Valorization of Aromatic Plant Distillation Residues: Phenolic Composition, Antioxidant Capacity, and Antimicrobial Activity of Rhododendron tomentosum Harmaja Extracts

Hydrodistillation of aromatic plants for essential oil production generates substantial amounts of solid and liquid residues that are commonly discarded despite their potential value as sources of bioactive compounds. In this study, the essential oil and post-distillation residues of Rhododendron tomentosum Harmaja were evaluated within a waste-to-value framework to recover phenolic compounds with antioxidant and antimicrobial properties. Dry extracts obtained from liquid (DEA) and solid (DEE) residues were characterized in terms of total phenolic and flavonoid contents, antioxidant capacity (DPPH assays), and antimicrobial activity against selected microorganisms. Quantitative HPLC–PDA analysis revealed multiple phenolic compounds. Extracts derived from solid residues exhibited significantly higher phenolic and flavonoid contents and stronger antioxidant activity than those obtained from liquid residues, indicating that solid by-products constitute a richer phenolic matrix. Antimicrobial assays revealed pronounced activity for extracts prepared from plant material harvested in October, particularly those based on propylene glycol and glycerin, which were effective against both Gram-positive bacteria and selected Gram-negative clinical isolates. The essential oil showed broad-spectrum antimicrobial activity, including inhibition of Aspergillus niger. Stability studies demonstrated that the phenolic composition and bioactivity of the dry extracts were largely preserved after one year of storage. These findings demonstrate that R. tomentosum hydrodistillation residues represent a promising source of natural antioxidants and antimicrobial agents, supporting their potential utilization as value-added ingredients in food and cosmetic applications and contributing to circular economy strategies.