Optimizing Ethanol-Water Cosolvent Systems for Green Supercritical Carbon Dioxide Extraction of Muscadine Grape Pomace Polyphenols

Abstract

This study evaluated the ethanol-water modified (50%, v/v) supercritical carbon dioxide (SC-CO₂) for the extraction of polyphenols from muscadine grape (Vitis rotundifolia Michgx.) pomace and compared with conventional solvent extractions (ethanol-water and HCl-methanol). The process was optimized with a central composite response surface design consisting of three levels of three independent variables: pressure (20-40 MPa), temperature (40-60 °C), and cosolvent concentration (5-15%) to maximize three responses: total phenolic content (TPC), total flavonoid content (TFC), and resveratrol yields. The optimal conditions were determined as 20 MPa, 60 °C, and 15% cosolvent concentration with TPC, TFC, and resveratrol yields of 2491 mg/100 g, 674 mg/100 g, and 1.07 mg/100 g, respectively. The surface plots indicated that a 15% cosolvent concentration maximized extraction efficiency, producing red-brown colored extracts. In contrast, a 5% cosolvent resulted in poor extractions, yielding yellow-green extracts under all conditions. The yields increased with higher temperatures (i.e., 60 °C) and lower pressures (i.e., 20 MPa). TPC and TFC obtained through cosolvent-modified SC-CO₂ were similar to those obtained through conventional extractions. Moreover, the resveratrol yield was lower than the HCl-methanol extraction, even though it was not different from any ethanol-water extractions at any solvent-to-solute ratios. The analysis of antioxidants indicated that the ABTS values of the cosolvent-modified SC-CO₂ extract were lower than those of the HCl-methanol extract. However, there were no significant differences in the DPPH values between the two extracts. Thus, this study optimized the sustainable technology of SC-CO₂ extraction by employing only food-grade cosolvents-ethanol and water-as a more environmentally friendly method for isolating polyphenols from the underutilized waste product of muscadine grape pomace utilizing statistical methodologies in the extraction process.

Figure 1

Pictures of the extracts obtained through ethanol–water cosolvent-modified supercritical carbon dioxide (SC-CO₂) at the designed conditions and conventional extractions performed with different solvents (E1, E2, and E3: 50% ethanol–water (v/v), M1: HCl–methanol) at different solvent-to-solute ratios (E1: 5/1, E2: 10:1, E3: 15/1, and M1: 15/1 v/w).

Figure 2

Response surface plots of the TPC: total phenolic content (A–C); TFC: total flavonoid content (D–F); and resveratrol yields (G–I). Graphics A, D, G, were drawn at 10% cosolvent concentration; B, E, H at 50 °C; and C, F, I at 30 MPa). MGP: Muscadine grape pomace (on dry basis).