Thermal evaluation of solar dryers and the effect on the antioxidant and some physicochemical properties of Agastache mexicana and Rosmarinus officinalis

Lemon balm (Agastache mexicana) and rosemary (Rosmarinus officinalis) were dried using a mixed-mode solar dryer. A 2(3)-factorial design was carried out: operation mode (mesh shade and direct irradiance), airflow (natural convection and forced convection), and type of flat plate solar collector cover (polycarbonate and glass). The drying kinetics of lemon balm and rosemary ranged from 4.5 to 6.5 h and 4 to 7 h, respectively, according to the operation mode of the solar dryer. The highest percentage of antioxidant activity in the 2,2-diphenyl-1-picrylhydrazil tests was lemon balm, with values between 88% and 93% (100 mu g/mL), and rosemary, 88% and 92% (100 mu g/mL). When forced convection was applied, solar collector thermal efficiency increased from 3.97%-5.11% to 17.20%-24.75%, dryer efficiencies ranged from 4.78% to 6.05%, and drying efficiencies between 14.17% and 44.23%. The lowest color difference (6.01) for lemon balm was shown with the mesh shade, forced convection, and glass cover collector, and for rosemary, the lowest color difference (12.87) resulted from using the mesh shade and natural convection; according to the analysis of variance, the cover collector did not affect significantly the color difference. Dehydrated lemon balm and rosemary could be used as an additive for medicinal, gastronomic, and food preservative applications.