Optimization of Conventional and Ultrasound-Assisted Extraction to Maximize Recovery of Total Phenolic Content and In Vitro Antioxidant Activity from Crataegus almaatensis Leaves

Background: Crataegus almaatensis, an endemic hawthorn species from Kazakhstan, is known for its rich content of phenolic compounds and flavonoids with significant pharmacological potential. This study aimed to optimize and compare conventional solid–liquid extraction (SLE) and ultrasound-assisted extraction (UAE) processes for maximizing the extractability of bioactive compounds from hawthorn leaves powder. Methods: The effects of temperature, extraction time, ethanol concentration, and solid-to-liquid ratio (or ultrasound power in the case of UAE) on total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity (FRAP, DPPH, and ABTS assays) were systematically evaluated. Results: The UAE method yielded higher concentrations of TPC and TFC, with up to 16% improvement in TPC and reduced ethanol usage (40% (v/v)) compared to SLE (75% (v/v)), demonstrating its efficiency and sustainability. Optimal extraction conditions were identified as 70 °C, 75% ethanol, 34 min, and an S/L ratio of 0.05 g/mL for SLE, 70 °C, 40% ethanol, 44 min, and 100 W US power for UAE. High-resolution HPLC-DAD and LC-Q/TOF-MS analyses confirmed the presence of key phenolic acids and flavonoid glycosides, including chlorogenic acid and apigenin-8-C-glucoside-2′-rhamnoside as the most abundant compounds identified. Conclusions: These findings validate UAE as an innovative, eco-friendly method for extracting bioactive compounds from hawthorn leaves and highlight its potential for developing natural antioxidants for pharmaceutical and nutraceutical applications.