The research proved that the extrusion process of acid and rennet technical casein had a significant effect on improving the microbiological purity of those protein preparations making them possible to meet the requirements under the relevant Polish Standard. During the extrusion, iron, copper, zinc, and tin partially moved into the extruded casein preparations and some interactions occurred between casein and low-molecular nitrogen compounds. The extruded casein formed a solution in a 0.5 % di-sodium phosphate that was twice as viscous as the solution of non-extruded casein. During the extrusion process, the compact and tight structure of casein was modified into a porous structure with an extended surface, and the acid casein demonstrated a higher susceptibility to structuralization compared to the rennet casein.