Electric field-driven fabrication of anisotropic hydrogels from plant proteins: Microstructure, gel performance and formation mechanism

Soy protein isolate (SPI) hydrogels with anisotropic structures were fabricated under a low voltage electric field (1.5 V/cm) and different levels of NaCl (100–500 mmol/L). Under the weak direct-current electric field, the negatively charged protein molecules orderly arranged and accumulated around the positive electrode, where the surface charges of the amino acids were screened by the local H+ and added ions. Meanwhile, the random coil in the protein secondary structure disappeared to form ordered structures (α-helix and β-turn), and the protein exposed more hydrophobic and sulfhydryl groups. Intermolecular interactions, including disulfide bonds, hydrogen bonds and hydrophobic interactions facilitated the cross-linking of protein molecules to form hydrogels. All the hydrogels showed anisotropic networks, and the fraction of SPI attached to the gel phase (36.80% ∼ 62.17%) was found to increase with the rise of NaCl level in the protein solution. Moreover, the hardness (96.13–210.54 g), springiness (33.88% ∼ 68.30%), chewiness (121.59–292.12 g⋅s) and swelling resistance (142.79% ∼ 54.20%) of electrogels were all enhanced with increased NaCl level. Conversely, a higher ionic strength compromised the water holding stability (99.62% ∼ 66.75%) of electrogels, which could be attributed to the increased pore size within their networks that allowed moisture to be transferred. These findings may provide a novel insight for design and fabrication of plant protein hydrogels with desirable structures and characters through a green and sustainable approach.