In this study the heat induced synergistic gelation of different hydrocolloid solutions, xanthan gum types (XG) in mixture with galactomannans like guar gum (GG), locust bean gum (LBG) and konjac glucomannan (KGM) is investigated. The physical mechanism of the synergy in thickening and gelling of blends depends on the monomer structure, the molecular weight, the charge, the polarity, and the chain stiffness of the hydrocolloids. Particularly the properties of the electrically neutral galacto- and glucomannans mixed in combination with xanthan gum strongly affect the synergistic effects. These are influenced by the number and distribution of mannan side chains and thus their flexibility. While the pure components do not show gelation on their own, they form viscoelastic solutions or even gels when mixed together and heated. In this study, rheological properties of the resulting composite gels of 0.5% (w/w) were examined under different physicochemical and thermal conditions. Focus was laid on thermally induced gels, as these gels showed higher synergistic effects compared to the non-heated ones. The gelation mechanisms were investigated by strain and temperature dependent oscillatory rheological measurements. Blends with XG-GG (20:80) showed the weakest synergism, followed by XG-LBG blends (20:80), whereas XG-KGM (60:40) blends showed the highest increase of the storage modulus. This can be explained by different local interactions in combinations with the flexibility of the various components. Furthermore, the impact of monovalent salt on the interactions was investigated. Addition of sodium chloride at 0.05% and 0.5% (w/w) concentrations influenced the gelling due to Coulomb screening of the negative charges of XG. Consequently, the synergism, in particular the storage modulus, is strongly affected by variation in salt concentration. We propose specific models based on the gel formation in case of XG-LBG and XG-KGM blends, whereas XG-GG shows an entropic phase separation due to flexibility of GG.
Several studies have investigated xanthan-guar gum (XG-GG), xanthan-locust bean gum (XG-LBG), and xanthan-konjac glucomannan (XG-KGM) blends but little attention has been paid to the physical interactions between the hydrocolloids on a molecular basis. This requires a consistent sample preparation. Often, LBG is heated up to dissolve completely and then xanthan is added, whereas mixtures with guar gum are prepared at room temperature. To understand the synergy during gelation it is necessary to investigate the xanthan-hydrocolloid solutions in the non-heated state because it sets and controls the preferred initial conditions for the given interactions by chain stiffness, charge and polarity under different concentrations In this first part of the publication we focused on blends which are all prepared at room temperature and analysed the molecular interaction in these cold mixed systems. Regarding this, we used Rheology and AFM measurements to characterise the single molecules and the mixing behavior and synergism of the blends. We found, that the cold mixed systems are not stable at room temperature and show a phase separation after one and two days, according to the sample, but are stable when stored at 4 °C. Further, these mixing and demixing properties are highly corresponding to the synergism. Blends with xanthan-guar gum with the weakest mixing properties show the weakest synergism, whereas xanthan-konjac blends with a good mixing behavior exhibit the highest synergism. From the AFM micrographs it was observed that XG-KGM gave most homogeneous mixtures, whereas XG-LBG and XG-GG showed strong phase separation. Based on our experimental results and the characteristics of the molecules such as molecular size, shape and side chains we propose molecular models to explain the physical interactions in these systems which are supported by atomic force microscopy.
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