Optimization of the Extraction Methodology of Grape Pomace Polyphenols for Food Applications

This study aims to take advantage of the wine industry by-products and extract bioactive compounds from grape pomace by applying methodologies susceptible to be integrated easily into industrial workflows because of the association with standard instrumentation and facilities, while the main factors affecting the efficiency of the process have been optimized. The sampling consisted of two grape varieties: 'Touriga Nacional' and 'Sousão'. A response surface methodology (RSM) method was used to optimize the extraction conditions based on three independent variables according to the chemical characteristics and stability/lability traits associated with polyphenols; the main bioactive phytochemical in grape pomace: solvent (50%, 70%, and 90% ethanol); temperature (20 °C, 40 °C, and 60 °C); and pH (0.5% HCl, 2% HCl, and 3.5% HCl). The phytochemical profile, as well as the radical scavenging and reducing powers were determined on 27 different samples. The highest yield and antioxidant activity corresponded to extracts obtained at 60 °C using 3.5% HCl and 70% ethanol. The values for total phenols and flavonoids were 44.93 mg of gallic acid equivalents (GAE) and 22.95 mg of catechins equivalents (CE) per gram, respectively. Concerning the evaluation of antioxidant capacity using various assays such as ABTS, DPPH, and FRAP, the results obtained were 0.30, 0.43, and 0.36 mmol of Trolox equivalent antioxidant capacity (TEAC) per gram, correspondingly. The analysis of the extract obtained with the best extraction performance using these parameters via High-Performance Liquid Chromatography-Mass Spectrometry has been also performed, allowing us to identify fourteen (14) compounds, including phenolic acids (n = 3), flavonols (n = 7), and anthocyanins (n = 4). As a result of this process, the best conditions for the production of a natural and environmentally friendly dye, not only avoiding waste but also reusing these by-products, were achieved.