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 (W-c.) was investgated 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 (W-nf), (2) four types of freezing bound water (W-fb), designated W-fb1, W-fb2, W-fb2* and W-fb3 and (3) free water (W-f). Glassy water was closely related to W-nf and the amount of glassy water was influenced by the cooling rate. The cold crystallization (T-cc) of glassy water caused an under estimation of the weight of sorbed water when calculated for the first cycle. Thereafter, the effect of T-cc was eliminated by cyclic measurements and the transition of W-fb2* could be readily observed. The proportion of each type of sorbed water changed with increasing W-c. The equivalent value of W-nf per pyranose ring was ca. 5.2 (mol/mol). W-fb1, W-fb2 and W-fb2* were the Wfb that were influenced by interaction with the KM matrix at lower W-c regions and were transferred to W-f in the high W-c regions. W-fb3 was strongly bound water, maintaining interaction with KM chains even in the high W-c region; the equivalent value of W-fb3 per pyranose ring was ca. 1.4 (mol/mol). Most of the sorbed water in the system with high W-c was held as W-f. (C) 2006 Elsevier Ltd. All rights reserved.