Sample preparation and testing methods affect the physical properties and evaluation of plasticized zein

The objective of this research was to generate zein ribbons on an extruder in which physical property analyses can be performed to determine whether physical properties were dependent on the testing direction. The extruded zein ribbon samples had between 10 and 15% triethylene glycol (TEG) and were conditioned at relative humidities between 23 and 70% before testing to determine the impact of sample testing direction on physical properties. Tensile strengths in the machine direction were between 18 and 64% higher than in the transverse direction. Elongations in the machine direction were between 33 and 291% higher than in the transverse direction. The magnitude of the difference between the machine and transverse direction was dependent on relative humidity and formulation. The impact of relative humidity was larger when the samples were tested in the machine direction; for example, tensile strength was reduced by 36% in the transverse direction and by 49% in the machine direction for the 15% TEG formulation when relative humidity was increased from 23 to 70%. With increased amounts of TEG, the magnitude of these differences was increased. The transverse direction tensile strength was reduced by 22% for a 10% TEG formulation, and by 36% for a 15% TEG formulation when relative humidity was increased from 23 to 70%. Polarized IR spectra of zein ribbons displayed differences between the spectra taken in the machine direction versus that taken in the transverse direction. Examination of the ribbons using polarizing light microscopy demonstrated that the ribbon was birefringent (δn = 0.0004). These results demonstrate that molecular orientation occurred during extrusion and that orientation impacts physical properties.