Optimization of Ultrasonic-Assisted Enzymatic Extraction, Purification, and Antioxidant Activity of Polyphenols from Almond Hull

Almond processing byproducts are rich in bioactive polyphenols but severely underutilized due to inefficient conventional extraction methods. This study presents the first systematic optimization of an integrated ultrasound-assisted enzymatic extraction and AB-8 macroporous resin purification process for almond hull polyphenols, addressing the limitations of low yield, high impurity content, and bioactivity loss in traditional approaches. Extraction parameters were optimized via single-factor experiments combined with Box–Behnken response surface methodology, while purification conditions were refined through static and dynamic adsorption–desorption tests. Structural characterization and antioxidant evaluation were performed using Ultraviolet-Visible Spectroscopy (UV-Vis), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and Ferric Reducing Antioxidant Power (FRAP) assays. Under optimal conditions, the polyphenol yield reached 23.67 mg/g. After purification, polyphenol purity increased 5.88-fold, flavonoid purity improved 4.62-fold, and DPPH/FRAP antioxidant activities were enhanced 5.0-fold and 6.5-fold, respectively. Purified polyphenols retained intact phenolic structures and exhibited a loose porous microstructure. This green process provides a technical basis for high-value utilization of almond hulls. Limitations include lack of polyphenol monomer identification, in vivo efficacy validation and industrial economic feasibility assessment.