Stable and pH-Sensitive Protein Nanogels Made by Self-Assembly of Heat Denatured Soy Protein

In this study, we examined the possibility of preparing stable soy protein nanogels by simply heating homogeneous soy protein dispersion. The protein nanogels formed were characterized by z-average hydrodynamic diameter, polydispersity index, turbidity, ζ-potential, morphology, and their stability to pH and ionic strength change. Soy protein dispersion (1% w/v) was homogeneous around pH 5.9 where it had the lowest polydispersity index (∼0.1). Stable and spherical nanogels were formed by heating soy protein dispersion at pH 5.9 under 95 °C. They sustained constantly low polydispersity index (∼0.1) in the investigated pH range of 6.06–7.0 and 2.6–3.0. The nanogels were pH-sensitive and would swell with pH change. They were stable at 0–200 mM NaCl concentration. Denaturation of soy glycinin was the prerequisite for the formation of stable nanogels. Soy protein nanogels had a core–shell structure with basic polypeptides and β subunits interacting together as the hydrophobic core; and acid polypeptides, α′, and α subunits locating outside the core as hydrophilic shell. The inner structure of soy protein nanogels was mainly stabilized by disulfide bonds cross-linked network and hydrophobic interaction. Soy protein nanogels made in this study would be useful as functional ingredients in biotechnological, pharmaceutical, and food industries.