| Acid hydrolysis increases the accessibility of polysaccharides to enzymes and chemical reagents and improves their physicochemical properties,thus being used as the pretreatment method to enhance their biological and chemical modifications.However,acid hydrolysis of polysaccharides is commonly inefficient and time-consuming due to their compact structure and low water solubility.Electric field treatment could shorten reaction time of organic synthesis and increase the yields.Currently,the applications of electro-technologies in polysaccharide processing focus on electro-assisted self-assembly in non-conductive media.Although the possibility of electro-assisted modification of polysaccharides in conductive media has been confirmed,the mechanism is still unclear.In addition,metal electrodes,which are necessary in traditional electro-technologies including pulsed electric field and ohmic heating,may cause undesirable electrochemical reactions,leakage of heavy metal ions and contamination of the sample.Induced electric field?IEF?,which was designed by our research group,is developed by Farady's Law of Induction.The basic configuration is a transformer rather than metal electrodes,thus exhibiting a great potential in polysaccharide modification.Therefore,this thesis firstly explored the effect of IEF on acid hydrolysis of polysaccharides by controlling the migration direction of polysaccharides;then enhanced the efficiency of IEF-assisted hydrolysis by altering the migration rate of polysaccharides;finally,clarified the mechanism of IEF-assisted hydrolysis of polysaccharides by analyzing the migration modes of charged species.The research content and results are as follows:IEF produced thermal and non-thermal effects to enhance acid hydrolysis of polysaccharides.Thermal effect played a dominant role and non-thermal effect was closely related to the charge type of polysaccharides.The changes in molecular weight?Mw?of polysaccharides during acid hydrolysis with or without IEF treatment were determined by HPSEC-MALLS-RI?a high-pressure size-exclusion chromatography system consisting of a multi-angle laser light-scatteringdetector,and a refractive index detector?.The changes in temperature of polysaccharide-acid solutions were monitered by a temperature probe.Results suggested that thermal effect enhanced the hydrolysis of polysaccharides,regardless of their charge type.During IEF-assisted hydrolysis,the temperature of polysaccharide-acid solutions initially increased?4 h?and then kept constant?24 h?;correspondingly,the Mw of polysaccharides rapidly decreased at initial 4 h but then decreased slowly.For example,at initial 4-h hydrolysis stage,the temperature of guar-acid solution rapidly increased from 24.5? to 42 ? with IEF treatment,while the Mw of guar rapidly decreased from 17.23×105g/mol to 4.728×105 g/mol;as hydrolysis time extended to 24 h,the temperature stably increased to 43 ? while the Mw of guar slowly decreased to 1.357×105 g/mol;compared with the control,the Mw of guar decreased by 15.423×105 g/mol after IEF-assisted hydrolysis at an excitation voltage of 75 V for 24 h.Non-thermal effect strengthened acid hydrolysis of negatively-charged and non-charged polysaccharides,but weakened the hydrolysis of positively-charged polysaccharide.When thermal effect was eliminated,the reducing sugar content in potato starch hydrolysate increased by 20.8%,the Mw of guar gum decreased by25.89%,but the Mw of chitosan increased by 6%after IEF treatment at an excitation voltage of 75 V for 24 h.Electrical conductivity of reaction medium was a key factor that determined IEF-assisted hydrolysis of polysaccharides.IEF showed no significant effect on the hydrolysis of polysaccharides conducted in non-conductive media but greatly improved the hydrolysis performed in conductive media.As the conductivity increased,IEF-assisted hydrolysis enhanced.When inconductive methanol,ethanol,2-propanol and 1-butanol were used as hydrolysis media,the changes in Mw of guar gum as a function of hydrolysis with or without IEF treatment were determined and results suggested that IEF treatment showed no significant effects on the hydrolysis rate of guar gum.When conductive water was applied as reaction medium,as KCl content increased,thermal effect enhanced,electrical conductivity significantly increased,impedance(ZS and Zload)dramatically decreased,and hence the output power of IEF system(Pouti=???,n=1,2,3…)increased.When KCl content increased from 0 to 6%,IEF-assisted hydrolysis rate constant increased from 2.739×10-91/?h·g·mol?to 3.417×10-9 1/?h·g·mol?.The effect of IEF-assisted hydrolysis on the structure of guar gum was investigated by Fourier transform infrared spectroscopy?FT-IR?,nuclear magnetic resonance spectrum?1H NMR?,and X-ray diffraction?XRD?.No new group formed,the ratio of mannose and galactose did not change,crystal