Study on sorbed water in konjac mannan

Konjac mannan (KM) is a water soluble glucomannan with high molar mass. The solubility of KM in water is low due to its strong structural hydrogen bonding and the aqueous solution shows extremely high viscosity. Here, the phase transition behavior of sorbed water in KM-water system with various water content (Wc) was investigated using differential scanning calorimetry (DSC). At least five types of sorbed water can be identified in the KM-water system: 1) non-freezing water, 2) three types of freezing bound water designated W-fb1, W-fb2 and W-fb3, and 3) free water. Glassy water was also found in with Wc less than 1.6. The proportion of each type of water changed with increasing Wc. The equivalent value of non-freezing water per pyranose ring was ca. 6.6 (mol/mol). W-fb1 and Wfb2 were categorized as freezing bound water that strongly received interaction from KM molecules at lower Wc region and transferred to W-f in the high Wc region. W-fb3 was strongly bound water, maintaining interaction between KM chains; the equivalent value of W-fb3 per pyranose ring was ca. 0.7 (mol/mol). Most of the sorbed water in the system with high Wc was held as W-f. The system with lower Wc exhibited lyotropic liquid crystalline (LC) behavior, but not thermotropic LC property. The endothermic transition observed at temperatures over 0 degrees C should reflect conformation change of KM induced by breakdown in hydrogen bonding.