The product and process design of protein composite materials continuously produced by reactive extrusion are studied in detail. A suitable process design and operating window are presented. Potassium nitrate (KNO3) was used as a model additive and as a tool for modifying the physiochemical properties of the extruded protein films. The crystalline structures were identified and correlated to the tensile properties of the films. Uniformly distributed trigonal phase-III KNO3 crystals were observed within the film, which led to significantly higher elongation values of the films. Appropriate downstream operations are able to prevent the undesired polymorph transition to the orthorhombic phase II of KNO3 which would strongly reduce the mechanical performance.