structure showed no changes,but its granular structure was completely destroyed into small fractions after hydrolysis.thermal gravity analysis?TGA?showed that IEF-assisted hydrolysis slightly decreased the thermal stability of guar.Rheology results suggested that the viscosity of guar dramatically decreased.Induced electrical parameters exhibited significant effect on acid hydrolysis of polysaccharides.As excitation voltage and reactor number increased,the input power(Pini=???,n=1,2,3…)of IEF system significantly increased,and thus IEF-assisted hydrolysis significantly enhanced.The reducing sugar content increased from 0.46 g/L to 5.28g/L as reactor number increased from 1 to 4;when excitation voltage increased from 15 V to75 V,the reducing sugar content increased from 0.59 g/L to 5.28 g/L?60 h?.Moreover,the hydrolysis rate also increased with the addition of salt,which further proved that electrical conductivity of reation medium was closely related to IEF-assisted hydrolysis.The granular,crystal,and chemical structure of potato starch was analyzed by X-ray diffraction?XRD?and scanning electron microscope?SEM?,laser particle size analyzer,and HPSEC-MALLS-RI.Results suggested that IEF-assisted hydrolysis included three stages:First,IEF-HCl treatment assisted hydrolysis roughened smooth surface,destructed external crystalline shell,and decreased molecular weight of potato granules.Second,the internal amorphous and crystalline regions were hydrolyzed simultaneously.The increased crystallinity and rapidly decreased molecular weight were caused by rapid hydrolysis of amorphous region and slow destruction of crystalline region.Finally,the remaining crystalline part containing densely packed double helices of amylopectin chains was further hydrolyzed along with a slight reduction in granular size and molecular weight.Compared to native starch,the swelling power and pasting viscosities of hydrolyzed starches dramatically decreased.Effect of charge contents of polysaccharides on IEF-assisted hydrolysis was not significant.Chitosan samples with various degrees of deacetylation?DD?were prepared by re-acetylation and then the changes in Mw of dissolved chitosan?0.15 mol/L HCl?and insoluble chitosan?1.5 mol/L HCl?as function of IEF-assisted hydrolysis time were determined.Results suggested that the output power of IEF slightly increased with increasing DD,but the increase was insignificant.As a result,no significant difference in IEF-assisted hydrolysis rates was observed among dissolved chitosan samples with various DDs.XRD and SEM results suggested that re-acetylation destroyed the granular and crystal structure of chitosan,made it more susceptible to chemical reagents.Hence,as the DD increased,IEF-assisted hydrolysis of insoluble chitosan decreased.After IEF-assisted hydrolysis,the changes in structure and physico-chemical properties of chitosan were investigated by FT-IR,SEM,XRD,rheology and TGA and results suggested that the DD of chitosan was slightly changed,the crystal and granular structure was partly destroyed,the structural compactness was decreased,the viscosity of chitosan-acid solution was greatly decreased,and the thermal stability was slightly decreased.IEF alters the migration direction and rate of charged species,thus producing thermal and non-thermal effects to affect the hydrolysis rate of polysaccharides.Migration direction depends on electric field direction and the charge type of species.IEF is a periodic alternating electric field.Charged species exhibit alternating migration under IEF.Polysaccharides with different charge type exhibit different migration direction under IEF,their collosion possibility with hydrogen ions is different,and thus the produced non-thermal effects is different:the migration direction of negatively-charged polysaccharide is opposite to hydrogen ions,the collosion possibility increased,and thus non-thermal effect strengthens the hydrolysis of negatively-charged polysaccharide;IEF has negligible effects on non-charged polysaccharide,but accelerates the migration of hydrogen ions,thus hydrogen ions attack the non-charged polysaccharide at a more rapid rate;the migration directions of positively-charged polysaccharide and hydrogen ions are same,collosion possibility decreases,and thus non-thermal effect inhibites the hydrolysis of positively-charged polysaccharide.The migration rate of charged species under IEF depends on the strength of the field and the charge content of the species.The effect of charge content is not significant.The strength of IEF increases with increasing excitation voltage,reactor number and electrical conductivity of reaction medium.Thermal effect dominates during IEF processing.Therefore,IEF enhances the acid hydrolysis of polysaccharides,regardless of their charge type. |
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