The Impact Mechanism Of Freezing Process On Deacetylated Konjac Glucomannan Gel And Its Application

Konjac glucomannan (KGM) is a kind of natural neutral hydrophilic polysaccharide from the tuber of amorphophallus konjac plant. High elastic rubber-like deacetylated konjac glucomannan (Da-KGM) gel could be obtained by adding a certain amount of alkali in konjac glucomannan sol to remove the acetyl groups attached to the saccharide units and then heating the mixture for a certain time. A series of unique phenomenon for this gel exist in the low-temperature range such as increase of gel strength after the freezing and thawing treatment that can be applied in food processing. There is no systematic study of change of structure and properties for Da-KGM gel so far. In view of its potential and special application, this paper examined the effects of freezing treatment on texture (TPA) parameters and syneresis of Da-KGM gel, characterized the structure and properties of the KGM sol and gel after the freezing treatment using rheology, FT-IR, DSC, SEM and XRD, in order to explore the mechanism that freezing process affect the structure and properties of the Da-KGM gel. This study could provide reference to the control of gel texture in actual production process. Furthermore, the theoretical research was extended to the preparation of a series of functional materials in order to expand its scope of application, and promote high value-added processing and utilization of konjac glucomannan.The main results are as follows:1. The parameters of textural property analysis (TPA) of Da-KGM gel with different concentrations were changed after freezing process, and the increase in hardness were greater when the concentration of the sample were larger. When the initial concentration of KGM was5wt%, hardness of frozen gel increased by2950%. It was found that the increase of the gel concentration is far less than the increase of the hardness after freezing, indicating that the change of gel texture mainly resulted from the change of gel structure rather than syneresis. Frozen treatment exerted a great impact on the textural properties of the samples. In the set temperature range of the experiment, hardness was the largest as the gel was frozen in-30℃. The hardness of the sample reached a maximum value after1cycle of freezing-thawing, then decreased with increase of freezing-thawing cycles, along with the increase of syneresis. The results show that appropriate freezing process is an effective measure to regulate the gel texture.2. With increase of alkali concentration, the deacetylation degree (DD) of KGM raised, and gel hardness and syneresis also showed an upward trend. The other parameters of TPA also changed with the amount of added alkali varied. This indicated that the deacetylation process was one of the most important factors affecting the texture properties and syneresis of frozen Da-KGM gel. The eutectic point of deacetylated konjac gel dropped from-11.9℃to-13.4℃while the thawing temperature increased at first and then decreased in this process with increasing degree of deacetylation.3. The concentration of the initial KGM, the freezing temperature and the amount of alkali all affected the microstructure of the Da-KGM gel, as these factors are associated with the growth of ice crystal. Rheology experiments of frozen Da-KGM sol indicated the freeze concentration and squeezing effect due to growth of ice crystals promoted self-aggregation of Da-KGM molecules resulting in decrease of entanglement among glucomannan molecules.4. Substances that may affect intermolecular forces such as urea, SDS or NaCl were added during the gel preparation. The TPA and peptization reulsts suggested not only hydrogen bonding but also hydrophobic interaction were the maintain force of the frozen Da-KGM gel.5. In order to increase the porosity of KGM material fabricated by freezing process and expand the application scope of frozen KGM gel, surfactant template method combined with freezing process was utilized to prepare KGM porous materials. The addition of SDS affected the porosity of the porous material, absorption rate of distilled water and saline. The freezing conditions (time and temperature) have impact on the structure and properties of KGM porous materials. The tensile strength of the foam increased while the break elongation and porosity decreased with the extension of the freezing time. The tensile strength of the foam frozen at-10℃was obviously higher than that of other group and reached a maximum tensile strength (49.12KPa) when the freezing time is9d. The porosity is higher when the freezing temperature is higher. XRD and DSC analysis indicated freezing induced change of crystal structure and thermal properties of KGM.6. The composite PVA-ε-PL antibacterial material based on konjac porous material could slowly release ε-PL and exhibited good inhibition effect against Staphylococcus aureus and Escherichia coli. Adsorption experiments of tannic acid showed the maximum adsorption capability of tannin from aqueous solution was653.97mg/g. Adsorption kinetic parameters of tannic acid by konjac porous material were better described by pseudo-second-order equation. Tannic acid adsorption by this porous material is more favorable at low temperature. Isotherm experiments indicated tannic acid adsorption by this porous material is a multilayer adsorption process. On this basis, a nano Ag-loaded foam was prepared by in situ reduction of Ag+using the tannin acid adsorbed on konjac porous material and exhibited highly effective catalysis activity and strong antibacterial ability. These studies mentioned above can lay the foundation for the exploitation of a class of degradable biological antibacterial, catalytic and multilayer adsorption porous materials.7. KGM-montmorillonite composite aerogel can be prepared using the facile freeze-drying method. The obtained composite aerogel is white with a low density (<0.09g/cm3). Addition of sodium montmorillonite (Na+-MMT) into the aerogel greatly improved the mechanical properties and thermal stability and changed microstructure of the aerogels. The compression modulus and specific modulus of the composite aerogel increased with increase of alkali within the system, and the addition of alkali improved the thermal stability of the composite aerogel. The compression modulus and specific modulus of the composite aerogel gradually increased with decreasing of freezing temperature when the initial content of KGM and Na+-MMT were fixed. When the initial content of KGM and Na+-MMT were2wt%,4wt%respectively, and pre-freezing temperature was-80℃, the compression modulus and specific modulus of composite aerogel reached a maximum value in the experiment, namely3.480MPa and53.80MPa cm3/g, respectively.