Submitted to: Polymer Degradation and Stability
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/25/2010
Publication Date: 11/2/2010
Citation: Selling, G.W. 2010. The effect of extrusion processing on zein. Polymer Degradation and Stability. 95(12):2241-2249. Available: http://dx.doi.org/10.1016/j.polymdegradstab.2010.09/013 Interpretive Summary: This research provides the first results on how zein, the major protein in corn, can degrade during extrusion processing. Extrusion processing is the preferred method for producing ‘plastic’ parts of mass consumption. Extrusion processing utilizes high temperatures (to melt the material) and high mixing (to provide a homogenous product) to provide the desired part. The benefits extrusion supplies are due to the relatively low equipment cost, high throughput and low chemical waste. Research efforts at processing zein using extrusion techniques has increased over the past ten years and defining how zein responds to the temperatures that it may see during processing will help manufacturers develop improved products. Specifically it was found that major changes occur in three temperatures ranges. Above 120 deg. C, the size of protein increases. This increase in size may result in high power consumption to run an extruder. Between 160 and 180 deg. C the color of the extrudate becomes darker more quickly which could lead to parts that do not meet specifications. Around this same temperature, the size of the protein begins to become smaller as the protein begins to degrade. Finally, zein extruded above 220 deg. C has undergone so much degradation that it cannot be processed into a part. These results demonstrate that the process box for un-modified zein, where it can provide a suitable part, has a maximum temperature of around 160 deg. C. This information will be beneficial to manufacturers interested in producing zein articles using extrusion processing.
Technical Abstract: Extrusion processing has been carried out on zein where extrusion temperatures were varied between 100 and 300 deg. C. By differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) thermal degradation begins around 220 deg. C. The color of the extrudate changed the most above temperatures of 160 deg. C. Sodium dodecyl sulfate polyacrylamide gel electrophoresisanalysis (SDS-PAGE) analysis shows that the protein begins cross-linking at 120 deg. C and chain cleavage begins above 180 deg. C. Examination of the structure of the extrudate using near and far-ultraviolet (UV) circular dichroism (CD) shows a slow loss in alpha-helix and beta-sheet content between 100 C and 240 deg. C; above 240 deg. C the rate of secondary structure loss is increased. Infrared (IR) 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 deg. C after which its loss accelerates. Nuclear magnetic resonance (NMR) spectroscopy demonstrated the formation of new carbonyl peaks and the lost 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 deg. C; extruding above 220 deg. C yields a material that cannot be molded.