Seedless table grape residues as a source of polyphenols: comparison and optimization of non-conventional extraction techniques

Grape skins are one of the most important leftovers of grape juice production, and are also a good source of bioactive compounds, especially phenolic antioxidants and fiber, because they are not stressed as the winemaking process occurs. Their extracts may be used as functional components of enriched foods and beverage, both to color the products and to supplement with bio-functional metabolites. Therefore, in this work, ultrasound assisted extraction (UAE) and microwave assisted extraction (MAE) were optimized and compared using response surface methodology (RSM) and desirability function (D) statistical tools, at selected temperature and solvent type (close to 50 °C and water/ethanol/phosphoric acid 70:30:1) but varying contact time (t) and sample-to-solvent ratio (S/L), to find the best conditions for the extraction of the main polyphenols present in table grape skin (Apulia Rose cv.) residues from juice processing. The mathematical models built in this investigation showed that the highest significant factor (P < 0.001) was t, influencing the extraction of all compounds irrespective of the technique used, with the optimal results obtained at intermediate levels (10.5 and 21 min for MAE and UAE, respectively). On the contrary, the only S/L factor was not always significant, even though higher amount of polyphenols were generally recovered at low solid/liquid ratio (0.05 and 0.07 g/mL for MAE and UAE, respectively). Finally, UAE extracts exhibited higher content of anthocyanins, procyanidins, flavonols, and stilbenes than MAE, with values ranging from 1.5 to 69.6 mg/100 g of fresh weight.