A mixed system composed of different molecular weights konjac glucomannan and κ-carrageenan. II. Molecular weight dependence of viscoelasticity and thermal properties

Abstract

Rheological and thermal properties for 5:5 mixed gels of konjac glucomannan (KGM) with five different molecular weights and κ-carrageenan (CAR) were studied. Molecular weight distribution was analysed by gel permeation chromatography (GPC). Dynamic and static viscoelastic moduli for the mixed gels were much smaller than those for gels of CAR alone and became larger with increasing molecular weight of KGM. Gel—sol transition for mixed systems with total gum content 1.5% w/w occurred at the temperatures between the transition temperature for 0.75% w/w CAR and that for 1.5% w/w CAR. Gel-to-sol and sol-to-gel transition temperatures for the mixed systems were slightly increased with increasing molecular weight of KGM in the mixture observed in differential scanning calorimetry (DSC). One exothermic peak was observed in cooling DSC curves, while two endothermic peaks appeared in heating DSC curves for 5:5 mixed systems. The intensity of the lower peak increased with increasing KGM proportion in the mixture and with increasing molecular weight of KGM at a given mixing ratio. This fact confirms that the mixed gels involve two crystalline regions: some are made by CAR molecules in the helical structure and others are formed by the interaction between helical CAR and KGM. On heating, the latter is broken at a lower temperature and then the former melts at a higher temperature. The junction zone formed by CAR and KGM is easily made in a gel containing KGM with a higher molecular weight or in a higher ratio to CAR, or a system of lower total gum content.