Distinctive Dissolving Behaviors Of Deacetylated Konjac Glucomannan At Low Temperature And Its Mechanism

Konjac Glucomannan (KGM) is the main functional component of konjac which is a kind of important and abundant hydrophilic colloid resource with monopolistic advantages in China. When part of the acetyl on the molecule chain was removed, the original highly hydrophilic heteropolysaccharide became less soluble or even insoluble. However, under the low temperature area, deacetylated KGM (da-KGM) possessed a serious of special low temperature processing properties. Few reports have been found on the structures and properties of da-KGM under low temperatures.In view of the potential and unique utilization potentialities, this paper researched the solubility of da-KGMs with various deacetylation degrees into10wt%urea solution, showing that the optimum dissolving temperature was-4℃. The effect of different salt, alkali, hydrogen cracking agent on the deacetylation degree of da-KGM was studied, showing that the best solvent was9wt%KSCN aqueous solution. The circular dichroism spectra (CD) and ultraviolte visible spectra(UV-Vis) of congo red which is a kind of fluorescent probes was investigated to discuss the molecular structure in the solution. Sol-gel transition properties was studied. Furthermore, several methods including DSC, FI-IR, CP/MAS13C NMR, TEM and viscometry were employed to research the structural properties of solvent, solute and solution in the Da-KGM/KSCN system dealt with different low temperature conditions.The main contents and results are indicated as follows:1. The optimum condition of the dissolution of da-KGM was determined. The solubility experiment showed that the best dissolving temperature was-4℃. Also, the solubility curve between-4-10℃followed the Arrhenius equation2. The activation energy was the activation energy being-21.2kJ/mol, manifesting that the low temperature dissolution was a entropy driven process. The optimum solvent was9wt%KSCN, with the dissolution kinetics following pseudo-first order model.3. The molecular structure of da-KGM in solvent was analyzed. The UV-Vis spectroscopy of da-KGM/10wt%urea aqueous solution and da-KGM/9wt%KSCN aqueous solution showed that the da-KGM molecule produced self crimping structure, which contributed to a more ordered structure. The intramolecular interactions was strengthened by the two solvents. Also,9wt%KSCN was more effective to form the ordered structure, to weaken the intermolecular interactions and then to dissolve. In addition, the result of CD showed that da-KGM molecules had a even stronger unsymmetry and notable conformational transition.4. The rheological characteristic of da-KGM solution was researched. Steady shear sweep showed that higher deacetylation degree made lower Zero shear viscosity of da-KGM, more gentle flat area in low frequency range and more applying to cross function. The dynamic rheological analysis showed that under the shearing action, the gelation time of da-KGM rose with increasing KGM concentration, and fell with decreasing deacetylation degree and temperature. The result of temperature sweep showed that da-KGM had a typical heat induced gel behavior.5. The existing form of water in KSCN under low temperature was determined to research the dissolution promoting mechanism of da-KGM under low temperatures. According to DSC、FT-IR and viscometry, KSCN hydrate existed in KSCN aqueous solution. Under low temperatures, compound structure with large size formed by hydrogen bonds network between KSCN hydrate and free water stopped the intermolecular auto-agglutination, leading to the effect of dissolution. The result of CP/MAS13C NMR showed no obvious change of molecular chain structure after the dissolution of da-KGM. There was no new derivative appearing, manifesting that KSCN was a non-derivative solvent of da-KGM.