Plant Protein-Based Nanocomposite Materials: Modification of Layered Nanoclay by Surface Coating and Enhanced Interactions by Enzymatic and Chemical Cross-linking

Highly intercalated or exfoliated nanoclay montmorillonite (MMT) has promises to improve mechanical and barrier properties of nanocomposite materials that may be further improved by strengthening interactions between matrix polymers and nanofillers. In this work, water-soluble proteins extracted from hominy feed and soy flour were utilized to modify the structures of MMT layers by surface-coating. Following coating at 60 °C using different MMT:protein mass ratios (49:1-2:1) and pH (2.0-10.0), the nanoclay was triple-washed for zeta potential analysis and lyophilized for X-ray diffraction and Fourier transform infrared spectroscopy analyses. Results showed that protein adsorption on MMT occurred at all pH conditions by coulumbic and/or non-coulumbic forces. With a sufficient amount of protein, highly intercalated or fully exfoliated MMT structures were achieved. MMT coated by soy protein was incorporated in soy protein dispersions for enzymatic and chemical cross-linking. Dynamic rheological tests were applied as a non-destructive method to illustrate the gel network formation and the interactions between protein-coated MMT and matrix proteins. Variables in enzymatic cross-linking included concentrations of NaCl and microbial transglutaminase (mTGase), and the absence or presence of protein-coated-MMT at pH 6.5. Without MMT, the maximum storage modulus was achieved at 100 mM NaCl and stronger gels with shorter gelling times were observed at higher mTGase concentrations. Conversely, the incorporation of coated-MMT inhibited the effect of ionic strength on soy protein gelation, and further shortened the gelation time and increased synergistic development of storage modulus with the combined treatments of 100 mM NaCl and mTGase. For chemical cross-linking, glutaradehyde was used as a cross-linker and studied for the impacts of pH, temperature and cross-linker concentration on dynamic rheological properties. Without MMT, storage moduli gradually increased with increasing glutaradehyde concentration; while the increase of storage modulus was in a higher order of magnitude in the presence of MMT. The practical approach and established parameters from this work can be used to manufacture nanocomposite materials with improved properties.