Comparison of drying methods for the retention of phenolic antioxidants in post-distillation solid residues of aromatic plants

Abstract

This study is focused on valorizing solid waste materials (SWM) obtained after the essential oil distillation of four aromatic plants (oregano, satureja, rosemary and salvia) to provide functional ingredients rich in phenolic compounds, by applying microwave-heating (MW), infrared-radiation (IR), oven-dried heating (DH), sun-drying (SD), shade-drying (SHD) and freeze-drying (FD) methods. The effect of the drying method on the bioactive components of SWM was analyzed and compared to the raw materials (before distillation). MW showed the best results in preserving phenolic antioxidants compared to the other drying processes, whereas FD provided dried SWM with the lowest antioxidant activity. Rosmarinic acid was detected in all SWM, while carnosol and carnosic acid were identified in rosemary and salvia SWM. The amount of these bioactive compounds remains very high in the post-distillation residues, and is worth exploiting their valorization, after optimization of the thermal processes. Overall, MW-drying is recommended as an eco-friendly method, applicable to drying of SWM of aromatic plants for their subsequent use as a source of valuable bioactive compounds in the food and pharmaceutical industries.