Study On Mechanism And Optimum Parameters For The Preparation Of Phosphate Monoester Of Sweet Potato Starch

The factors influencing the degree of substitution in preparation ,structure changes,physical and chemical properties of phosphate monoesters of sweet potato starch were systematically studied.The starch phosphate monoesters were produced with sweet potato starch by the wet process,which were esterified by disodium hydrogen phosphate(Na2HPO4) and sodium dihydrogen phosphate(NaH2PO4). We studied the effects of different factors in preparation on degree of substitution(DS) of phosphate monoesters,the results indicated that the DS increased with the level of esterifying agent,reaction temperature and time,the amount of catalyzer;however,when pH increased,DS first increased and then decreased,the combination of Na2HPO4 and NaH2PO4 and vacuum had no marked influence on DS. On the basis of one-factor experiments,we select the ratio of Na2HP04 and NaH2PO4,pH value,esterization temperature and time,amount of catalyzer as the variables,DS as the experiment index. Using the five-factors quadratic regression orthogonal rotary method,we can obtain the following optimum processing conditions for preparation:Na2HPO4:NaH2PO4=3:1,pH value was about 5.5,reaction temperature was 130-140C,reaction time was 2-3h,and the concentration of catalyzer was 4-5% of starch dry weight,the DS of final products were about 0.012.The structure of products were token by FTIR spectrum,and the infra-red spectrogram verified that after esterization,the P=O and P-O-C groups were added in the starch glucose units indeed. The analysis of scan electronic microcopy (SEM) showed that there existed fallings and splits of different degree on the starch granules surface,moreover,the broken degree increased as the DS increased. With the feature of peak diffraction in the diffractogram weakened,the feature of dispersion diffraction increased,these results demonstrated that esterization not only occurred in the amorphous region,but also damaged the crystalline region.Compared with the native sweet potato starch,the paste clarity,solubility and swelling power of the phosphate monoesters increased dramatically. Esterization can also improved the retrogradation and freeze-thaw stability of native starch notably. Furthermore,the properties of lower DS products were superior to those of higher DS products. The apparent viscosity of phosphate monoesters decreased rapidly withthe increase of shear rate,the shear-thinning behavior was found,and it belongs to the character of pseudoplastic fluid. The paste viscosity of phosphate monoesters catalyzed with urea were higher than those of the products which were produced without urea,but the paste clarity value of former were much lower. When the phosphate monoesters were dispersed into the sodium chloride (NaCl),the properties of these paste changed:paste clarity,freeze-thaw stability,solubility,swelling power and apparent viscosity all decreased,and the retrogradation increased. All these changes made the paste properties inferior to those of native starch. While in the sucrose dispersion,we observed the contrary phenomenon,which showed that the paste properties had improved in some degree.