<p>Aggregated emulsion gels composed of soy protein isolate (SPI) and soybean oil have been developed as structured fat inks for extrusion-based 3D food printing. The formulation was designed to mimic adipose tissue by generating oil-in-protein microstructures with the hypothesis that the SPI/oil ratio governs the balance between interfacial stabilization and gelation, enabling extrusion-compatible rheology with shear-thinning flow and rapid recovery. The screening of various plant proteins (soy protein isolate, soy protein concentrate, pea protein isolate, faba bean protein isolate, and whey protein isolate) demonstrated that SPI produced the most stable high-oil gels, enabling mechanistic evaluations. Rheological tests revealed that the SPI/oil ratio governs the interplay between interfacial stabilization and bulk gelation, resulting in emulsification- or gelation-dominated regimes. Among the formulations, 6% SPI/70% oil exhibited the best overall performance, combining fine droplet dispersion with low dimensional error and clear edge/layer contours. Confocal laser scanning microscopy confirmed stable droplet structures before and after extrusion, whereas a higher SPI content promoted droplet bridging and over-gelation. Overall, the results demonstrated that the SPI/oil ratio governed the emulsification–gelation balance and defined the compositional criteria for designing robust printable fat-mimetic inks for 3D food printing.</p>