The effect of different concentrations of kappa-carrageenan on the gel stre
ngth, gelation kinetics, and gel microstructure of bovine serum albumin (BS
A) gels was investigated at different conditions of pH and ionic strength w
ith and without the addition of potassium chloride. Two types of strengthen
ing effects were observed: a major strengthening effect at the isoelectric
point of BSA and slightly above, low ionic strength, and high carrageenan c
oncentration (similar to 0.4-1.0% (w/v)); and a minor strengthening effect
at higher pH and all ionic strengths at lower carrageenan concentration (si
milar to 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 gel
s at low pH and ionic strength, suggesting associative phase separation. Th
e gelation kinetics of these gels differed from that of purr BSA or carrage
enan 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 denat
ured BSA-kappa-carrageenan gels by dynamic oscillation measurements. These
effects are explained by segregative phase separation. The results also sho
w that the presence of potassium ions during the gelation of BSA is importa
nt for the strengthening effect at associative conditions. This strengtheni
ng effect was observed up to higher pH values when sodium ions were partly;
replaced by potassium ions, and vanished completely when the potassium sal
t of carrageenan was replaced by the sodium salt. This suggests that synerg
istic interactions are present between the carrageenan and the BSA gel netw
orks, but not between BSA and carrageenan in the sol state. (C) 2000 Elsevi
er Science Ltd. All rights reserved.