Segregative interactions and competitive binding of Ca 2+ in gelling mixtures of whey protein isolate with Na +κ-carrageenan

Abstract

Gelling mixtures of Na + κ-carrageenan with whey protein isolate (WPI) at pH 7.0 have been studied rheologically and by differential scanning calorimetry (DSC), with comparative measurements for the individual constituents of the mixtures. The concentration of WPI was held fixed at 10.0 wt% and carrageenan concentration was varied in the range 0.05–3.0 wt%. Ca 2+ cations, which have been shown previously to be particularly effective in inducing gelation of κ-carrageenan, were introduced as CaCl 2. The concentration of CaCl 2 used in most of the experiments was 8 mM, but other concentrations were also studied. Mixtures were prepared in the solution state at 45 °C, and showed no evidence of either phase separation or complex formation. Rheological changes were monitored by low-amplitude oscillatory measurements of storage modulus, G′, during (i) cooling (1 °C/min) and holding at 5 °C, to induce gelation of the carrageenan in the presence of non-gelled WPI; (ii) heating and holding at 80 °C to dissociate the carrageenan network and induce gelation of WPI; (iii) cooling and holding again at 5 °C, to give composite networks with both components gelled; and (iv) re-heating to 80 °C to dissociate the carrageenan network. Gel structure was characterised further by creep–recovery measurements at the end of each holding period, and by torsion measurements at 5 °C, before and after thermal gelation of WPI. In 8 mM CaCl 2, the presence of 10.0 wt% WPI promoted earlier gelation of κ-carrageenan during cooling, and displaced the disorder–order transition in DSC to higher temperature. These changes are attributed to segregative interaction with non-gelled WPI. The resulting networks, however, were weaker that those formed by κ-carrageenan alone, which was traced to binding of Ca 2+ cations to the WPI. At higher concentrations of CaCl 2, where availability of Ca 2+ cations was no longer a limiting factor in gelation of the κ-carrageenan, the effect of segregative interactions could be seen as an increase in gel strength. On heating to gel the WPI, the presence of κ-carrageenan led to the earlier appearance of an aggregation exotherm in DSC and to network formation at shorter times, which can similarly be explained by enhanced self-association of denatured WPI in response to segregative interactions with disordered carrageenan. An initial large (∼10 fold) increase in gel strength with increasing concentration of carrageenan to ∼0.25 wt% was followed by a reduction at higher concentrations, attributed to excessive aggregation (incipient precipitation) of the WPI. The magnitude of the changes in G′ observed on subsequent cooling suggest that gelation of κ-carrageenan within the pores of an existing network of gelled WPI gives rise to a bicontinuous co-gel structure. Comparison of moduli at 80 °C before and after cooling to 5 °C demonstrated that formation and dissociation of the carrageenan network caused no disruption of the network of gelled WPI.