Brown onion peel as a functional ingredient: antioxidant and anti-inflammatory potential after in vitro digestion

Onion peel is an abundant byproduct of the food industry with high content of bioactive compounds, so its valorization is of interest. In this work, the objective was to evaluate the bioaccessibility and the biological activity of breads supplemented with 7% of brown onion peel (BOP). BOP and breads (with and without the byproduct) were subjected to in vitro simulation of digestion by INFOGEST protocol, obtaining after centrifugation the bioaccessible (supernatant) and colonic (precipitate) fractions (BF and CF, respectively). BF and CF antioxidant capacity was evaluated by total phenolic content (TPC, Folin reaction) and ABTS assays. Intracellular antioxidant activity was performed by measuring reactive oxygen species (ROS) in intestinal Caco-2 cells and anti-inflammatory activity by inhibition of NO production in RAW264.7 macrophages.

BOP BF and CF exhibited high TPC (72.2 and 56.7 mgGAE/g) and ABTS [656.9 and 472.1 µmol Trolox equivalents (TE)/g] values, respectively. Regarding onion peel-enriched bread (OPEB), BF and CF showed higher TPC compared to the control bread (5.5 and 3.7 mgGAE/g, respectively, p<0.05). Similarly, ABTS values were higher for OPEB (102.9 and 270.1 µmolTE/g in the respective fractions, p<0.05). Cell assays were performed with concentrations of BF and CF up to 0.5 mg/mL of BOP and OPEB as no cytotoxicity was shown by MTT. Intracellular ROS levels were reduced at concentrations above 0.05 mg/mL for BOP fractions compared to AAPH control (p<0.05). In the same line, OPEB showed inhibition of intracellular ROS levels compared to the AAPH control, whereas control bread showed no inhibition. Furthermore, the BF and CF of BOP and breads inhibited the production of nitric oxide when compared to LPS control (p<0.05).

In conclusion, OPEB exhibited enhanced antioxidant properties and showed potential biological activity after in vitro digestion, highlighting its potential as an ingredient for the development of sustainable functional foods.