Acid-induced formation of soy protein gels in the presence of NaCl

Although it is well known that ions can have a strong impact on repulsive and attractive forces in colloidal systems, the acid-induced formation of soy protein gels in the presence of NaCl is poorly analyzed. This study gives a deeper insight into the mechanisms of acid-induced gelation of soy protein isolate under varying conditions of acidification (gelation velocity, temperature) and different concentration of NaCl in the system. It is shown that increasing the acidification rate results in lower gel stiffness by giving soy proteins less time to arrange and aggregate at a certain pH. Because of their isoelectric precipitation, protein solutions without addition of NaCl showed a stiffness maximum (Gmax′) at a similar pH. Below that pH G′ decreased due to the dissolving of mainly hydrophilic polypeptides. The presence of NaCl screened electrostatic interactions and enhanced protein solubility leading to lower gel stiffness. Additionally, Gmax′ shifted to lower pH presumably because of repulsive hydration forces at high ion concentrations that counteracted hydrophobic interactions. Rearrangements were detected in gels with NaCl and seemed to be more dominant at higher salt concentration.