Structural reorganization of hydration-limited soft-matter: mechanistic and sustainability insights for micro-aqueous extraction of fish oil

Conventional lipid extraction from fish by-products typically relies on high water input and disruption by thermal or hydrolysis to break emulsions and release oil. This study introduces a fundamentally reversed mechanism by micro-aqueous extraction (MAE). This method induces oil release via limited hydration and reassembly of the soft matter without heat or dilution. This MAE approach first dehydrates fish by-products until moisture content reaching ~18%, reassembly forming a protein–lipid matrix of high internal phase emulsion gel (φ ~75%). Upon controlled micro-aqueous hydration and low-shear agitation, structured proteins aggregate and destabilize oil droplets and release entrapped oil. Compared to heat extraction (HE), MAE achieved higher oil recovery (94.3% vs. 76.4%), shortened processing time by 35%, and decreased oil total oxidation value (TOTOX 1.33 vs. 2.88). Life cycle assessment confirmed lower environmental burden with 62% reduction in CO₂ emission. The fish meal obtained by MAE retained superior emulsifying activity and structural integrity. Mechanistically, micro-aqueous hydration reduced α-helix content of proteins and decreased oil droplet stability, thereby promoting demulsification. In contrast, drying at 60 °C resulted in lower turbidity and smaller aggregation size of myofibrillar proteins compared to drying at 30 °C, which enhanced their interfacial stabilization. Rheological analysis further revealed that water-limited restructuring of the myofibrillar network played a critical role in controlling viscoelasticity (as reflected by G', G'') and extraction efficiency. This work reveals a counterintuitive pathway for oil recovery, governed by moisture-regulated soft-matter reorganization, which presents a low-energy, high-yield, and sustainable approach for fish by-product valorisation.