A Comparison Study Of Natural Polysaccharide Emulsifiers

Emulsifier is the kind of material that owns surface activity and makes the emulsion stable.It has been widely used in the fields of food,cosmetics,textile,printing and dyeing,medicine,plastics and petrochemical.Food emulsifier is the type of emulsifiers with large consumption and market demand.High requirements are need for food emulsifier as it has to be efficient,multifunctional,nonirritating,biodegradable,and safe.Therefore,the development of new natural polysaccharide emulsifiers has received more and more attention nowadays.Corn fiber gum(CFG),also called arabinoxylan and hemicellulose B,is extracted from corn bran with highly branched structure using the alkaline hydrogen peroxide method.CFG is a new type of natural emulsifier which has great potential in food and beverage flavor emulsification.Compared to other early developed and widely used polysaccharide emulsifiers,especially the ones with highly branched structure such as octenyl succinate anhydride-modified starch(OSA-s),gum arabic(GA)and soybean soluble polysaccharide(SSPS),CFG is lack of studies on the molecular structure characterization,the rheological properties evaluation and the emulsification mechanism clarification.Both bulk and interfacial rheology especially the latter have been focused nowadays due to the close relationship between these properties to emulsifying stabilities.Bulk shear viscosity and surface shear rheological properties have been proved to be significant in evaluating the stability of bubble and emulsion.In this thesis,multiple methods were used to characterize the physicochemical properties,molecular parameters and structure of CFG and other three commercial polysaccharides.The bulk and interfacial rheological properties were systematically compared.The stability of limonene emulsion prepared by these gum were evaluated and the interface and emulsion stabilizing mechanisms were illustrated according to the molecular structure of these polysaccharide.The complex relationship between interfacial viscoelasticity and emulsion stability were indicated.The comparative studies on the physicochemical properties and emulsion stabilized mechanisms of polysaccharide emulsifiers with heterogeneous structure not only help to develop the emulsification theory of natural polymer emulsifiers,but also provide firm technical support to broaden and promote the industrialized application of CFG and other branched polysaccharides.The main contents and conclusions were as follows:1 Spectral characterization was conducted to confirm the contained polysaccharide composition of araboxylan and glucuronic acids in CFG.The protein contents,Mw,[η] and molecular structure were measured by Dumas Combustion method,SEC and AFM,respectively.The results proved that these four kinds of polysaccharides all had high Mw,low [η] and compact structure.The Mw,Rh and [η] for CFG were 2.0?105 Da,14.1 nm and 1.12 d L/g,respectively.Branched chains could be clearly observed by AFM using Tween 20 as the diffusion.Besides,high decomposition temperature and amorphous nature of CFG were identified by the results of TGA and XRD.2 The bulk rheological properties of the four biomacromolecule were significantly different.CFG and OSA-s showed Newtonian fluid behavior even at a high concentration of 15% w/w,while shear thinning and thixotropic behavior were observed of GA and SSPS even at a low concentration of 1% w/w.The results of various modes of rheological tests confirmed the viscoelastic nature and structural rebuilding ability of polysaccharides aqueous solution.The critical overlap concentration(c*)and critical micelle concentration(CMC)were measured to clarify the bulk rheological properties.GA and SSPS formed intermolecular micelles above CMC and intramolecular aggregation below CMC.Their shear thinning behavior could be attributed to the breaking down and rebuilding of aggregated structure rather than disentanglement and orientation.Time-thickening phenomenon of CFG,GA and SSPS had close connection with the protein contents and linkage between protein and polysaccharide chains.Highly branched and compact structure together with short branched chains was the main reason for the Newtonian behavior of OSA-s.3 The surface tension and air/liquid interfacial rheological responses of biomacromolecule were compared and the influences of concentration(c),temperature(T),hydration time,enzyme and salts were considered.The surface tension decreased with increasing c and T,while the interfacial viscoelastic moduli increased with c but decreased with T.The films formed by CFG and OSA-s were predominantly viscous(G″ > G′)while elastically for GA and SSPS and the layer formation kinetics were not the same.The best hydration time for CFG(~ 40 h)to exhibit the highest film strength was longer than the others.The enzyme digestion experiments showed that G′ and G″ for OSA-s,GA and SSPS decreased severely compared with CFG,as the proteins in CFG had been proved to be inaccessible and difficult to be digested.The addition of electrolyte(Na Cl,Ca Cl2)would affect the aggregated and stretch extent of the molecule and the intermolecular interaction.Although adding Na+ and Ca2+ to CFG solution would resulted in the increase of interfacial moduli,the supplemented c did not always show positive correlation with the increase of viscoelastic moduli.The distinguished molecular configuration,protein contents and their linkage with polysaccharides all had influences on their rheological properties.4 The interfacial shear rheological properties at the limonene/liquid interface were similar as air/liquid interface.CFG showed specificity exhibiting a sustained rising of G′ while still maintaining viscous during the test time.OSA-s produced predominantly viscous,fluid-like interfacial layers while GA and SSPS formed elastic,solid-like films.Different modes of interfacial rheological tests,including time sweep,amplitude and frequency sweep,steady shear,creep and stress relaxation were performed and used to identify the viscoelastic properties of the interface.We found that the films formed by CFG,GA and SSPS all showed elasticity and the G′ of CFG was much lower than GA and SSPS.The empirical Cox-Merz rule was not obeyed for these formed layers(η < η*)and hysteresis were seen in the measurement of η versus shear rates.The elasticity of the layers followed the order of SSPS > GA > CFG > OSA-s according to the values of γe/γv(elastically recovered strain / viscous strain).The creep compliance was fitted to Maxwell-Voigt type model and the value of time scale(λm)for CFG film equaled to 0.44 s,indicating the weak physically cross-linked network at the interface.A power law was used to interpret the relaxation behavior and the calculated value of gel strength for CFG was 1.87?10-4,which was magnitudes lower than GA and SSPS(2?10-2).5 According to the interfacial rheological properties,a schematic mechanism involving molecular structure and intermolecular interaction was proposed and compared.The hydrophobic amino acid-rich portions of CFG,GA and SSPS adsorb and anchor onto the surface while the hydrophilic hydrate blocks protrude into the solution.The increase of G′ and G″ and the reinforcement of the formed film can be attributed to electrostatic interaction among negatively molecule and the entanglement of polypeptide chains.Certain amount of entangled branched chains and multilayer adsorption may also have contribution to the enhanced interfacial viscoelastic response with time.As for OSA-s,partial of alkenyls introduced during its modification provide the hydrophobic property and the grafted short,substituted branched chains eliminate the possibility to form strong intermolecular interaction or structured network at the surface.Thus no increase was observed in its interfacial viscoelasticity and the strength of the formed film stayed still.6 The limonene emulsions(5% w/w)were prepared by the four branched polysaccharide emulsifiers and the effect of bulk concentration and storage temperature on their stability were evaluated by monitoring accelerate destabilization,droplet-size distribution and droplets patterns.Droplets aggregating and emulsion creaming would appear at low c of emulsifier(1%)and high storage T(40 oC).Increasing c and decreasing T would result in high stability of the emulsion.7 The interfacial rheology is a sensitive and comprehensive way to estimate the complex corresponding relationship between interfacial viscoelasticity and emulsion stability.It plays the key role in confirming the viscoelastic property of the formed film,analyzing emulsification dynamics and revealing the stabilization mechanism.The relationship between interfacial moduli and emulsion stability can’t be simply established.The emulsions with low formed film strength may also exhibit high stability.