Effect of convective and vacuum drying on some physicochemical and phytochemical characteristics of peppermint leaves

This study examines the effects of convective air and vacuum drying at 40, 50, and 60 ℃ on the drying behavior, color, pigments, phenolic content, and antioxidant capacity of peppermint leaves. The drying data were modeled using eight drying models, with the Midilli model being the best fit for both drying methods with the highest R² (>0.99) and lowest values of χ² (<0.003) and root mean square error (RMSE) (<0.035). Results showed that convective drying at 60 ℃ had the highest drying rate (0.62 d.b./h) compared to vacuum drying (0.25 d.b./h) at the same drying temperature. Effective moisture diffusivity increased with the increase in drying temperature and ranged from 1.00 × 10⁻¹³ to 5.16 × 10⁻¹³ (m² s⁻¹). Activation energy ranged from 39.72 to 41.46 (kJ mol⁻¹). Furthermore, vacuum drying resulted in higher lightness and lower redness (a*) values than convective drying at higher temperatures. Both methods increased chlorophyll a and b contents, while β-carotene and phenolic contents significantly decreased, particularly at higher temperatures. This study highlights that both convective and vacuum drying methods affect the drying behavior and quality of peppermint leaves, with lower temperatures being more effective in preserving color and antioxidant properties. Future studies should focus on optimizing drying conditions to further enhance the retention of key bioactive compounds and explore the potential of other drying techniques for improved peppermint preservation.