Sequential Pressurized and Supercritical Extraction Strategies for the Recovery of Phenolic Compounds and Grape Bagasse Valorization

Grape bagasse is an abundant agro-industrial by-product and an important source of phenolic compounds with antioxidant properties. This study evaluated Soxhlet extraction, supercritical fluid extraction (SFE), pressurized liquid extraction (PLE), subcritical water extraction (SWE), and sequential extraction strategies for recovering bioactive compounds from grape bagasse. Box–Behnken designs were applied to SFE and PLE to evaluate process effects on extraction yield, while total phenolic content (TPC), total anthocyanin content (TAC), and antioxidant activity (ABTS) were additionally determined for PLE extracts. Hydroethanolic extractions showed greater selectivity toward phenolic compounds, whereas water-based extractions promoted higher yields associated with additional polar constituents. In SWE, increasing temperature enhanced extraction yield and phenolic recovery, although anthocyanin contents decreased under more severe thermal conditions. SWE provided higher extraction yields than PLE with comparable phenolic content and antioxidant activity, suggesting the recovery of additional highly polar non-phenolic compounds, whereas PLE resulted in higher extraction yields than SFE. Sequential extraction demonstrated that the first step accounted for most of the phenolic recovery and antioxidant activity, while the second aqueous step increased overall extraction yield. The sequential PLE–SWE route resulted in the highest TPC (198.0 mg GAE g−1) and antioxidant activity (2321 μmol TE g−1), demonstrating the potential of sequential extraction for grape bagasse fractionation and valorization.