Bigel formation through surfactant-free co-gelation-in-emulsion without thermal activation of the hydrogel: Expanding the toolbox for customizable, and food-grade fat replacers

Replacing saturated and trans fats with healthier alternatives remains a major challenge for the food industry. This study presents a simple method to fabricate food-grade bigels with improved structural and functional properties. A surfactant-free co-gelation-in-emulsion approach was applied, in which basic pullulan, sodium trimetaphosphate (STMP) solutions were emulsified with molten candelilla wax/extra virgin olive oil solutions under mild agitation. Hydro- and oleo-gelation ocurred simultaneously once emulsion was prepared, withput thermal activation of the hidrogel, directly forming the bigel. Bigels with hydrogel-to-oleogel ratios of 25:75, 50:50, and 75:25 were produced and compared with systems obtained by conventional high-shear mixing of the separately prepared hydro- and oleo-gels. Co-gelated bigels exhibited homogeneous phase distribution, solid-like rheology, and high oil-binding capacity (>80 % at 25 % hydrogel; no oil leakage at ≥50 %). When the hydrogel acted as the continuous phase (≥50 %), the systems displayed marked increases in elasticity (G′ > 6000 Pa), hardness (≈450–500 g), and swelling (up to 55 %), closely resembling pure hydrogels. In contrast, mixed bigels prepared by high shear mixing of single gels presented fragmented, heterogeneous structures with lower stability and weaker texture. FT-IR, WAXS, and confocal microscopy confirmed improved hydrogen bonding, maintenance of oleogel crystallinity, and more continuous networks in co-gelated samples. This thermally independent route of the hydriphilic domain enables the use of biopolymers with slow chemical cross-linking kinetics (such as pullulan-STMP) and yields bigels that are more homogeneous, mechanically stronger, and oil-retentive. Overall, the method broadens the possibilities of food-grade bigels as customizable fat replacers. • Bigels were prepared by co-gelation-in-emulsion without hydrogel thermal activation. • Method enables stable, elastic bigels as potential food-grade fat replacers. • Surfactant-free strategy broadens biopolymer use in bigel formulation. • Co-gelated bigels show higher oil retention and elasticity than mixed Systems with the same formation.