Mechanical and Physical Properties of Soy Protein Films with pH-Modified Microstructures

Mechanical, physical, and barrier properties of films obtained from soy protein isolate solutions at different pH were studied and correlated with the structural properties and the microstructure of films. Films obtained at pH 2 and 11, which had denser microstructures and a higher amount of disulfide bonds, showed a higher tensile strength — of about 1.05 MPa — and a higher Young's modulus — of at least 0.15MPa — than the one at pH 8. However, films formed at alkaline pH (8 and 11) exhibited a higher deformation than films at pH 2 — by about 70%. The presence of at least a protein fraction in native state allowed macromolecules to unfold during the mechanical test, reaching greater deformation before breaking. Acidic films exhibited higher water vapor permeability — of about 7 × 10 -11 g/m s Pa — and water content — of about 1.5% — and a lower glass transition temperature — of at least 15 °C — than basic ones, due to their higher hydrophilic nature.