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 strategy 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 content (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 protocatechuic 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, reaching 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 retained. 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 intestinal 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.