Selective chemical modification of soy protein for a tough and applicable plant protein-based material

Soy protein, one of the most abundant plant proteins, has gained considerable attention and popularity in recent years due to its biocompatibility, biodegradability, and renewability. However, the poor mechanical properties and high moisture sensitivity of soy protein-based materials have limited their wider application in practical use. In the present study we have overcome these shortcomings by the use of an aqueous reagent, tetrakis(hydroxylmethyl) phosphonium chloride (THPC), to modify the amino groups of the lysine (Lys) and arginine (Arg) residues present in soy protein. Solid state ¹³C cross polarization/magic-angle spinning nuclear magnetic resonance (CP/MAS NMR) and ³¹P NMR spectroscopy have confirmed the success of the chemical reaction by replacement of the hydroxymethyl arm of THPC, both qualitatively and quantitatively. Fourier-transform infrared (FTIR) spectroscopy, rheological observations, transmission electron microscopy (TEM) and atomic force microscopy (AFM) have been used to confirm the changes in the coherent tertiary structure of soy protein after modification, increasing the interconnection between the molecular chains. Finally, we have been able to produce a modified soy protein film, which provides a good combination of tensile strength and extensibility under either dry (10 ± 2 MPa and 25 ± 3%) or wet (5 ± 1 MPa and 200 ± 20%) conditions. In addition the modified soy protein film has unexpectedly been found to exhibit antimicrobial properties, and this adds to the merits of the final product. We believe that the method described further broadens the application of natural soy protein-based materials, for example in antimicrobial packaging films.