Aggregation kinetics and ζ-potential of soy protein during fractionation

The mechanism of soy protein fractionation was explored. A Focused Beam Reflectance Measurement (FBRM) technique was used to study the effects of Ca2+ and Mg2+ concentrations on the aggregation/precipitation processes of soy globulins. An electrophoretic technique was used to provide information about the electrical charge of the resulting individual protein precipitate. FBRM measurements demonstrated the presence of a two-step process for glycinin (around 90% purity) aggregation. First, in the absence of Ca2+ and Mg2+, about 9730 counts of the primary particles (~ 3.33 μm) per second were formed immediately, with the pH being adjusted to 5.8 for glycinin precipitation. Second, more than 90% of the primary particles aggregated slowly within approximately 15 min into larger secondary particles (~ 54.72 μm). The addition of Ca2+ and Mg2+ significantly increased the amount of the primary aggregate of soy globulins and altered their aggregation process. However, the number of primary particles induced by Ca2+ was smaller than that by Mg2+. The aggregation kinetic pattern for soy globulins in the second precipitation step (mainly β-conglycinin) significantly differed from that of the first precipitation step (mainly glycinin) in the stability of secondary particles. Electrophoretic measurements showed that, as Ca2+ and Mg2+ concentrations were increased, the net ζ-potential of the glycinin-rich fractions decreased, and the reduced effect by using Ca2+ was greater than that by using Mg2+. The different influences on the ζ-potential for soy protein between Ca2+ and Mg2+ were probably due to their different influences on phytate precipitation, respectively. The combined effects of Ca2+ and Mg2+ and phytate led to the reduction in the ζ-potential and thus to colloidal stability for soy globulins, resulting in more protein precipitation.