Zero-Waste Hydrogel Design via Integral Biomass Valorization of Protein-Rich Spirulina Microalgae

Interest in alternative protein sources has grown, with Spirulina, a microalga belonging to the genus <i>Limnospira</i> (formerly <i>Arthrospira</i>), emerging as a key option. Guided by sustainability principles, this study explored the gelling capacity and hydrogel-forming properties of integral Spirulina biomass (SpB), targeting applications in structured foods. Two experimental designs (DoE) were employed. One to identify key factors influencing hydrogel formation, and another to optimize the formulation (22 wt%, pH 5.6, thermal gelation at 90 °C). Syneresis analysis revealed that high SpB hydrogels experienced less water loss, with the 22% sample losing just 2.51% after 14 days, due to its dense, particulate morphology as observed by means of scanning electron microscopy. Rheological analysis confirmed the optimized formulation's superior mechanical properties, with a storage modulus (G') 24-times higher than the low concentration reference sample (~1890 Pa), remaining dominant over the loss modulus (G″) (G' > G″) across the analysed frequency range, corroborating a strong elastic behaviour. Although the recovery tests showed partial recovery (27.1%) after high shear, the high residual stiffness (≈515 Pa) confirmed the material's ability to maintain its shape. These results enabled successful 3D printing tests with the optimized hydrogel, pointing out its potential for innovative food applications in structured food design.