Deep eutectic solvents-synergistic ultrasonic-assisted extraction of polyphenols from flaxseed (Linum usitatissimum L.) meal: Optimization, mechanisms, in vitro activity, and structural characterization.

Flaxseed meal (FSM), a nutrient-rich by-product of oil extraction, was valorized using an ultrasound-assisted deep eutectic solvents (UAE-DES) system. Among nine deep eutectic solvents with varying hydrogen bond donors and two conventional solvents (70 % ethanol and water), choline chloride-fructose (Chcl-D-Fru) exhibited the highest extraction efficiency. Under optimized conditions (30 % water content, 40 min of ultrasonication at 60 °C, and a solid-to-liquid ratio of 1:50), the yields of free phenolics (FP) and bound phenolics (BP) reached 680.19 and 499.05 μg/g, respectively, significantly surpassing those obtained from conventional solvents (p < 0.05). The extract demonstrated potent bioactivities, including DPPH (99.31 % for BP) and ABTS (99.61 % for FP) radical scavenging activities, and high inhibition rates of α-glucosidase (69.12 %) and α-amylase (75.73 % for BP). The UPLCI-MS/MS analysis identified 43 phenolic compounds, with vanillic acid, p-hydroxycinnamic acid, and syringic acid being most abundant. FTIR and DSC analyses confirmed the formation of DES and its interaction with polyphenols, which was enhanced by ultrasound. SEM revealed that the synergy between DES and ultrasound critically disrupted the microstructure of FSM. Ethyl acetate was identified as the optimal antisolvent, achieving recovery rates exceeding 70 % for FP and over 60 % for BP. Moreover, the DES demonstrates promising potential for cyclic utilization, maintaining approximately 50 % of its extraction efficiency after five reuse cycles. The decline in extraction rate is primarily attributed to impurity accumulation and weakened hydrogen bonding. This study establishes UAE-DES with ChCl-D-Fru as a novel and sustainable technology for converting FSM into high-value, bioactive ingredients.