Valorization of Onion-Processing Waste: Digestive Fate, Bioavailability, and Cellular Antioxidant Properties of Red and Yellow Peels Polyphenols

Onion (Allium cepa L.) peels represent a major agro-industrial by-product and are a rich source of polyphenols, with recognized antioxidant properties. This study compared the polyphenolic profile of two onion cultivars peels: red “Rossa di Tropea” and yellow “Recas”. Their digestive stability, intestinal bioavailability, and antioxidant activity were evaluated. Hydroalcoholic extracts were characterized by HPLC-DAD, subjected to a static gastrointestinal digestion model, and assessed for transport across differentiated Caco-2 monolayers. Antioxidant properties were determined using DPPH, FRAP, Cellular An- tioxidant Activity (CAA), and intracellular glutathione (GSH) assays. Red peels contained a higher total polyphenol content (28.44 mg/g DW) than yellow peels (15.61 mg/g DW), including anthocyanins uniquely present in the red cultivar. Digestive stability varied markedly between cultivars, with yellow peels showing greater intestinal recovery (72.7%) than red peels (49.1%). Glycosylated flavonols were more stable and exhibited moderate intestinal transport (Papp = 1.1–9.9 ×10−6 cm·s−1), whereas quercetin aglycone showed low permeability. Red peel extracts demonstrated stronger chemical antioxidant activity, while yellow peels were more effective in cell-based assays, displaying higher CAA values and inducing a pronounced increase in intracellular GSH. Overall, onion peel extracts exhibit promising antioxidant and biological properties. However, their limited bioavail- ability highlights the need for formulation strategies to enhance gastrointestinal stability and intestinal uptake, supporting their potential use as sustainable functional ingredients.