Effect of cross-linking on microstructure and physical performance of casein protein

The development of advanced materials from biorenewable protein biopolymers requires the generation of more exogenous bonds to maintain the microstructure and durability in the final products. Casein is the main protein of milk, representing about 80% of the total protein. In the present investigation the casein protein was solubilized and/or emulsified in aqueous alkaline solutions, and 2D films and 3D matrices were produced. The effects of silane (3-aminopropyl triethoxy silane), DL-glyceraldehyde and glutaraldehyde on tensile properties and water swelling/absorption of 2D casein films and also the microstructure of the freeze-dried 3D matrices were analyzed. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that there were no significant changes in the molecular weight (19-23.9 kDa) of the casein proteins on exposure to alkaline solutions of sodium hydroxide and silane. The casein films produced without glycerol plasticizer and with heat treatment (130 °C for 18 h) were fragile. However, the fragile films were transformed into ductile and tough materials on exposure to moisture (i.e., conditioned for one week at 50 ± 2% relative humidity and 22 ± 2 °C) and showed a maximum average tensile strength of 49-52 MPa and modulus of 1107-1391 MPa. The chemical cross-linkers (i.e., DL-glyceraldehyde and glutaraldehyde) improved the microstructure of glycerol plasticized casein protein, when analyzed under scanning electron microscope (SEM). Furthermore, these chemical cross-linking agents enhanced the mechanical properties and water resistant properties of casein films.