Differential scanning calorimetry (DSC) and electron spin resonance spectroscopy (ESR) have been used to probe the mechanism of gelation in gels formed by mixtures of konjac mannan and «-carrageenan and have indicated that an association occurs between konjac mannan chains and (aggregated) «-carrageenan helices. On cooling mixed systems at low ionic strength, a coil-helix transition of «-carrageenan, as evidenced by a DSC exothermic peak (peak I), occurs at a temperature significantly higher than the "normal" transition temperature observed for «-carrageenan alone. This transition is accompanied by a partial immobilization of the konjac mannan chains, as evidenced by ESR spectroscopy. When «-carrageenan is in excess, a second transition (peak II) appears at the normal transition temperature. Both transitions are ion specific, and they shift in the same way with added salt, which strongly suggests that they both reflect the formation of normal «-carrageenan helices. At sufficiently high ionic strength, only one transition is apparent in the DSC curve, and its effect on the mobility of the konjac mannan chains is considerably reduced. Both the peak I and peak II transitions show a marked thermal hysteresis, as evidenced by DSC heating experiments, which demonstrate that both transitions are accompanied by aggregation processes subsequent to the coil-helix transition. The transition enthalpy per mole of carrageenan is significantly less in the mixtures than for «-carrageenan alone, indicating that the «-carrageenan self-aggregation is affected by the presence of konjac mannan. The results are interpreted in terms of the formation of mixed aggregates of «-carrageenan helices and konjac mannan, possibly involving bundles of self-aggregated «-carrageenan helices covered with surface-adsorbed konjac mannan chains.
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