This study develop a relationship between protein structure, rheological properties and plasticization of Novatein. Novatein is a biomass-based thermoplastic in which the main constituent is blood meal, a highly aggregated protein-rich biopolymer, which is a by-product from the meat industry. However, its properties makes more challenging processing methods such as sheet extrusion limited. This is considered a rheology problem and is related to the protein’s structural characteristics that can be modified by using polyol plasticizers. Synchrotron-based FT-IR and XRD were used to explain the rheological performance. Blood meal’s protein structure is highly aggregated, consisting of up to 50% -sheets that do not melt into a fully amorphous state at suitable processing temperatures and the behaviour of Novatein was therefore considered to be closer to filled polymers and made sufficient plasticization of the amorphous fraction crucial. Increasing polyol content decreased extensional viscosity, but increased shear viscosity due to better flow development in the capillary. In other words, poor elongational properties of compositions without polyol led to flow behaviour closer to plug flow. Thus, with longer capillaries, the apparent shear viscosity of polyol plasticized samples (at the same water content) could become even higher, but raises the question whether fully developed flow is a desired property for processing Novatein. Plasticization was classified into primary and secondary plasticization. In primary plasticization, the plasticizer interacts directly with the protein network by replacing the protein’s hydrogen bonding sites with water. In secondary plasticization, the polymer network becomes saturated, leading to phase separation. Water provided the ability to form ideally mixed phases, explaining the applicability of the free volume-based plasticizer theory. The Novatein network consisted of protein-rich, plasticizer rich and an intermediate phase and the fractional composition and the relative magnitude of each phase was determined by using the Couchman-Karasz model. The role of the intermediate phase was found to make the biggest difference in plasticizer performance and behaved in accordance to the observed Elongational flow was dominated by primary plasticization of the protein-rich and intermediate phases whereas secondary plasticization played a significant role in the reduction of the shear viscosity. With a fundamental understanding of plasticization and rheology, significant process improvements were made by increasing temperature and combining different polyols, leading to efficient sheet extrusion.