The binding of calcium ions to phytic acid may occur above pH
5.0 and the extent of binding is dependent upon both pH and the
ratios of calcium and phytic acid (Martin and Evans, 1986).
Theoretically, one mole of phytic acid can chelate a maximum of
six moles of calcium. Between pH 7 and 12, the phosphate group
removed from phytic acid by phytase has approximately two
negative charges due to the dissociation constants (pKa2 of 7.21;
pKa3 of 12.67) within the pH range (Fig. 1). Therefore, calcium
hydrogen phosphate (CaHPO3) would be the predominant species.
At pH 5.4 (used in the present study for the phytase treatment),
where phosphate carries a single negative charge, the form of
calcium dihydrogen phosphate [Ca(H2PO3)2] would, in theory, be
prevalent. However, the electron withdrawing effect of the pair of
hydrogen ions (H+) associated with the phosphoric anion greatly
weakens the anion’s capability to attract Ca2+. Therefore, the bulk
of the divalent cation is expected to be free in the aqueous solution,
thus, explaining the significant increase in the level of detected free
calcium in soymilk upon the phytase treatment.