κ-carrageenan mediated microwave 3D printing of Spirulina: Realizing immediate curing through temperature-dependent phase transition

Spirulina, a nutrient rich microalga, is predominantly utilized in powdered form, limiting its direct role in structuring food matrices. This study developed a Spirulina based printable system by incorporating κ-carrageenan and applying microwave 3D printing to achieve rapid solidification and high fidelity shaping. Rheological results showed that κ-carrageenan not only ensured the shear-thinning behavior of the slurry at high temperatures, but also imparted a modulus response that strengthened with decreasing temperature, thereby facilitating smooth extrusion and deposition during microwave 3D printing. Successful printing of complex structures was achieved at κ-carrageenan contents of 2% or higher and temperatures above 60 °C. The textural properties of the printed products, including gel strength, hardness and gumminess, increased with κ-carrageenan content and printing temperature. Meanwhile, the compressive stress increased markedly with both parameters, with distinct yielding behavior emerging under high-temperature conditions with ≥2% κ-carrageenan. Low-field NMR and scanning electron microscopy revealed that microwave-induced synergistic crosslinking further enhanced water binding and network crosslinking density. Finally, numerical simulations revealed the coupling of viscosity, temperature, and shear fields during extrusion, elucidating the synergistic mechanism of κ-carrageenan and microwave heating on the instant solidification and surface fidelity of the printed products.