Impact of Acid and Alkaline Pretreatments on the Molecular Network of Wheat Gluten and on the Mechanical Properties of Compression-Molded Glassy Wheat Gluten Bioplastics

Abstract

Wheat gluten can be converted into rigid biobased materials by high-temperature compression molding at low moisture contents. During molding, a cross-linked protein network is formed. This study investigated the effect of mixing gluten with acid/alkali in 70% ethanol at ambient temperature for 16 h followed by ethanol removal, freeze-drying, and compression molding at 130 and 150 °C on network formation and on types of cross-links formed. Alkaline pretreatment (0-100 mmol/L sodium hydroxide or 25 mmol/L potassium hydroxide) strongly affected gluten cross-linking, whereas acid pretreatment (0-25 mmol/L sulfuric acid or 25 mmol/L hydrochloric acid) had limited effect on the gluten network. Molded alkaline-treated gluten showed enhanced cross-linking but also degradation when treated with high alkali concentrations, whereas acid treatment reduced gluten cross-linking. β-Elimination of cystine and lanthionine formation occurred more pronouncedly at higher alkali concentrations. In contrast, formation of disulfide and nondisulfide cross-links during molding was hindered in acid-pretreated gluten. Bioplastic strength was higher for alkali than for acid-pretreated samples, whereas the flexural modulus was only slightly affected by either alkaline or acid pretreatment. Apparently, the ratio of disulfide to nondisulfide cross-links did not affect the mechanical properties of rigid gluten materials.

Controlling wheat gluten cross-linking for high temperature processing

“…It appears that large differences in the degree of WG cross-linking prior to moulding do not necessarily have an effect on the flexural properties of moulded products in the case of glassy materials. This finding is in agreement with results by Jansens et al (2013b) who showed that pre-treatment of WG at increasing alkaline concentration lead to significant differences in the protein network with various degrees of LAN formation. However, the properties of moulded articles from these samples showed little dependence on the concentration of the base.…”

Macromolecular changes and nano-structural arrangements in gliadin and glutenin films upon chemical modification

“…2b and Supplementary Fig. S1a), and with lysine, forming the LAL crosslinks [5,33,34]. Formation of irreversible crosslinks has previously been described in gliadin at alkaline pH and in extruded WG when NH 4 OH and SA were used as additives [35,36].…”

Importance of crosslinking and disulfide bridge reduction for the mechanical properties of rigid wheat gluten bioplastics compression molded with thiol and/or disulfide functionalized additives

“…Different types of gluten crosslinks are present in rigid, glassy gluten materials, but disulfide bonds are predominant . To improve the strength and failure strain of rigid gluten materials, molecules with multiple terminal thiol (SH) groups can be incorporated into the protein structure .…”