Polyphenolic Content, Antioxidant and Antimicrobial Activity of Hydrosols from Some Aromatic Lamiaceae Plants ^†

Hydrosols, also named hydrolates or aromatic waters, are considered distillation by-products obtained during essential oil extraction from herbs or spices, which are usually discarded. However, they contain certain amounts of bioactive molecules, such as water-soluble components of the essential oils and other polar plant components with valuable biological activities [1,2]. This study aimed to evaluate the polyphenolic content, as well as the antioxidant and antimicrobial activity of three aromatic plant hydrosols obtained from Lavandula officinalis, Rosmarinus officinalis and Salvia officinalis, belonging to the Lamiaceae family, in order to develop new products for pathogenic microorganism control management. Aerial parts of lavender, rosemary and sage were subjected to steam distillation in an essential oil extraction equipment for 3 h. The resulting essential oils and the corresponding hydrosols were separately collected and stored in the dark at 4 °C. The hydrosols were analysed in terms of total phenolic and flavonoid content by the Folin–Ciocalteu assay and aluminium chloride colorimetric method, respectively [3]. The antioxidant activity of the hydrosols was evaluated using Trolox equivalent antioxidant capacity (TEAC) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical assays [3]. The hydrosols were also tested for their antimicrobial activity against the Gram-positive Staphylococcus aureus strain and Gram-negative Pseudomonas aeruginosa strain by agar disc diffusion assay. Sage hydrosol had the highest content of polyphenols, followed by rosemary and lavender hydrosols. A significant level of flavonoids was found in sage hydrosol, while rosemary and lavender samples contained similar amounts of flavonoids but lower than that of sage hydrosol. Rosemary and sage hydrosols exhibited the strongest antioxidant activity, as assessed by TEAC and DPPH assays. Antimicrobial tests showed that rosemary hydrosol exerted antimicrobial activity against S. aureus. Sage and lavender hydrosols were less effective against S. aureus growth. The three hydrosols did not show any inhibiting activity against P. aeruginosa. In conclusion, among the tested plant hydrosols, those obtained from sage and rosemary contained significant amounts of bioactive molecules, in terms of polyphenols, exhibiting at the same time good antioxidant activity. Rosemary hydrosol also showed antimicrobial activity against G-positive bacteria. All these results indicate rosemary hydrosol as a useful by-product that could be recycled to develop novel products for the prevention of plant diseases caused by microbiological spoilage.