Effect of multiple extrusion passes on zein

Zein was processed up to seven times using a single screw extruder at a temperature of 145 °C and at approximately 15 g per minute to determine the extent of degradation that occurs with multiple extrusion passes. SDS-PAGE shows that with the second pass, and each additional pass, the amount of high molecular weight material increases. Given that a reducing agent reduces the molecular weight of the protein, then this increased molecular weight is predominantly due to disulfide bond formation between proteins. Examination of the secondary structure of the protein using far-UV CD illustrated that after the third pass there was a significant reduction in the amount of α-helix and β-sheet content. Examining the near-UV CD spectra, which reflects changes in protein tertiary structure, illustrates significant changes in structure after the first and second pass through the extruder. IR and NMR spectroscopy revealed small differences with multiple passes through the extruder. Examining color using the δE scale displays a change in color after four extrusion passes. The physical properties of compression molded samples from each pass are similar with statistical differences being observed only in modulus. While some changes in zein properties can be observed with multiple extrusion passes, if physical properties are of the highest importance, than zein can be processed at least seven times and still provide a quality material.