A Comprehensive Analysis of Non-Thermal Ultrasonic-Assisted Extraction of Bioactive Compounds from Citrus Peel Waste Through a One-Factor-at-a-Time Approach

Food waste presents a critical environmental and economic challenge across Europe. In the Mediterranean region, the agricultural industry generates considerable quantities of citrus fruits, leading to significant byproduct waste, which remains underutilized. To help address this, this study explored the valorization of orange peel waste using non-thermal ultrasonic-assisted extraction (UAE) and a one-factor-at-a-time experimental design to investigate the effects of nine chemical and physical UAE parameters. The goal was to identify ideal operational ranges for each parameter using several responses (bioactive compound recovery, antioxidant activity, and radical scavenging activity), thus elucidating the most influential UAE factors and their role in co-extracting various classes of natural compounds. The key findings revealed that the polarity and ionic potential of the extraction medium, tuned through ethanol:water or pH, significantly influenced both the chemical profile and bioactivity of the extracts. Notably, citric acid and citrates appeared to stabilize co-extracted compounds. Lower solid-to-liquid ratios increased yields, while particle sizes between 1400 and 710 µm enhanced phenolic recovery by approximately 150 mg/L GAE. In contrast, increases in pulse, probe diameter, immersion depth, and extraction time led to degradation of bioactive compounds, whereas the maximal amplitude improved phenolic acid recovery by up to 2-fold. Collectively, these insights provide a foundation for optimizing non-thermal UAE to valorize orange peel waste.