Specific combination of mixtures of two different polysaccharides can form a gel even if each polysaccharide aqueous solution can not form a gel. The mixture of xanthan and galactomannan is one of the oldest and most extensively studied system of this kind of synergistic gelling system. The origin of the gelation mechanism has been in dispute in the past 30 years. Experimental findings obtained by using HPLC, NMR5, X-ray diffraction and rheology, suggest an intermolecular binding. However, the formation of gels does not mean direct binding of these two different polysaccharides, and there still exist different possibilities for the gelling mechanism. For instance, it is well known that thermodynamic incompatibility causes phase separation, promotes self-association of each polymer, and leads to formation of a gel. X-ray fibre diffraction patterns from the stretched fibre was different from that from each component, suggesting that these polysaccharides bind to each other. However, the fibre contains only a small amount of water, and is almost in the solid state, and far from the liquid or gel state. Additionally, the stretching of the fibre or film potentially induces a new structure which is absent in the aqueous solution. Although most of the suggested models are based on the indirect methods, it has been believed that the direct binding model is correct. We recently have performed NMR measurements using low molar mass samples, and succeeded in finding a direct evidence of intermolecular binding. Degraded samples are also useful to consider the gelling mechanism. The most commonly accepted model is that the binding region for galactomannan is the continuous unsubstituted mannose region, the so called 'smooth region', since galactomannan with lower galactose content, locust bean gum, has higher gelling abilities than that with higher galactose content, guar gum. If the smooth region hypothesis is correct, the number of smooth regions depends on the galactose content, and also does on molar mass of galactomannan. We performed DSC and rheological measurements using different molar mass galactomannans, and analyzed the effect of molar mass in terms of the number of smooth region.
展开▼
