Exploring the Influence of Extraction Methods, Solvents, and Temperature on Total Phenolic Recovery and Antioxidant Capacity in Olive Leaf Extracts: A Systematic Review with Quantitative Synthesis

Abstract

Background: Olive leaves are a rich source of bioactive phenolic compounds, but extraction yields vary depending on methodological choices. The aim was to identify optimal parameters for maximizing recovery and preserving antioxidant activity. Methods: Fourteen studies (149 samples) were included, following predefined eligibility criteria and PRISMA guidelines for systematic review. Data on TPC, TFC, and antioxidant assays (DPPH, FRAP, ABTS) were extracted and analyzed according to extraction method, solvent type, and processing conditions. Results: Soxhlet extraction and shaking achieved the highest TPC and antioxidant capacity, whereas ultrasound-assisted and high-voltage electrical discharge extractions showed lower averages unless intensity or duration was increased. Solvent polarity was critical: ≥75% aqueous methanol provided the highest TPC and FRAP, while ≥75% ethanol yielded the greatest TFC and ABTS activity. Pure water consistently gave the lowest yields. Extractions at >50 °C increased TPC up to fivefold compared to room temperature but did not proportionally improve radical-scavenging capacity. Most phenolic compounds were recovered within ≤1 h under optimized, heated, or assisted conditions, with longer times offering no significant advantage. Conclusions: Optimizing solvent composition, temperature, and extraction time is essential for maximizing yield and maintaining antioxidant quality in olive leaf extracts, and standardized protocols are needed to enable direct comparisons across studies.