Morphological Changes Of Blocklets During The Gelatinization Process And The Properties Of Insufficient Gelatinization Starch Granules

The structure of starch can be easily divided into four hierarchies:molecules,blocklets,shells,and granules.Blocklets comprise the building blocks of the granules whose structure is a difficult issue in starch structure research.Since the method for separating blocklets from granules has not been established,most studies for blocklets concentrate on in-situ observation and size estimation.The study of starch gelatinization process only restrict to the granular level.The properties of amylose content,particle size,thermodynamic property,crystalline property of tuber(sweet potato,potato,tapioca),legume(pea,mung bean),and cereal(buckwheat,corn,wheat)starches have been tested in our previous studies.The blocklets were exhibited in eight starches after insufficent gelatinization or enzymolysis.However,little information was available for the morphological changes of blocklets in gelatinization process.The pasting temperatures of sweet potato,potato,tapioca,pea,mung bean,buckwheat,corn and wheat starch in turn were: 74,70,72,71,72,72,89 and 93?,which were determined by using a Rapid Visco Analyzer(RVA).A series of programs were set up by reducing the starch concentration(from 5% to 1%)and temperature,based on the standard program for determination of pasting temperature.The maximum temperature of the programs were 95?,form pasting temperature to 30?lower than the pasting temperature 5,10,15,20,25 and 30?.Each starch treated by 6-8 programs to interrupt the gelatinization process of starch.The above mentioned suspensions were settled for 1 h,the upper and the lower layers were separated and freeze dried respectively for SEM observation.The method for observing the morphological changes of the blocklets were established during the gelatinization process.The major research contents and results were as follows:Firstly,the swelling ability of blocklets varied among the eight starches,which led to big differences in the morphological changes of blocklets during the gelatinization process.The blocklets swelled;deformed from spherical to olive shape formed bead-like structure and no longer existed alone.The blocklets deformation process of tapioca starch was the slowest.At the temperature of 52?,the spherical blocklets were observed;at the temperature of 62?,a lot of bead-like blocklets were observed;at the temperature of 72?,the blocklets fused together were observed.After treated by the setting programs,bead-like and spherical blocklets were observed for pea,mung bean and corn starch;bead-like blocklets were observed for potato and sweet potato starch.The blocklets deformation process of buckwheat and wheat starch was the fastest,only spherical blocklets were observed.When heated in 95?,the eight starch formed starch gels and the structures were different.Visible network structure and sparse holes were observed for sweet potato,tapioca,mung bean,pea and corn starch,and loose network were observed for tapioca starch.Lamellar gels with some folds and no network were observed for potato,buckwheat and wheat starch.Secondly,through the morphological changes of blocklets during the gelatinization process of tapioca starch,gelatinization theory was put forward on the blocklet level for the first time.The blocklets were spherical with different size in starch granule.During the gelatinization process,the starch absorbed water and ruptured,blocklets were released from the granules.At the same time water molecular entered into the starch molecule chains,which made the tightly folded starch molecule chains gradually extended.The blocklets fused together after absorbed water and some blocklets were existed in spherical.When heated higher than the pasting temperature,the blocklets completely lost their shapes and fused together forming three dimensional network structure due to the highly extended molecular chains.Thirdly,the properties of insufficient gelatinization starch granules of the eight starches were tested and compared with the native starches.The degree of gelatinization of the eight insufficient gelatinization starch granules were 7.8%-32.6%.Tuber,legume and buckwheat starches werechosen the lower layer after heated in 15? lower than the pasting temperature,while corn and wheat starches were chosen the lower layer after heated in 30? lower than the pasting temperature.Thermal properties tested by Differential Scanning Calorimeter(DSC)showed that the absorption peak of wheat starch disappeared,the peak intensity of the rest seven starches decreased after treat with RVA.So,the gelatinization enthalpy decreased for the rest seven starches.Mung bean starch showed the biggest change,the gelatinization enthalpy decreased from 18.2 J/g to 5.1J/g,reduced by 72.0%.Crystallization properties tested by X-Ray Diffraction(XRD)indicated that the peak intensity and the relative crystallinity decreased.Potato starch showed the biggest change,the relative crystallinity decreased from 30% to 8%,reduced by 73.3%.The crystal form of mung bean and pea starch transformed from C to A.The ordered structures tested by Infrared Spectrum(IR)indicated that no new absorption peaks presented,which indicated that the starch molecule structure were not changed.The ratios of the absorption intensity of the 1047 cm-1 represent the crystallization area and 1020 cm-1 represent the amorphous area were decreased,which showed the part of the ordered structures were destroyed.