Selective Preparation Of Gellan Gum With Different Acylation And Its Molecular Conformation,rheological Behavior And Application Properties

High acyl gellan gum is a linear,anionic heteropolyaccharide produced by a microorganism Sphingomonas elodea.It has been broadly applied in food products to provide various functional properties such as gelling,thickening,stabilizing,etc.However,there still lie some shortcomings in gelling and stabilizing of domestic high acyl gellan gum.Previous studies have revealed that the defects of gelling and stabilizing properties may be attributed to the composition of acyl group in gellum gum.Further studies to clarify this issue seems to be of high necessity.In this study,a series of gellan gum with different degrees of deacetylation without degradation were obtained by deacetylation reaction in Na₂HPO₄/Na OH buffer system.The acquired samples were subjected to further characterization,such as the solution properties and rheological behaviour.Then multiple beverage systems containing natural pigment,such as carotene,anthocyanins,were prepared with the addition of each gellan gum sample.The roles of gellan gum in system emulsification and stability were determined.The main results are shown as follows:The sodium salt of high acyl gellan?NA-GG?was obtained from a commercial product,LT-100,by ionic exchange,which could improve the purity and water solubility of gellan gum significantly.Then the gellan gum with various glyceral and acyl contents were prepared by modulating reaction temperatures,reaction duration,and alkali types.Through the determination of glyceryl and acetyl content in the acquired samples,we found that the glyceryl was easier to dissociate and removed from the gellan gum in the Na₂HPO₄/Na OH buffer system,comparing to the acetyl.Thus,Na₂HPO₄/NaOH could serve as an ideal system to remove glyceral and acetyl gradually from high acyl gellan gum.Based on the data matrix of temperature,duration,glyceryl content,and acetyl content,we took advantage of BP-artificial neural network?ANN?to establish a mathematical model.The established ANN model can be used as useful tool to precisely prepare gellan gum with specific acetyl and glyceryl content,which could be used in personalized gellan gum preparation.According to the ANN model,six gellan gum samples with different acyl compositions?named G0,G1,G2,G3,G4,G5?were prepared and characterized.FT-IR spectras showed that the main structure of gellan gum samples remain stable after the deacylation processing.Their molecular parameters?weight-average molecular weight M_w,molecular weight distribution M_w/M_n,z-average radius ofgyration R_g ,intrinsic viscosity [?] and viscosity average molecular weight?andconformation characteristics in the good solvent of 50 m M NaNO₃ /DMSO solution were determined by using capillary viscometry and GPC-MALLS.It showed that the insertion of glyceryl group increased the chain extension of gellan gum.In order to make comparisons with the experimental data,gellan gum samples with different molecular weights were selected for the molecular dynamic modeling.The conformational parameter?slope of the curve?calculated from modeling data?0.8494?was similar to that obtained from GPC-MALLS?0.9092?,indicating gellan gum macromolecules tend to adopt a nearly rod semi-flexible chains conformation in solution.The conformational properties of gellan gum samples with different glyceryl group?named a,b,c,e?were also considered.The calculated R_g values were 4.93 ±0.10,4.28 ± 0.25,4.51 ± 0.15,and 4.02 ± 0.09,respectively,which were consistent with the experimental data.For the completely deacetylated gellan gum samples?d and f?,the calculate R_g values were 4.90 ± 0.14 and 7.47 ± 0.14,respectively.Theabnormal increase of R_g indicated the great role of acetyl group on the molecularconformation of gellan gum.Rheological tests showed that the sol-to-gel transition temperature(T_tr)of gellangum was mainly influenced by the glyceryl content,but not the acetyl group.However,the removal of acetyl groups,to some extent,would increase the storage module and cause the thermal hysteresis.The existence of acetyl group could improve the ion tolerance of gellan gum.The potential gelation mechanism of partial deacylated gellan gum was then proposed based on the molecular conformation and rhelogical properties: gellan gum could form the stable "high acyl" structure by the association of glyceryl groups.However,when the glyceryl content was less than6.65 %,gellan gum present the "deacylated" structure instead of the "high acyl" structure.The excellent emulsification properties of gellan gum on carotene,as well as the effect of acylation differences in gellan gum on its emulsification properties,were revealed.It was found that glyceryl and acetyl could sustain the excellent emulsification properties of gellan gum synergistically but not identically.When the glyceryl content is less than 6.65 %,even if the acetyl content remains unchanged,gellan gum could no longer maintain a stable emulsion system,and the milking phenomenon at the bottom of the system occur.Removal of acetyl groups would be helpful to alleviate the milking phenomenon,but it would cause local flocculation of the system and lost the emulsion ability.Full consideration about the protection of acetyl groups must be taken during the emulsion process.Related research results have been patented.Previous work in our laboratory found that the acyl group of gellan gum show great advantage in improving the suspension performance and maintaining the anthocyanin thermal stability of the cloudy juice beverage.Here,the different effects of low and high acyl gellan gum on the thermal stability of anthocyanin-containing model beverage systems in the presence of ascorbic acid were investigated.The addition of gellan gum,especially high acyl gellan gum,resulted in higher absorbance values,which suggested higher anthocyanin stability during heat treatment.Thethermal stability of each anthocyanin was significantly improved by the addition of gellan gum?P < 0.05?.Moreover,according to HPLC/MS-MS results,di-acylated anthocyanins exhibited higher heat resistance than mono-acylated anthocyanins.The molecular dynamics simulations revealed that the improvement in the anthocyanin stability following the addition of gellan gum was due to the interaction between the gellan gum and the anthocyanins.High acyl gellan gum was found to have stronger interactions with the anthocyanins than low acyl gellan gum,which was attributed to the increased steric hinderance and flexibility caused by the acyl groups.In summary,by the selective preparation of various acylated gellan gum samples,the synergy and specific roles of glyceryl and acetyl in the gellan gum sol-gel properties were revealed,which could be helpful to the product modification,quality improvement and application promotion of gellan gum.