Ultrasound-Assisted Convective Drying of Stinging Nettle (Urtica dioica L.): Kinetics, Energy Consumption and Quality Attributes

To extend shelf life and enjoy the aroma and bioactive properties of wild nettle all year long, a drying operation is needed. This research aimed to compare conventional drying with modern drying technology using ultrasound to investigate intensification of the process together with the quality of sensitive leafy herbs. The kinetic, energy, and selected quality attributes typical for dry products of plant origin were analyzed: color, water activity, and polyphenol retention. Drying tests on wild nettle were carried out at two temperatures of 50 and 70 °C, with and without airborne ultrasound (100 and 200 W). The results showed that the application of ultrasound contributed to higher drying rates throughout the moisture content range compared to convective drying at the same air temperature. The higher the ultrasound power, the higher the increase in drying rate and shorter drying time, confirming intensification of moisture diffusion due to ultrasound assistance. The higher temperature and ultrasound level strongly influenced the final appearance and antioxidant properties of dry nettle, leading to color change (dE00 = 8.2) and polyphenol degradation (36%). However, all of the analyzed drying variants reduced the risk of microbial development (aw = 0.444). In conclusion, ultrasound can be effectively applied to intensify the drying of nettle leaves by providing more favorable drying conditions, including a 53% reduction in drying time and a drying rate more than twice as fast as that of convective drying.