Egg white protein‑sodium alginate smart labels based on pH-driven method and nonphase change 3D printing in mackerel freshness response.

The smart labels prepared via the casting method and molten 3D printing method have a long heating time at high temperature and a dense network structure, resulting in a decrease in the color response ability of the labels. Therefore, this study uses a nonphase change foam 3D printing method with a shorter heating time to improve the color sensitivity of smart labels. By the pH driving method, the blending and pregelation of sodium alginate (Alg) can extend the drainage time of the interfacial film to the maximum extent, thus further improving the foam stability of egg white protein (EWP) and endowing the interfacial adsorption layer with better flexibility and fluidity. The pregelled Alg-EWP foam has good 3D printing adaptability, shows obvious shear thinning behavior, and has excellent shear recovery and creep recovery properties. The 3D-printed smart label has significantly higher swelling rate (275 % → 400 %), porosity (19.86 % → 42.86 %) and phenolic retention rate (55.51 % → 97.26 %). In addition, the sensitivity of the smart labels prepared via foam 3D printing significantly increased, indicating mackerel freshness. Therefore, the method of nonphase-change foam 3D printing provides a new strategy for preparing smart labels with increased porosity and color sensitivity.