Effects Of Molecular Structure And Solution Environment On Spatial Conformation And Functionality Of Citrus Pectin

The molecular structure of pectin is the internal reason that determines its spatial conformation,and the solution environment is the main external factor that causes the change of noncovalent interactions between molecular chains.At the same time,the spatial conformation of pectin determined by the molecular structure and solution environment directly affects the functionality of pectin.In this paper,by regulating the solution environment and the molecular structure of pectin,the intermolecular and intramolecular interactions of pectin were changed,and new gelation mechanisms of high methoxyl pectin（HMP）was explored by constructing different solution environments,which aimed to improve the functional properties of pectin.The following conclusions can be drawn in this paper:（1）In this study,the conformation of HMP in a binary system（water/ethanol）was investigated,which was found have positive effect on the stability of HMP emulsions.According to the results,increasing ethanol concentration（<21%）could enhance intermolecular and intramolecular noncovalent interactions,which could result in an increased viscosity and limit the migration of oil droplets.HMP also undergoes conformational contraction in ethanol solutions and adsorbed at O/W interface more quickly,which could be beneficial to the stability of the emulsions.However,an ethanol concentration of more than21%could lead to over aggregating of HMP,which could will be detrimental to the stability of O/W emulsion.This study ignored the restriction of molecular structure on the emulsification of HMP,and provided a new idea for the functional improvement of pectin.（2）Based on the improved emulsifying stability of HMP in binary system（water/ethanol）,the gelation of HMP emulsion could be realized by acidification.A gelling mechanism of HMP based on H⁺/ethanol was proposed,and the influence of molecular structure on the gelling properties of pectin were discussed.Results show that the HMP gelling in H⁺/ethanol environment was driven intermolecular hydrogen bonds.Comparing the gelling behavior of HMP and LMP under H⁺/ethanol,it can concluded that RG-I structure play an important role in the stabilization of HMP gels.Under the condition of 25%ethanol and p H=3.0,the emulsion gel prepared by HMP can stabilize the oil phase withΦ=0.2～0.75,but LMP can only prepare the emulsion gel with high internal phase withΦ=0.75.In H⁺/ethanol system,The changed solvation state and aggregation behavior of pectin molecules endows the emulsion with good freeze-thaw stability.（3）High concentration ammonium sulfate（AS）was used as co-solute,and a gelling mechanism of HMP based on H⁺/AS was proposed.The effect of AS concentration on the solubility of HMP was investigated,results indicated that AS destroy the hydration layer and shield the negative charge of HMP which could promote the aggregation of pectin molecules.The gelling conditions of HMP in H+/AS environment（AS:25～30%,p H≤3.0）were preliminarily determined,and it is claimed that HMP content（0.25～1%）is the main factor to regulate gel strength.The microstructure of HMP gels was analyzed,and it was found that dense cavity structure existed in HMP gel,which could limit the fluidity of AS solution.The stability of HMP gel was investigated in a dynamic water environment model,which suggested that HMP gel could slowly release NH₄⁺in dynamic water environment,and the potential as a of HMP gels be used as slow-release fertilizer gel.（4）Oxidized pectin（OP）with dialdehyde structure was prepared by oxidizing HMP with periodate,and adipic dihydrazide（ADH）covalently crosslinked OP gel was prepared by the Schiff base reaction between amino group and active aldehyde group.Oxidized pectin with different degrees of modification could be obtained by controlling the amount of oxidant,and the molecular structure and chain conformation of HMP and OP were compared.Results suggested that oxidation destroyed the molecular chain（mainly RG-I structure）of HMP,and the conformation HMP altered from random coil to compact sphere.HMP and OP were applied to three gel preparation methods(Ca²⁺crosslinking,acid/sugar gelling and ADH crosslinking),and the gelling ability of HMP and OP were discussed.The gel morphology and rheological results indicated that OP gels crosslinked by ADH could from an elastic gels,and oxidized pectin could not gelling in acid/sugar condition but retains some responsiveness to Ca²⁺.Large amplitude oscillation shear（LAOS）confirmed that double gelation of ADH+Ca²⁺could further improve the gel network structure of OP.（5）Colloidal bismuth pectin（PB）was fabricated with citrus HMP,and the structure of PB was characterized.The aggregation behavior of PB in acidic environment was investigated for the first time.Results indicated that alkali processing lead to the desertification of HMP,and confirmed the ionic binding between Bi（III）and carboxyl groups of pectin.In acidic solution,Bi（III）gradually dissociated from carboxyl groups which could lead to a much larger volume of flocculation.At a p H value close to the stomach environment（p H 2.2～1.5）,pectin forms a gel network,in which colloidal bismuth can be trapped.Rheology and SEM indicated that PB particles have strongest cohesion in p H 2.2 and 1.5,and a dense gel network structure could be observed.This study investigated the aggregation behavior of bismuth pectin in acidic environment and the function of pectin in PB,which provides a theoretical basis for the application of colloidal pectin.In summary,based on different solution environments and molecular structures,this paper puts forward new gelation mechanisms of pectin,which provides a theoretical basis for the functional development of citrus pectin and enriches the application scenarios of pectin gels.