Phase transition behaviour of sorbed water in Konjac mannan

Phase transition behaviour of sorbed water in Konjac mannan (KM)-water system with various water contents (Wc) was investigated using differential scanning calorimetry (DSC). The measurements were performed repeatedly for 3 cycles. At least six types of sorbed water together with glassy water can be identified in the KM-water system: (1) non-freezing water (Wnf), (2) four types of freezing bound water (Wfb), designated Wfb1, Wfb2, Wfb2* and Wfb3 and (3) free water (Wf). Glassy water was closely related to Wnf and the amount of glassy water was influenced by the cooling rate. The cold crystallization (Tcc) of glassy water caused an under estimation of the weight of sorbed water when calculated for the first cycle. Thereafter, the effect of Tcc was eliminated by cyclic measurements and the transition of Wfb2* could be readily observed. The proportion of each type of sorbed water changed with increasing Wc. The equivalent value of Wnf per pyranose ring was ca. 5.2 (mol/mol). Wfb1, Wfb2 and Wfb2* were the Wfb that were influenced by interaction with the KM matrix at lower Wc regions and were transferred to Wf in the high Wc regions. Wfb3 was strongly bound water, maintaining interaction with KM chains even in the high Wc region; the equivalent value of Wfb3 per pyranose ring was ca. 1.4 (mol/mol). Most of the sorbed water in the system with high Wc was held as Wf.