 16.63 and 5.18% higher, respectively. MWD at 700 W was the best condition for minimising decomposition of the flavonoids (26.0%) and antioxidant activity namely DPPH assay (27.9%), FRAP scavenging (37.1%), reducing power (29.8%), Fe2+ chelating ability (22.9%), and Fe2+-1,10-Phenanthroline reducing (33.6%) of these medicinal leaves as compared to FD. Chlorophyll content, total phenolics, ABTS assay, and tyrosinase enzyme inhibition of leaves dried at MWD 700 W were not significantly different from those of the FD sample.