The effect of extrusion processing on Zein

Extrusion processing was carried out on zein where extrusion temperatures were varied between 100 and 300 °C. Differential scanning calorimetry and thermogravimetric analysis showed that thermal degradation begins around 220 °C. The color of the extrudate changed the most above temperatures of 160 °C. Sodium dodecyl sulfate polyacrylamide gel electrophoresis shows that the protein begins cross-linking at 120 °C and chain cleavage begins above 180 °C. Examination of the protein structure of the extrudate using near and far-UV circular dichroism shows a gradual reduction in α-helix and β-sheet content between 100 and 240 °C; above 240 °C the rate of secondary structure loss is increased. Infrared spectroscopy displayed differences in the carbonyl absorption with the carbonyl peak becoming narrower and shifting towards higher wavenumber with increased extrusion temperature. The peak at 1533 cm−1 becomes slowly smaller with higher extrusion temperature until 220 °C after which its loss accelerates. Nuclear magnetic resonance spectroscopy demonstrated the formation of new carbonyl peaks and the loss of alkoxyl carbons suggesting that in addition to protein backbone cleavage the alcohol moieties of serine and threonine are oxidizing to carboxylic acids. Tensile properties begin to deteriorate when extruding above 140 °C; extruding above 220 °C yields a material that cannot be molded.