Improved corn protein (zein) extrusion processing

Authors: Boddu, Veera; Selling, Gordon

Submitted to: Corn Utilization Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 5/10/2016
Publication Date: 6/6/2016
Citation: Boddu, V.M., Selling, G.W. 2016. Improved corn protein (zein) extrusion processing [abstract]. Corn Utilization Conference Proceedings.

Interpretive Summary:

Technical Abstract: Melt processing using a single and twin screw extruder has been carried out on zein where extrusion temperatures were varied between 100ºC and 300ºC. In addition, melt reprocessing (up to seven times) of zein was undertaken using a single screw extruder. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) revealed that the thermal degradation of zein begins around 220ºC. The color of the extrudate changed the most above temperatures of 160ºC for both extrusion techniques, however rate of color change was more slower using the twin screw extruder. Using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), it was seen that the molecular weight of the zein changes more quickly with increasing temperature on the single screw extruder relative to the twin screw extruder. By using chemical reduction techniques and nuclear magnetic resonance (NMR), this increased molecular weight was due to disulfide bond formation as well as general oxidation and ester formation. The increased residence time in the single screw was the cause for the increased rate of molecular weight growth. At higher temperatures (above 180ºC and 220ºC for the single and twin screw, respectively), chain cleavage began to dominate. Examination of the structure of the extrudate using near and far-ultraviolet circular dichroism (CD) spectroscopy showed a slow loss in alpha-helix and beta-sheet content between 100ºC and 240ºC; above 240ºC, the rate of secondary structure loss increased. Infrared spectroscopy displayed differences in the carbonyl absorption with the carbonyl peak becoming narrower and shifting towards higher wavenumber with increased extrusion temperature. Tensile properties began to deteriorate when extruding above 140ºC and 220ºC for the single and twin screw, respectively. Melt processed zein, which had been produced using the single screw extruder, was reprocessed through the single screw seven times. With each pass through the extruder, the exit pressure and the torque required to provide the desired throughput increased in a linear fashion. With each pass, the molecular weight of the zein increased due to disulfide bond formation. The color of the extrudate also changed with each pass. The tertiary structure began to change after the first pass while the secondary structure began to change after the third pass, as observed by NMR analysis. The physical properties such as tensile strength did not change significantly with melt reprocessing. This research experimentally confirmed the melt processabilty of zein. The extrusion process window is expanded versus historical processing with temperature ranges of 80-140°C or 80-220°C on single or twin screw extruders, respectively; throughput will have a large impact on the temperature which can be tolerated. Zein can be reprocessed numerous times using extrusion processing.