Thermal stability and bioavailability of bioactive compounds after baking of bread enriched with different onion by-products

This study investigated the effects of the addition of onion waste fractions into gluten-free (GF) bread to promote its health benefits. 5% of the control (C) GF flour blend was replaced with three waste fractions in the form of: fried onion (FO), dried onion (DO) and onion peel (OP). Antioxidant activity, content of flavonols and total polyphenols of breads increased in the following order: C < FO < DO < OP. No differences were observed in sensory analysis. We found that quercetin glycosides, dimers and trimer in OP-bread, determined according to their mass spectra, decomposed during baking and released free quercetin, which points to their thermal instability. Cross-over study revealed that consumption of OP-bread significantly increased (p < 0.05) antioxidant activity of consumers' blood compared to control bread consumption, indicating good bioavailability of flavonols. Results suggest incorporation of OP into GF bread can increase its biological value with satisfactory sensory acceptance.