Rheological behavior and 3D printing performance of high internal phase emulsions stabilized by Maillard-type hazelnut protein–polysaccharide conjugates

High internal phase emulsions (HIPEs) are structured, semi-solid systems with high dispersed-phase volume fractions and complex viscoelastic behavior, making them attractive for advanced food applications. In this study, Maillard-type hazelnut protein isolate (HPI) conjugates with gum Arabic (GA) or sodium alginate (SA) were used to stabilize highly concentrated emulsions and HIPEs formulated at different water:oil ratios (20:80–40:60, w/w) and conjugate concentrations (2.5–3.5%, w/w). All formulations exhibited PDI values ≤1 and negative ζ-potential values (−27 to −41 mV), indicating acceptable colloidal stability and droplet dispersion within the emulsion systems. Polysaccharide type markedly influenced emulsion structure, with HPI–SA formulations yielding smaller droplet sizes, whereas HPI–GA systems supported higher oil loadings. Steady-shear analysis showed that increasing oil fraction and conjugate concentration increased apparent viscosity and consistency in both systems, with HPI–SA formulations exhibiting higher viscosity levels. In contrast, HPI–GA systems showed stronger shear sensitivity (lower n), indicating a more shear-responsive structure favorable for extrusion. Frequency sweep analysis further revealed that HPI–GA–stabilized systems exhibited a more elastic and weakly frequency-dependent response, whereas HPI–SA formulations showed greater frequency sensitivity. Lower shift factor (α_SF) values for HPI–GA–stabilized HIPEs indicated a stronger deviation from the Cox–Merz rule, reflecting a more elastic and structurally organized network. These rheological differences were reflected in extrusion-based 3D printing performance, with HPI–GA conjugates favoring cohesive, gel-like HIPE networks, while HPI–SA conjugates primarily contributed to droplet-level stabilization via reduced droplet size and enhanced electrostatic repulsion.