Eco-Efficient Ultrasound-Assisted Extraction of Polyphenols from Date Palm Kernels Using Natural Deep Eutectic Solvents: Antioxidant and Antibacterial Activities

Abstract

This study optimized the extraction of polyphenols from Phoenix dactylifera L. kernels by combining hydrated natural deep eutectic solvents (NADESs) with sonotrode ultrasound-assisted extraction (S-UAE), aiming to obtain a ready-to-use antioxidant and antimicrobial liquid extract. A sequential optimization strategy based on Box–Behnken designs identified the solvent-to-solid ratio as the main extraction driver. The optimal conditions were 55% hydrated NADES, a 5 mL/g solvent-to-solid ratio, 60% amplitude, 170 J/mL specific energy, and an 80% duty cycle. Under these conditions, experimental concentrations reached 309.78 mg/L total polyphenols and 265.81 mg/L flavan-3-ols, the predominant phenolic family. Compared with conventional agitation extraction, optimized S-UAE increased total polyphenols by 65.78%, flavan-3-ols by 68.65%, hydroxycinnamic acids by 48.47%, and flavonols by 35.05%. Antioxidant activity was markedly enhanced, reaching 15.47 and 14.70 mg Trolox eq./L in DPPH and FRAP assays, respectively, representing approximately 16-fold and 9-fold improvements over agitation extraction. The extract also showed antibacterial activity, producing inhibition zones of 16.0 mm against Bacillus sp. and 12.0 mm against Pseudomonas fluorescens, while complete inhibition was achieved against several strains in broth assays. These findings demonstrate that hydrated NADESs coupled with S-UAE is an efficient and scalable green strategy for valorizing date kernels into multifunctional polyphenol-rich food additives.