Effect of drying methods on mucilage, anthocyanin content, and antioxidant activity of black hollyhock (Alcea rosea var. nigra)

This study investigates the impact of different drying methods on the phytochemical composition, antioxidant activity, anthocyanin content, and mucilage percentage of Alcea rosea var. nigra. Drying techniques, including shade drying, sun drying, oven drying (40 °C and 60 °C), and microwave drying (540 W, 720 W and 900 W), were evaluated. The results demonstrated that shade drying preserved the highest levels of total phenol (171.75 mg GAE/g DW in flowers), flavonoids (68.97 mg RE/g DW in flowers), and antioxidant activity (59.61 µmol Fe(II)/g DW in flowers). However, it required the longest drying duration (up to 89 h for roots). Oven drying at 40 °C effectively retained phytochemicals while significantly reducing drying time. Microwave drying (540 W) offered the fastest drying process with acceptable retention of bioactive compounds, whereas higher microwave power (900 W) led to a decline in mucilage content. Overall, shade drying and low-temperature oven drying (40 °C) were the most effective methods for preserving bioactive compounds, while microwave drying provided a rapid alternative with some compromise in quality. These findings offer practical insights for optimizing post-harvest processing to enhance the pharmaceutical and nutritional value of A. rosea var. nigra.