Rheological Properties and Structural Stability of Sodium Caseinate-Based Inks Reinforced with Pectin and κ-Carrageenan for 6-Axis Robotic 3D Printing

Formulating printable and structurally stable edible inks is a challenge in 3D food printing. This study investigated the effects of combined pectin and κ-carrageenan concentrations on skim milk and sodium caseinate-based inks. Three formulations with varying polysaccharide complex (PC) concentrations (Low-PC, Med-PC, and High-PC) were characterized using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), rheological tests, and texture profile analysis (TPA) followed by six-axis robotic printing. Higher PC concentrations produced more compact freeze-dried matrices and increased rheological strength and textural hardness. FTIR spectral changes suggested possible non-covalent interactions. All formulations exhibited shear-thinning behavior, while High-PC showed the highest mean structural recovery, although the difference between High-PC and Med-PC was not statistically significant (p > 0.05). During printing of a star geometry, Low-PC spread beyond the target area, whereas High-PC produced smaller structures. Med-PC achieved the closest agreement with the target area (96.16% area-based shape fidelity). After 2 h of immersion, swelling ratios were 267.81%, 313.81%, and 285.12% for Low-PC, Med-PC, and High-PC, respectively, without statistically significant differences (p > 0.05). After 5 h, High-PC retained a star-like structure, whereas Low-PC and Med-PC largely lost their printed geometries. These findings highlight a trade-off between printing accuracy and post-printing structural stability.