Rheology of iota-carrageenan gels containing caseins

Small strain oscillatory techniques were used to study the effects of added sodium caseinate or casein fractions on the rheology of 1% iota-carrageenan gels containing 0.01 mol/dm(3) CaCl2 or 0.02 mol/dm(3) KCl at 5 degrees C. The mechanical spectra (frequency dependence of G' (storage modulus), G'' (loss modulus) and tan delta (G''/G')) of all mixed casein/iota-carrageenan systems differed from those of corresponding iota-carrageenan gels. Added kappa-casein transformed a relatively weak Ca2+-free iota-carrageenan gel to a strong, elastic gel, indicating increased crosslinking which is attributed to electrostatic interactions between kappa-casein and iota-carrageenan. Changes to the rheology of Ca2+-free iota-carrageenan gels on addition of alpha(s)- or beta-casein are attributed to excluded volume effects, with monomeric beta-casein exerting a greater excluded volume effect than aggregated alpha(s)-casein. Added alpha(s)- or beta-casein also altered the rheology of Ca2+-containing iota-carrageenan gels, attributable to Ca2+-mediated interactions of alpha(s)- and beta-caseins with iota-carrageenan. The effects of these caseins were reduced on dephosphorylation of the caseins indicating the importance of phosphate groups for the interaction of these caseins with iota-carrageenan in systems containing Ca2+. Low levels of sodium caseinate increased both the rigidity and elasticity of Ca2+-free iota-carrageenan gels and Ca2+-containing iota-carrageenan gels. However, higher levels of sodium caseinate reduced elasticity in both cases. Sodium caseinate had a greater effect on the rheology of iota-carrageenan gels in the presence of Ca2+, presumably due to Ca2+-mediated interactions of alpha(s)- and beta-caseins with iota-carrageenan.