Fractionation of Soybean Globulins Using Ca2+ and Mg2+: A Comparative Analysis

The substitution of CaCl2 by MgCl2 was undertaken in Deak's two-step process of separating the soybean 11S and 7S globulins, aiming at higher purities and lower phytic acid (PA) contents of recovered protein fractions. The effects of pH and the addition of NaCl were also evaluated. Compared with CaCl2, MgCl2 reduced the PA content of the 11S-rich fraction by 63-71% but increased that of the 7S-rich fraction by 14-28%, depending on pH. Correspondingly, more Ca2+ was recovered in the 11S-rich fraction, while more Mg2+ co-precipitated with the 7S-rich fraction. NaCl increased the purity of the 11S-rich fraction and reduced its PA content, but the purity of the 7S-rich fraction was reduced by using 50-100 mM NaCl. Lowering pHs from 6.4 and 4.8 to 5.6 and 4.0 in the two precipitation steps increased the yield of both fractions. The optimized fractionating procedure was as follows: the 11S-rich fraction was precipitated at pH 5.8 by using 5 mM MgCl2, 10 mM NaHSO3 and 20 mM NaCl, followed by the precipitation of the 7S-rich fraction at pH 4.5. The new method provided both fractions with satisfactory protein yields (22% for 11S and 16% for 7S), purities (88% for 11S and 80% for 7S) and PA contents (0.356% for 11S and 0.882% for 7S).