Conformation, Rheology And Homogeneous Esterification Of Starch In Aqueous ZnCl 2 Solution

Starch is a kind of natural biomacromolecule.It is cheap,renewable and degradable,so it is widely used in industry.However,due to the compaction of starch granules,it is difficult to dissolve in water before gelatinization.And due to the complexity of starch granules,after gelatinization,the starch molecules still cannot disperse in water homogeneously.As a result,most chemical reactions for modified starch are reacted in the non-homogeneous system,which result in the low reaction efficiency,the low substitution degree,and the high energy consumption.Focused on this problem,in our former work,it found that the zinc chloride solution was a good solvent for starch.Therefore,this dissertation did the further study work,which concentrated on the dissolution process of starch granules in zinc chloride solution,the existence of starch molecule chains in zinc chloride solution,and the effect of zinc chloride solution on starch chemical modification.Specifically,the study results were as follows:1.It observed the dissolution process of starch granules in zinc chloride solution.First,when starch granules were dissolved in 29.6% zinc chloride solution?the critical concentration for dissolution?,the dissolution process was similar as that in pure water,which the granules swelled and expanded from the hilum irreversible,until the granules disintegrated completely.This process lasted for a very short time,and was accompanied by the disappearance of the polarizing cross.Second,when starch granules dissolved in 65% zinc chloride solution,the dissolution process lasted for a long time.The whole granules were swelled slowly,and then they were disintegrated instantly.On the other hand,comparing the dissolution process of corn starch with different amylose content in zinc chloride solution,it found that the higher amylose content in granules,the more quickly the granules dissolved.This indicated that the combination of ion with amylose molecule was faster?or easier?than the combination with amylopectin.2.It studied the rheological properties and conformation of cassava starch in zinc chloride solution.The results showed that the cassava starch-zinc chloride solution was a pseudo plastic fluid,and the shear thinning occurred.Based on the power-law model,the rheological equation was established for starch-ZnCl₂ solutions?starch content: 1%–5%;ZnCl₂ concentration:29.6%–65%;temperature: 25°C–60°C?,and was expected to be useful to predict the rheological properties in other conditions.After that,by discussing and analyzing the equations and the main curves,the phase diagram of starch molecule in zinc chloride solution was established,and three states of the conformation of starch in solution were defined.In the extremely dilute area,the entanglement and the interaction between molecules was small.In the semi-dilute area,there wasa certain degree of entanglement between the molecules.In the viscous region,the entanglement between starch chains was enhanced sharply.The reason for this was that there were two ion space orbits in Zn2+,and starch molecule hydroxyl groups had one lone electron pair,so the Zn2+could attract two starch molecule hydroxyl groups and played as the cross-linking agent to promote the entanglement between starch molecule chains.3.It investigated the effect of zinc chloride solution?namely the homogeneous conditions?on the chemical modification of starch.Specifically,it used the 29.6% zinc chloride solution as the homogeneous reaction medium,used the cassava starch and the succinic anhydride as reactants to prepare esterified starch.After reaction,the substitution degree of starch ester was0.063,under the parameters that the material ratio was 1.2,the reaction temperature was 70 °C and the reaction time was 3h.Under the same parameters,used water as heterogeneous reaction medium,the substitution degree of starch ester was only 0.018,which was clearly lower than that of homogeneous system.