Wheat Gluten -Based Materials and Composites: Extrusion, Casting and Antimicrobial Properties
Sammanfattning: The use of bio-based polymers as packaging materials has gained much attention due to increasing environmental concerns regarding non-biodegradable petroleum-derived plastics. Wheat gluten (WG) is a valuable renewable resource for the production of bio-based materials because of its low price, biodegradability, good film-forming properties and good gas barrier properties under dry conditions. This study presents the processing and development of WG-based materials and composites.The first part presents a novel approach to obtain high quality WG films in a solvent-free extrusion process. Extrudability of WG was significantly improved by using urea and the films were flexible with remarkable barrier properties. Results indicated that urea-containing films were aggregated/polymerized and contained a considerable amount of b-sheet structure with a high degree of hydrogen bonding. Adding urea changed the protein structure, and it was found that urea seemed to work as a plasticiser, as observed by the way in which the barrier and mechanical properties changed with increasing urea concentration (Paper I).In the second part, wheat gluten/montmorillonite (WG/MMT) nanocomposite films were prepared in a solvent-free extrusion process with urea. The oxygen permeability (OP) and water vapor transmission rate (WVTR) of the films were lowered by the use of montmorillonite clay. At the same time, the stiffness of the films increased without any critical loss of extensibility and the thermal stability of the extrudates was improved by the addition of the clay. Results indicated that the clay particles were layered mainly in the plane of the extruded films and the clay existed as individual platelets, intercalated tactoids and agglomerates (Paper II).The third part describes the development of WG/silica hybrid materials obtained by the sol-gel process. The presence of silica constrained the WG component to such an extent that moisture-induced aggregation/denaturation was small. Results suggested that the constraint came from interactions between the silica and wheat gluten phases, mainly due to hydrogen bond interactions. A substantial improvement in the thermal properties was observed as the silica content increased. It was found that reduction of the moisture diffusivity in the presence of silica can be explained by the geometrical impedance imposed by the interpenetrating silica phase (Paper III).The fourth part focuses on thermoplastically produced antimicrobial wheat gluten materials. Antimicrobial WG films containing potassium sorbate (PS) were successfully produced by compression moulding. Films containing more than 10 wt.% PS showed antimicrobial activity against Aspergillus niger while films containing 2.5 wt.% or more PS showed antimicrobial activity against Fusarium incarnatum. It was observed that when no seeding with spores was adopted, the PS films, in contrast to the PS-free films, resisted antimicrobial growth for at least one week. It was also found that PS was dissolved in WG films and that when the films were exposed to an agar solution most of the PS was released. In addition to the antimicrobial properties, results also indicated that PS acted as a plasticiser in the WG films (Paper IV).
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