Fractionating soybean storage proteins using Ca2+ and NaHSO3

Individual soybean storage proteins have been identified as having nutraceutical properties, especially beta-conglycinin. Several methods to fractionate soy proteins on industrial scales have been published, but there are no commercial products of fractionated soy proteins. The present study addresses this problem by using calcium salts to achieve glycinin-rich and beta-conglycinin-rich fractions in high yields and purities. A well-known 3-step fractionation procedure that uses SO2, NaCl, and pH adjustments was evaluated with CaCl2 as a substitute for NaCl. Calcium was effective in precipitating residual glycinin, after precipitating a glycinin-rich fraction, into an intermediate fraction at 5 to 10 nM CaCl2 and pH 6.4, eliminating the contaminant glycinin from the beta-conglycinin-rich fraction. Purities of 100% beta-conglycinin with unique subunit compositions were obtained after prior precipitation of the glycinin-rich and intermediate fractions. The use of 5 mM SO2 in combination with 5 mM CaCl2 in a 2-step fractionation procedure produced the highest purities in the glycinin-rich (85.2%) and beta-conglycinin-rich (80.9%) fractions. The glycinin in the glycinin-rich fraction had a unique acidic (62.6%) to basic (37.4%) subunit distribution. The beta-conglycinin-rich fraction was approximately evenly distributed among the beta-conglycinin subunits (30.9%), 35.8%, and 33.3%, for alpha', alpha, and beta subunits, respectively). Solids yields and protein yeilds, as well as purities and subunit compositions, were highly affected by pH and SO2 and CaCl2 concentrations.