Valorization of Grape Seed By-Products Using Subcritical Water Extraction: A Sustainable Approach for Bioactive Compound Recovery

Grape seeds are a major by-product of the winemaking industry and a great source of bioactive compounds such as polyphenols and proteins. These compounds have a wide range of applications including those in nutraceutical products and cosmetics and within the wine industry itself. Subcritical water extraction (SWE) was explored as a global method to valorize grape seed by-products for their different bioactive compounds in the context of waste valorization, green chemistry (solvent-free extraction), and circular economy. A Box–Behnken design was applied to generate mathematical responses and the ANOVA analysis determined the optimal extraction conditions (pressure, temperature, and time of extraction) for different responses such as total polyphenol content (TPC), antioxidant activity (AA), and total protein (Tprot). Extraction temperature was found to be the most significant factor influencing all responses while pressure had no significant impact on them. Optimal conditions were derived from the mathematical models for each response. For polyphenol extraction, the optimal conditions were as follows: 170 °C and 20 bar for 39 min with 288 mg GAE/g DM. To achieve the highest AA, SWE parameters should be set at 165 °C and 20 bar for 51 min with 332 mg TROLOX/g DM. For the extraction of proteins, it is necessary to work at 105 °C and 20 bar for 10 min (78 mg BSA/g DM) to preserve protein functionality. In comparison, conventional solvent extraction was unable to outperform SWE with values under the SWE results. Given the high content of polyphenols found in the extracts, an HPLC analysis was conducted. The following compounds were detected and quantified: protocatechuic acid (7.75 mg/g extract), gallic acid (6.63 mg/g extract), delphinidin chloride (1.44 mg/g extract), catechin (0.36 mg/g extract), gentisic acid (0.197 mg/g extract), and some epicatechin (0.07 mg/g extract). Additionally, Maillard reaction products (MRPs) were detected at high temperatures, with 5-hydroxymethylfurfural (5-HMF) appearing in extracts processed at 165 °C and above. The presence of MRPs, known for their antioxidant and bioactive properties, may have contributed to the increased AA observed in these extracts. These findings are significant because a solvent-free extraction process like SWE offers a sustainable approach to repurposing winemaking by-products, with potential applications in the wine and food industries.