From Processing to Digestion: Polyphenols Stability, Antioxidant Activity, and Bioaccessibility in Onion Peel–Enriched Fresh Pasta

The valorization of agro-industrial by-products as sources of bioactive compounds represents a promising strat￾egy for developing sustainable functional foods. In this study, red onion peel powder (OPP), a polyphenol-rich by-product, was incorporated into fresh pasta at 3%, 6%, and 9% (w/w) to investigate the impact of cooking and in vitro gastrointestinal digestion on phenolic composition, antioxidant activity, bioaccessible polyphenol con￾tent (BPC), and polyphenol recovery during digestion, expressed as recovery index (RI). OPP showed a high total polyphenol content (3719 mg/100 g DW), mainly consisting of quercetin derivatives, anthocyanins, and proto￾catechuic acid. Pasta fortification significantly increased total polyphenol content in a dose-dependent manner, from 3.9 mg/100 g DW in control pasta to 534.4 mg/100 g DW in uncooked pasta enriched with 9% OPP, reach￾ing 608 mg/100 g DW after cooking. Cooking enhanced the extractable levels of several phenolic compounds, particularly quercetin aglycone and protocatechuic acid, while a substantial proportion of anthocyanins was re￾tained. Following in vitro digestion, BPC increased significantly, reaching 55.6, 104, and 134 mg/100 g DW in pasta enriched with 3%, 6%, and 9% OPP, respectively. The 6% formulation showed the highest RI (51.7%), indicating more efficient recovery of phenolic compounds after digestion, whereas the 9% formulation provided the highest amount of bioaccessible polyphenols. Phenolic compounds were also recovered in the residual intesti￾nal digesta, indicating distinct potential gastrointestinal fates. Antioxidant activity increased proportionally with polyphenol content. Overall, red onion peels appear to be a promising and sustainable ingredient for developing polyphenol-enriched pasta within a circular economy framework.