Microwave-Triggered 4D Automatic Color Change in 3D-Printed Food Materials Incorporating Natural Pigments

The feasibility of using microwaves to quickly stimulate automatic color change in 3D-printed food containing curcumin or anthocyanins was studied. Firstly, with a dual-nozzle 3D printer, stacked structures included mashed potatoes (MPs, upper part, containing anthocyanins) and lemon juice-starch gel (LJSG, lower part) were 3D-printed and post-treated using a microwave. The results indicated that the viscosity and gel strength (indicated by the elastic modulus (G') and complex modulus (G*)) of LJSG were improved with the increase in starch concentration, while water mobility was reduced. During microwave post-treatment, the color change speed was negatively correlated with the gel strength but positively correlated with the diffusion of H⁺ and anthocyanin concentration. Secondly, nested structures were 3D-printed using MPs containing curcumin emulsion and baking soda (NaHCO₃). During microwave post-treatment, the curcumin emulsion structure was destroyed, and NaHCO₃ was decomposed, along with an increase in alkalinity; thus, the automatic color change was achieved with the automated presentation of hidden information. This study suggests that 4D printing could enable the creation of colorful and attractive food structures using a household microwave oven, leading to more imaginative solutions regarding personalized foods, which may be particularly important to people with poor appetites.