Study On Modification And Emulsification Mechanism Of Alkaline Soluble Polysaccharides From Sugar Beet Pulp

Sugar beet pulp is a main by-product frombeet sugar industry,which could be used to prepare pectin with emulsifying properties.After pectin extraction,the pulp which still contains a large amount of alkaline soluble polysaccharides with amphiphilic nature is valuable.The alkaline soluble polysaccharides have been proved to contain hydrophobic components and have good interfacial activity.However,at present,the extraction,structure and properties of alkaline soluble polysaccharides from sugar beet pulp are still lack of systematic research and breakthrough,which greatly limits its further application.Under the background of the market demand and techinical bottleneck of the deep processing of sugar beet pulp,this paper,with alkaline soluble polysaccharides as its research subject,studies the relationship between physicochemical properties and emulsifying properties of alkaline soluble polysaccharides from sugar beet pulp.Focusing on the effects of key functional groups（protein and esterification group）on emulsifying properties,this paper aims at revealing the emulsifying mechanism of alkaline soluble polysaccharides and providing theoretical basis for its further application as food-grade emulsifier.The main research contents are as follows:1.SBP,ASP1 and ASP2 were extracted from sugar beet pulp by the following series of steps:acid method,alkaline method and alkaline hydrogen peroxide method.The yields of SBP,ASP1 and ASP2 were 11.1%,10.3%and 11.6%,respectively.The content of Gal A,NS,DM and DA of ASP1 was 75.8%,11.7%,23.6%and 14.0%,respectively;However,the Gal A of ASP2 was only 34.4%,and the NS was 30.0%,while Gal A was hardly esterified.The Mw（295 k Da）,hydraulic radius（239.4 nm）and Zeta potential（-38.1 m V）of ASP1 are also different from those of ASP2（238 k Da,110.3 nm,-32.3 m V）.FTIR and AFM analysis showed that there were obvious differences in chemical functional groups and molecular morphology among SBP,ASP1 and ASP2.The above analysis indicate ASP1 and ASP2probably possessing different emulsifying properties which need in-depth studies.2.ASP1 and ASP2 were used as emulsifiers to stabilize the oil-water interface to analyze their emulsifying properties by taking SBP and GA as references.The results showed that the critical concentrations of SBP,ASP1,ASP2 and GA were 1.25%,1.5%,1.5%and 9%respectively.The d_4,3value of the emulsion at the critical concentration of polysaccharides were 0.55,0.52,2.14 and 0.55μmrespectively.Due toof the effect of free carboxyl groups,ASP1 will form electrostatic shielding effect in the high Na⁺concent oil-in-water system and result in the reduction of its emulsifying activity.However,in the system containing Ca²⁺,the hydrophobic groups of ASP1 and ASP2 could be wrapped due to the calcium bridge effect,resulting in a bad emulsifying activity.ASP2 has excellent emulsification stability due to its high electrostatic repulsion and viscosity.3.The emulsifying property of ASP1 is good,and its molecular weight distribution display as obvious bimodal pattern,indicating different components in ASP1 with different molecular weight.Therefore,five subfractions（UR,UP,F50,F65,F75）were obtained from ASP1 by using hollow fiber membrane filtration combined with ethanol precipitation.The membrane filtration method can efficiently separate the UP from ASP1 in the process of concentrating ASP1.Furthermore,the ethanol precipitation method was used to separate ASP1 subfractions（F50,F65,F75）with different molecular polarity.The results showed that the performance of sub-fractions was varied with the differences of molecular structure（molecular weight,degree of esterification,protein content,etc.）,among which F75 had the best emulsifying activity and calcium tolerance,and F65 had the best emulsifying stability,while UP possessed the worst emulsifying properties.4.In order to study the effect of esterification group on the emulsifying properties and to overcome the poor salt resistance,especially the calcium-induced flocculation of ASP2,we used ASP2 as the template to esterify the free carboxyl and hydroxyl groups of galacturonic acid residues in ASP2.The results showed that molecular weight,Zeta potential,neutral sugar content were varied with the degree of methoxylation or acetylation.Based on solely or conbined modification,four representative modified products were selected as emulsifiers to study the relationships between esterification group and emulsifying properties.The emulsifying activity of A-ASP2 is the best,and the d_4,3 of its emulsion is 0.29μm,followed by M-ASP2.However,the emulsifying activity of combined modified sample was significantly lower than that of unmodified ASP2.Due to the decrease of zeta potential,the emulsifying stability of M-ASP2 is poor,while the emulsifying stability of A-ASP2 is equivalent to that of ASP2.M-ASP2 can effectively inhibit the calcium bridging flocculation,so that it can stabilize oil-in-water interface in the high calcium system(containing 50mmol·L^-1 Ca²⁺)with the d_4,3 of 0.53μm.However,the acetylation group had no obvious inhibition to the calcium bridge effect.The poor emulsifying properties of AMA and MAA could be attributed to the fact that the dually modified ASP2 samples may suffer unfavorable structural alterations,resulting in the inferior emulsifying activity of AMA and MAA.In addition,excessive hydrophobic groups distributing among ASP2 may induce unfavorable conformational transition,with comittant enhanced hydrophobic interactions between chain segments but less adsorption capacity onto the oil-water interface,which resulted the assessibility of proteinaceous moiety to the oil phase may also be hindered.5.Protein is a key structure for the emulsifying activity of polysaccharides.In order to study the effect of the binding modes of exogenous protein and protein content on emulsifying properties,self-crosslinking and ASP2-WPI conjugates were prepared to enhance the protein content of ASP2 through Maillard reaction and genipin crosslinking reaction.The result showed that both reactions occurred on theε-amino group of lysine.The ASP2-WPI conjugates prepared by Maillard reaction has relatively more protein anchors than the conjugates prepared by genipin crosslinking,as a result,the d_4,3 value reduced to 0.28μm.However,due to the decrease of electrostatic repulsion,the emulsification stability of ASP2-WPI conjugates is decreased.The ASP2-WPI conjugates prepared by genipin crosslinking can make the protein anchor larger to be easily exposed in water,the emulsifying activity could be slight increase.Furthermore,due to the increase of molecular weight of ASP2 and viscosity effect,it possess good emulsion stability.