Rheology of edible food inks from 2D/3D/4D printing, and its role in future 5D/6D printing

Food printing involving 2D, 3D, 4D and 5D printing methods has attracted extensive attention owing to the growing living standards and higher consumer demand for innovative food. Printable biopolymers with a special structure hierarchy are particularly appealing to formulate edible inks for food printing as they serve as essential structural components for constructing ink matrix and providing a supportive presence for the printed functions. Rheology (e.g., viscosity, yield stress, temperature ramp & sweep, oscillatory modulus, creep-recovery and thixotropy by methodology including rotation tests, oscillatory tests and mathematic rheological models) is important to redefine the relationship between edible inks and printability. It is essential to promote the practical mass food printing application with high consumer acceptance. The rheological principles of edible inks that underpin a variety of printing techniques are discussed systematically including flowability, shearing dependence, thermal endurability and structure maintainability for pre-evaluating the potential printing effectiveness. The related rheology models indicate the capacity to further deepen the link between ink structure and printing efficiency for printing multi-functional food products. It is found that rheology lays the foundation for design, development and food application of edible food ink printing. This review also summarizes recent advances in 2D/3D/4D/5D printing of rheologically-stable inks in food application, including food decoration, food customization and food intelligence. Additionally, prospects (like 6D printing) and key challenges (rheology with multidisciplinary integration) for edible ink printing are proposed and addressed for creative food production.