Rethinking rheology–printability relationships in protein-based food inks: critical insights into gaps and inconsistencies in correlations

Numerous 3D food printing studies have explored rheology-printability relationships as a means of efficiently optimizing inks for printing. Consequently, correlations are widely used to support and evaluate these relationships in the literature. However, there is a clear lack of consensus among reported correlations, and generalizations are often accompanied by inaccuracies. This paper reviews research advances that defined rheology-printability correlations and relationships in protein-based inks. Inconsistencies in these correlations and their contributing factors are critically discussed. The key rheological properties and their relation to the three main stages of the printing process – extrudability, recovery, and deposition – are highlighted. Additionally, the analytical methods used to characterize rheological properties are reviewed. To mitigate these inconsistencies, strategies and approaches that could improve the application of correlations are proposed. Generalized rheology-printability correlations for protein-based inks are inconsistent across studies due to high variability in the ink components, the rheological methods applied, and the lack of standardization in printing setups and printability assessment methods. To mitigate these inconsistencies, several approaches are suggested. These include a shift toward rheological methods that better simulate the printing process, such as complementary rheometry protocols, investigation of printing phenomena, including over-extrusion, filament non-uniformities, and die swell. Furthermore, variations in printability due to different types of extruders used for printing should be accounted for to improve assessment accuracy. Collectively, this review proposes a more comprehensive and defect-inclusive analytical approach for improving the reliability of rheology–printability correlations in protein-based food inks. • Rheology–printability correlations in protein-based inks are often inconsistent. • Linear viscoelastic properties are insufficient for printability correlations. • Certain printing defects are overlooked in printability assessments. • The need for a standardized framework to guide rheology-printability correlations.