Gel formation in heat-treated bovine serum albumin-sodium alginate systems

Abstract

The influence of pH and ionic strength on the gel strength of mixed bovine serum albumin (BSA, 8%)-sodium alginate (1%) gels and pure BSA gels has been investigated by longitudinal compression and shear measurements. A net strengthening effect of sodium alginate on the BSA-alginate gels was observed between pH 5 and 6 and at low ionic strengths (0.008–0.1 M NaCl at pH 5.9), passing through a maximum at 0.075 M NaCl at pH 5.9. An increase in ionic strength above 0.1 M NaCl caused a sudden drop in gel strength; at high ionic strengths this resulted in much weaker gels than those of pure BSA. At pH values higher than 5.9, the maximum gel strength decreased and was shifted towards lower ionic strengths, whereas at pH 5.3–5.7 (still above the BSA's isoelectric point of 5.0), strong gels were found over a more extended range of ionic strengths than at higher pH values. The strengthening effect of salts at low ionic strength may be caused by short-range electrostatic attraction between BSA and alginate, when long-range electrostatic repulsions between the net negative BSA and alginate are shielded. The results suggest that the decrease in gel strength at high salt concentrations was due to a phase separation and a more particulate gel structure. When Mg 2+ salts (MgCl 2, MgSO 4) were used instead of NaCl and Na 2SO 4, the same characteristics were observed as described above,but the strengthening effect occurred at lower ionic strengths. This should be expected since divalent cations reduce the effective charge density of negatively charged macromolecules more effectively than monovalent cations