Transforming aquatic processing by-products into multifunctional food structures can support the development of sustainable and nutrition-oriented blue foods. In this study, Pangasius belly oil was structured into a multiphase bigel by coupling a glycerol-monostearate oleogel with an iron-loaded protein–κ-carrageenan hydrogel. Hydrogel-to-oleogel ratios ranging from 30:70 to 70:30 were examined to establish how phase organization influenced oil retention, viscoelasticity, thermal behavior, microstructure, and iron delivery under simulated gastrointestinal conditions. The bigel formulations retained more than 98% of the incorporated oil. Among the formulations, the 50:50 system formed the most uniform bicontinuous-like architecture and provided a balanced combination of structural stability and spreadable texture. Approximately 65.74% of the incorporated iron was released during the eight-hour gastrointestinal simulation. Its release profile was best described by the Korsmeyer–Peppas model (R² = 0.9941), demonstrating that iron mobility could be modulated through the multiphase matrix. The selected bigel was subsequently incorporated into an Ulva-enriched seaweed mayonnaise. Evaluation involving 100 naïve panelists showed acceptable appearance, homogeneity, texture, and spreadability, with overall responses ranging from “like” to “like very much.” The findings demonstrate that higher-order organization of aqueous and lipid networks can convert Pangasius by-product oil into blue soft matter that simultaneously supports lipid structuring, iron delivery, and the development of value-added Ulva-based foods.

Keywords: blue soft matter; multiphase structuring; Pangasius by-product oil; bigel; Ulva; iron delivery