Modification Of Xanthan Gum And Formation Mechanism Of Xanthan Gum And Locust Bean Gum Gel

Xanthan gum,which is an extracellular polysaccharide secreted by Xanthomonas Campestris,is widely used as a thickener in the food and cosmetics industry because its electric potential and pseudoplasticity.The structure of xanthan is composed of the backbone and a side chain.The backbone is formed by the repeated glucose units,linked by the β-(1-4)glycosidic bonds.The side chain is a negatively charged trisaccharide [β-(1-3)-D-mannose-(1-4)-β-D-glucuronic acid-(1-2)-α-Dmannose].Most of the acetyl groups are linked to the inner mannose,and the remaining acetyl groups exist on the terminal mannose.Furthermore,the terminal mannose possesses a pyruvate group.The helix conformation and conformation transformation of xanthan are clear.The secondary structure of xanthan in solution is a right-handed helix with five-fold symmetry and the pitch is 4.7 nm.In this research,xanthan gum was treated by cooling and deacetylation methods,and the conformation change of xanthan was detected.On this basis,the difference of interaction force of syncretistic interaction between xanthan and galactomannan was studied.The main results of this research are as follow:(1)After low temperature treatment,the fibrillar degree of LG,the interfacial properties,and the hydrophobicity of CG and LG exhibit obvious differences compared to those of XG.These results revealed that the conformation of xanthan has been changed after cooling or lyophilization treatment.Xanthan treated by cooling or lyophilization exhibited aggregated property in the experiment of dynamic light scattering.In the rheological tests,the storage modulus and viscosity of xanthan was decreased after low temperature treatment.All of these phenomena were attributed to the conformation change of xanthan molecules: When the temperature decreases,the activation energy of water decreases and the destruction of water to intermolecular force became weaken.At the same time,xanthan molecules are close to each other and the part of double helix structure in solution is increased.When the temperature further decreased,ice crystals began to make xanthan gum molecules further approached to form clusters through hydrophobic interaction.Furthermore,the hydrophobic region was existed in the cluster.(2)The fibrillar degree and interfacial properties of deacetylated xanthan were decreased,and the its hydrophobicity was enhanced.This was attributed to the conformation changed of molecules.the results of electron spin resonance spectroscopy and rheological study demonstrated that the interaction between molecules was increased and the conformation of xanthan molecules was transformed to disordered when the acetyl group was removed.Xanthan possesses helix structure which is maintained by the hydrogen bond between acetyl group and O of the mannose of backbone.And the double helix conformation was formed by the interaction between molecules.When the acetyl group was removed,the force which is important to maintain the helix structure of xanthan was destructed,and the ordered conformation of xanthan became unstable in room temperature.(3)the gels formed between locust bean gum and the different conformations of xanthan had different structures,morphologies and properties.These differences were caused by the different xanthan conformations.When the ordered helix conformation of xanthan was dominant in solution,locust bean gum interacted with xanthan via electrostatic interactions.In contrast,locust bean gum interacted with the disordered conformation of xanthan through hydrogen bonding.In addition,a heterotypic gel structure was formed by hydrogen bonding,while a geometry gel structure was formed by electrostatic interactions.The results also show that the hydrogen bonding and the electrostatic interactions are not present at the same time,but instead inhibit each other.