Supermolecular aspects of xanthan-locust bean gum gels based on rheology and electron microscopy

摘要

The viscoelastic properties and supermolecular structure of synergistic gels, formed by xanthan and locust bean gum (LBG) of two different mannose:galactose ratios (M:G), have been investigated by small deformation viscoelastic measurements and by low angle rotary-shadowing for transmission electron microscopy. The rheological properties at 20℃ for mixtures subjected to heating and cooling cycles in the temperature range 30-80℃ were found to be dependent on the M:G ratio. Mixtures of xanthan and LBG mixed at temperatures ≤ 40℃ were found to form true gels with low phase angles. Blends of xanthan and LBG with a low M:G ratio did not show any increase in synergistic effects as the temperature was increased, whilst the mixture of xanthan and LBG with a high M:G ratio showed a strong increase in synergistic effects as the temperature was raised above 60℃. A difference in gelation temperature (T_g) of ～13℃ was observed between the mixtures of xanthan and the two LBG fractions. The T_g for xanthan with a high M:G ratio was ～53℃, whilst the T_g for mixtures of xanthan and LBG with a low M:G ratio was ～40℃. Results obtained using electron microscopy showed that the xanthan-LBG network was formed from xanthan supermolecular strands, and addition of LBG did not influence the xanthan structure. The observed structural features of the gels were independent of heat treatment and LBG fraction. The structural similarities and rheological differences observed between xanthan and the LBG fractions are discussed in comparison with existing interaction models at the molecular level. Based on these results, a speculative network model at the supermolecular level is presented.