Regulation of rheological properties of soy protein isolate-beeswax based bigel inks for high-precision 3D printing

Bigel inks were developed for high-precision 3D food printing using soy protein isolate (SPI) hydrogel and beeswax oleogel. The ratio of hydrogel to oleogel played a crucial role in determining the rheology and moisture state of the inks. When the ratio of hydrogel to oleogel reached 7:3, the ink was softer and showed stronger shear-thinning properties, which resulted in better recoverability and stronger self-supportability. These were demonstrated by the lowest flow index (n value) of 0.135, the largest recovery viscosity of 10.920 × 105 mPa s, and the biggest yield stress (1034.200 Pa), complex modulus (G* of 6608.066 Pa), and gel strength (A value of 488.34 × 103 Pa s1/z). Additionally, the ink contained more semi-solid water (T22) and had a higher proportion of moisture freedom (A23). Consequently, the ink had a smooth extrusion while exhibiting strong mechanical properties and better structure stability. As a result, a hollow-rotating structure was achieved with a height of 20.05 mm, a gap diameter of 10.11 mm, and an inclination angle of 46.13° with precision beyond 99%. This was attributed to the presence of more β′ and β crystals with high oleogel content, which might protect the completeness of the SPI hydrogel network structure and prevent the aggregation of oleogel particles. In this turn, it improved the microstructure of the SPI-based bigel inks and made them more resistant to shearing during extrusion. This work provides a new insight into the application of SPI in personalized diets.