Viscosity control of zein processing with sodium dodecyl sulfate

Abstract

Zein, the predominant protein in corn, may be used as an alternative to various commercial plastics. Plastics produced from zein tend to be brittle, so plasticizers such as triethylene glycol (TEG) and water were used in this study to alleviate brittleness. When zein is blended with water and subjected to heat and shear in a torque rheometer, zein readily generates aggregates that produce a rapid increase in viscosity. This rapid increase in viscosity is manifested as a torque increase, which can be detrimental to industrial processes such as extrusion. The primary objective of this study was to control the viscosity rise during torque rheometry by adding known amounts of water, TEG, and/or sodium dodecyl sulfate (SDS), a known protein denaturant. Our findings with torque rheometry showed that water alone allowed zein to generate higher molecular weight species in approximately 1 min. Varied combinations of TEG and water delayed the torque rise to 2–4 min. When SDS was added to zein and combined with water and TEG, it significantly delayed the viscosity increase in a linear relationship dependent on the amount of SDS added. Because extrusion is a standard method for processing plastics, the second objective of this study was to assess the mechanical properties of extruded, SDS-modified zein. To accomplish this, zein was blended with a fixed amount of water and TEG with varying amounts of SDS. The mixtures were subjected to extrusion processing on a single-screw extruder equipped with a 2 mm round die, and the resulting rope was pelletized. One half of the weight of pellets was re-extruded through a 2 cm ribbon die, and the other half was freeze-ground and compression molded. The mechanical properties of tensile bars from the compression mold and those cut from the ribbon die were evaluated at either 50 or 70% relative humidity on an Instron with a crosshead speed of 50 mm/min. Tensile strength and Young's modulus diminished, whereas elongation increased with increasing amounts of SDS. Therefore, the addition of SDS did not improve the strength of the tensile bars, but it did improve the ductility. The results from torque rheometry and the mechanical testing of extruded products show that the addition of SDS to zein blends of water and TEG can be used to develop an acceptable formulation for both processing and end-use requirements.