Gel formation in heat-treated bovine serum albumin–κ-carrageenan systems

The effect of different concentrations of κ-carrageenan on the gel strength, gelation kinetics, and gel microstructure of bovine serum albumin (BSA) gels was investigated at different conditions of pH and ionic strength with and without the addition of potassium chloride. Two types of strengthening effects were observed: a major strengthening effect at the isoelectric point of BSA and slightly above, low ionic strength, and high carrageenan concentration (∼0.4–1.0% (w/v)); and a minor strengthening effect at higher pH and all ionic strengths at lower carrageenan concentration (∼0.2–0.4% (w/v)). The liquid that could be removed from the gels by centrifugation had a lower carrageenan concentration than the original gels at low pH and ionic strength, suggesting associative phase separation. The gelation kinetics of these gels differed from that of pure BSA or carrageenan gels, suggesting strong interactions between the polymers. At high pH or ionic strength, the gel centrifugates were enriched in carrageenan, and the gelation and melting of carrageenan could be observed in the heat-denatured BSA–κ-carrageenan gels by dynamic oscillation measurements. These effects are explained by segregative phase separation. The results also show that the presence of potassium ions during the gelation of BSA is important for the strengthening effect at associative conditions. This strengthening effect was observed up to higher pH values when sodium ions were partly replaced by potassium ions, and vanished completely when the potassium salt of carrageenan was replaced by the sodium salt. This suggests that synergistic interactions are present between the carrageenan and the BSA gel networks, but not between BSA and carrageenan in the sol state.