Structure-Function Relationships of Bigels as Animal Fat Replacers: Phase Inversion-Induced Structural Diversity and...

Animal fat-based triacylglycerols contribute unique texture to foods, but their excessive intake poses health risks. Therefore, developing fat replacers that can mimic the plasticity and melting properties of animal triacylglycerols has emerged as a key challenge in soft matter research. In this context, bigels are promising fat replacers due to their comparable thermal and rheological properties. However, they undergo phase inversion modulated by the oil-to-water ratio and gelator concentration/type, which determines their final microstructure, namely oil-in-water (O/W), bi-continuous, and water-in-oil (W/O). Clarifying the structure-function relationship is critical for designing bigels as animal fat replacers. This review presents the formation of bigels, while also indicating the factors that control their phase inversion. Moreover, the physicochemical properties, long-term stability, and in vitro digestion behavior of phase inversion-induced bigels are compared, and current applications of these bigels are presented. Finally, this review summarizes promising research directions for developing bigels as advanced fat replacers. The key conclusions are as follows. First, hydrogelator assembly is driven by intermolecular interactions (e.g., hydrogen bonding and hydrophobic association). The structure and mechanical strength are determined by oleogelator crystallization and 3D network formation. The interface between the two phases is stabilized by emulsifiers. Second, with phase inversion induced by the oil-to-water ratio, W/O systems exhibit enhanced mechanical properties and low free fatty acid release, while O/W systems exhibit superior oxidative and freeze-thaw stability. Finally, W/O systems are predicted to be suitable for meat products, O/W systems for bakery products, and bi-continuous systems for 3D-printed foods.