Insight into the composition, in vitro activities and in vivo safety and efficacy of hydrolates from Hyssopus officinalis, Lavandula × intermedia , and Thymus vulgaris

Hydrolates are aqueous by-products of essential oil distillation that contain volatile, water-soluble compounds with potential biological activity. Although their utilization in traditional medicine and cosmetics is increasing, their phytochemical profiles and biological effects remain insufficiently investigated. This study assessed the phytochemical composition, biological activities in vitro, and safety and efficacy in vivo of Hyssopus officinalis, Lavandula × intermedia, and Thymus vulgaris hydrolates. Phytochemical analysis revealed notable differences among the hydrolates investigated. Thyme hydrolate contained the highest levels of phenolic compounds (190.92±1.219 mg gallic acid equivalents per L), flavonoids (1.17±0.155 mg rutin equivalents per L), and phenylpropanoid derivatives (0.53±0.014% hydroxycinnamic acid per mL), along with the lowest amount of reducing sugars (199.22±3.078 mg glucose equivalents per L). It also exhibited the most potent antioxidant activity in the DPPH and β-carotene tests, with IC50 values of 6.28±0.08% v/v and 9.36±0.008% v/v, respectively. Additionally, thyme hydrolate exhibited antimicrobial activity against both Gram-positive and Gram-negative microorganisms, with a minimum inhibitory concentration of 50% (v/v). Lavandin hydrolate demonstrated the highest anti-inflammatory activity, inhibiting protein denaturation by 83.15±0.018% in the bovine serum albumin assay. The safety and efficacy of gel formulations were further evaluated on artificially irritated skin in healthy volunteers by measuring skin parameters: skin pH, transepidermal water loss and erythema index. All samples reduced skin redness over 7 days. Overall, the results validate the traditional use of hydrolates from T. vulgaris, L. × intermedia, and H. officinalis as promising ingredients with antioxidant, antimicrobial, and anti-inflammatory activities suitable for broader application.