Effect of galactomannans on the thermal and rheological properties of sago starch.

The thermal and rheological properties of sago starch have been studied in the presence of various concentrations of locust bean gum and guar gum of various molecular masses. At the concentrations studied (<1%) the galactomannans gave rise to only a very slight increase in the gelatinization temperature (up to 0.6 degrees C), and the gelatinization enthalpy remained constant within experimental error. For the low molecular mass galactomannans, depending on the concentration, the storage modulus, G', of the mixtures remained constant or actually decreased, and tan delta remained very low (0.01-0.03 at 0.1 Hz), indicating strong elastic gels. For the higher molecular mass samples G' increased significantly; however, the loss modulus, G' ', increased proportionally to a greater extent, and at 1% galactomannan tan delta was approximately 0.20 at 0.1 Hz, indicating a reduction in elastic character. The systems were shown to undergo phase separation, and the variations in rheological properties have been discussed in the context of their phase behavior and the relative rates of the phase separation and gelation processes. The presence of galactomannans significantly improved the freeze-thaw stability.