Onion peel powder as an upcycled food ingredient: antioxidant, hydration, and soup application properties

Onion peel is a phenolic-rich byproduct with potential to be upcycled into a food ingredient. This study compared onion peel powder (OPP) with laboratory-prepared onion flesh powder (OFP) and commercial onion powder (COP) for total phenolic content (TPC), antioxidant capacity, hydration properties, color, and rheological characteristics in both aqueous dispersions and onion cream soup. OPP had markedly higher TPC (52.68 mg GAE/g), DPPH radical-scavenging activity (115.22 mg VCE/g), ABTS radical-scavenging activity (100.82 mg VCE/g), FRAP (25.45 mg FeSO4 eq/g), and quercetin content (60.42 μg/g) compared with COP and OFP. OPP also exhibited the highest water absorption index (WAI) and the lowest water solubility index (WSI), indicating strong water absorption with limited solubilization. All aqueous dispersions and cream soups showed shear-thinning behavior. Across the measured shear-rate range, OPP generally produced the highest apparent viscosity in both systems, suggesting that its peel-derived, poorly soluble particulate matrix increased water binding and flow resistance. However, OPP had the lowest L* and highest a* values and imparted a pronounced reddish-brown color to the soup, which warrants further investigation into concentration optimization and sensory acceptability. Overall, these findings support the potential of OPP as a multifunctional upcycled ingredient for phenolic enrichment, water binding, and viscosity modification.