Electrospinning has been used to produce edible fibrous mats from micro- or nanosized fibers of sodium caseinate (NaCAS) and calcium caseinate (CaCAS) with pullulan (PUL), a polysaccharide added to facilitate formation of the caseinate fibers. The effects of rheological properties on electrospun fiber morphology are known for many synthetic and water-soluble polymers; however, this information is not available for proteins in aqueous solution such as the milk proteins or in blends with polysaccharides, such as PUL. The objective of this study was to determine the dependence of specific viscosity on aqueous protein concentration, C, for neat NFDM, CaCAS and NaCAS and in blends with PUL, to identify the entanglement concentrations, Ce, the point at which the respective protein chains begin to interact, and to identify the C regions where fibers are formed. In this study, aqueous solutions of NFDM, CaCAS, NaCAS, and PUL were prepared at 20°C at C ranging from 1 to 20 wt% and then 1:1 blends of each protein and PUL solution were prepared with total C also ranging from 1 to 20 wt%. A syringe containing 3 mL of each solution was used to feed an electrospinning unit at flow rate of 3mL/h, and at voltage of 20 kV, with fibers forming a fibrous mat. From plots of specific viscosity as a function of C for the neat and blended protein solutions, 3 regions were identified: the semidilute unentangled (C < C*), a semidilute entangled (Ce > C > C*) and the concentrated entangled (C > Ce > C*). In general, electrospraying (drops) was observed for the blended solutions at C < C*; followed by semidilute entangled region of beaded fibers, and fully formed fibers just after the transition to Ce. Fully formed fibers were not identified at C > Ce for the neat protein solutions but formed powders since electrospraying behavior predominated over the C range due to the lower solution viscosities. This is the first detailed examination of the dependence of milk protein-based nanofiber morphology on the viscosity and concentration regimens.