Rheological studies on mixtures of agar ( Gracilaria changii) and κ-carrageenan

Abstract

The rheological properties of agar and κ-carrageenan mixtures (with a total polysaccharide concentration of 1.5% w/w) were investigated using dynamic oscillatory measurements, creep compliance tests and compressive deformation measurements. The effects of different agar/ κ-carrageenan ratios (100/0, 80/20, 60/40, 40/60 and 20/80) on the formation and properties of the gel mixtures were investigated at different pH values (pH 3.5, 4.5 and 5.5). Temperature dependence of G′ of mixtures showed one step like change during cooling, however, two-steps like change was observed on heating trace. Significant thermal hysteresis was observed in all mixtures, moreover, the observed hysteresis was influenced by those characteristics of κ-carrageenan. The gel point as determined by oscillatory measurements of storage and loss modulus ( G′ and G″) of these mixtures show a monotonic decrease in temperature as the proportion of κ-carrageenan increased. Additionally, the gelling temperature of the mixtures resembled that of pure agar system. Incorporation of κ-carrageenan caused a large reduction in gel rigidity, which arise from sharp drop in G′ determined by mechanical spectra. Reduction in gel rigidity has also been demonstrated by a marked increased in instantaneous compliance evaluated by creep compliance tests. Young's modulus, stress and strain at failure were also monitored. In comparison with agar gel, the mixed gels were much more deformable, with a higher failure strain, but had lower strength indicated by a marked decrease in Young's modulus and failure stress. On the other hand, a decrease in pH below 4.5 caused a sudden drop in the gelling temperature, G′ and instantaneous compliance. In general, reduction in pH caused gel to be weaker and more brittle, as shown by lower values for Young's modulus, failure stress and failure strain. These effects were attributed to an increase in the number of shorter chains, which disfavored the formation of junction zones. The results of this study suggest that the gelation of the mixtures appeared to have occurred through a segregative phase separation where agar-rich phase formed a continuous phase and κ-carrageenan rich-phase formed a discontinuous gelled phase.