Rheological and thermal properties of milk gels formed with κ-carrageenan.: I.: Sodium caseinate

Mixed gels, formed by kappa-carrageenan, and sodium caseinate were studied by differential scanning calorimetry (DSC) and rheometry. DSC showed that during gelation (i.e. cooling) the thermal behaviour of kappa-carrageenan was almost uninfluenced by the presence of sodium caseinate. Thus the interaction of kappa-carrageenan with sodium caseinate has little (or no) effect on the carrageenan's coil-to-helix transition. In contrast, during melting, added sodium caseinate strongly modified the thermal behaviour. The DSC peak became progressively broader with addition of sodium caseinate, indicating that the junction zones are highly heterogeneous in the mixed gel. Rheometry showed that sodium caseinate strongly influences the storage modulus (G'). In experiments in which the concentration of sodium caseinate was fixed and that of kappa-carrageenan varied, plots of G' vs. concentration of kappa-carrageenan were biphasic, with an abrupt change in slope at a concentration that increased linearly with the concentration of sodium caseinate. When the concentration of kappa-carrageenan was constant and that of sodium caseinate varied, G' as a function of concentration of sodium caseinate passed through a minimum. This behaviour could be modelled quantitatively, by assuming that: (a) the sodium caseinate adsorbs kappa-carrageenan, but with a limited adsorptive capacity; (b) sodium caseinate aggregates (sub-micelles) with adsorbed kappa-carrageenan can associate via interaction between free ends of adsorbed kappa-carrageenan chains and form a gel network; and (c) the contributions to G' from the sodium caseinate-kappa-carrageenan network and the network formed by kappa-carrageenan alone are additive. At low kappa-carrageenan to sodium caseinate ratios, the sodium caseinate and kappa-carrageenan combine to form a mixed gel. As the ratio of kappa-carrageenan to sodium caseinate increases, the sodium caseinate becomes saturated and no further association with kappa-carrageenan can occur-the increase in G', as further kappa-carrageenan is added, comes from a gel network formed by kappa-carrageenan alone. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.