Dehydration-induced 4D shape transformation in 3D printed egg yolk gels: Insights into the coupling between water redistribution and network restructuring.

The swift advancement of food 3D/4D printing requires inks with suitable rheological and structural properties. This research used calcium ions to improve the printability of salt-induced egg yolk gels, and subsequent dehydration enabled 4D transformations. Rheological analysis revealed that the viscosity of these gels increased from 107.4 Pa·s (control, without calcium chloride, all samples contained the same NaCl concentration) to 483.8 Pa·s (Ca-4, shear rate = 1 s-1), which enhanced extrusion and structural stability. Complementary low-field nuclear magnetic resonance and scanning electron microscopy results indicated that calcium ions restricted water mobility by the promoting conversion of free water to bound water and bound to immobilized water while also inducing protein conformational rearrangement and lipoprotein disassembly, thereby yielding denser gel networks. During dehydration, water redistribution and network contraction generate internal stress, which triggers the observed 4D deformation. These mechanistic insights reveal how calcium ions and drying jointly regulate the water state and gel architecture, providing a basis for controllable 4D printing effects. This work expands the functional diversity of egg-based products and underscores the potential of 3D/4D printing for smart food design and production.