The Relationship Between Blocklets And Molecular Structure Of Starch

Starch blocklet and molecule are two levels of its four structural hierarchy.At present,studies of starch blocklet were limited to its morphological observation and size estimation,but its separation methods,properties and molecular structure remain unclear.In this study,the separation methods of starch blocklets from potato,sweet potato,cassava,pea,maize and wheat starches were established.The morphology observation,crystalline property and molecular structures(molecular weight and chain length distribution)of starch blocklets were analyzed.Otherwise the relationship between starch blocklets and molecular structures were explored.The main results and conclusions were as follows:(1)Methods of insufficient gelatinization for separating starch blocklets were established.Detected by rapid visco analyzer,the gelatinization temperatures of potato,sweet potato,cassava,pea,maize and wheat starches were 70,74,72,71,89 and 93?,respectively.Based on this,the optimum conditions for separating starch blocklets were determined:starch concentrations were 1%and the highest temperatures were below 5-10?of gelatinization temperatures.The supernatants were separated from heated starch pastes after placed for 1 h at room temperature.After freeze-drying,starch blocklets were obtained.Under optimum temperatures,maize and wheat starches had higher gelatinization degrees(60.7%and 65.4%,respectively).The pasting degrees of sweet potato,cassava and pea starches were 43.8%,42.6%and 40.8%,respectively.The gelatinization degree of potato starch was the lowest(26.8%).It was insufficient gelatinization.(2)Using scanning electron microscopy,starch blocklets showed near-spheroidal and their diameters were between 150 nm-2?m.Separated starch blocklets from potato,sweet potato,cassava were mostly connected to each other and formed strings of beads,and separated starch blocklets from peas,maize and wheat were closely stacked.Detected by X-ray diffractometer,the relative crystallinities of native starches were 27.6%-38.0%,and that of starch blocklets from potato,sweet potato,cassava,peas,maize and wheat were 6.0%,8.5%,6.5%,14.9%,14.4%and 7.5%,respectively.It showed that the relative crystallinities of starch blocklets were lower than that of native starches.The diffraction patterns of starch blocklets were significantly different from that of native starches,and there was no obvious peak or a weak crystalline peak,all of which showed dispersion characteristics.(3)Starch blocklets separated by insufficient gelatinization were mainly made up of amylose.The amylose contents of starch blocklets from potato,sweet potato,cassava,peas,maize and wheat were 93.7%,85.6%,100.0%,81.1%,86.4%and 68.7%,respectively.Cassava starch blocklet had nearly no amylopectin,while wheat starch blocklets was higher.High performance gel permeation chromatography equipped with multi-angle laser light scattering and refractive index detectors was used to determine weight average molecular weight(M_w)and gyration radii(R_z)of native starches and starch blocklets.Range of M_w of native starches was 10~6-10~7 g/mol.The chromatogram of potato native starch showed two components(amylopectin and amylose),and that of other five native starches showed three components(amylopectin,intermediate fraction and amylose).Range of M_w of amylopectin was(3.47-4.75)×10~7 g/mol,maize was the biggest,but potato was the smallest.Range of R_z of amylopectin was 89.2-104.2 nm.Range of M_w of amylose was(4.70-9.33)×10~6 g/mol,cassava was the biggest,but maize was the smallest.Range of R_z of amylose was92.6-156.4 nm.The chromatograms of starch blocklets of sweet potato,cassava and wheat showed two components,and that of potato,peas and wheat showed one component.Range of M_w of component 1 was 2.23×10~5-6.92×10~6 g/mol,cassava was the biggest,but maize was the smallest.Range of R_z of component 1was 21.1-53.6 nm.Range of M_w of component 2 was 8.08×10~4-3.35×10~5 g/mol,cassava was the biggest,but sweet potato and maize was smaller.Range of R_z of component 2 was 33.8-53.9 nm.The molecular conformations of native starches liked mostly random coil and rodlike.The molecular conformations of starch blocklets of potato and cassava showed random coil,that of sweet potato was rodlike,that of pea,maize and wheat were globular.M_w and R_z of all starch blocklets were both lower than that of amylose of native starches.There was also difference in the molecular conformation between starch blocklets and native starches.(4)After debranched with isoamylase,high performance gel permeation chromatograms of native starches and starch blocklets showed three and two components,respectively.High performance anion exchange chromatograms of native starches and starch blocklets showed both 84 peaks.The chain length distribution of native starches was divided into three parts:DP 6-12,DP 13-24and DP 25-89,which accounted for 17.8%-20.9%,37.0%-42.1%and39.6%-41.9%,respectively.The chain length distribution of starche blocklets was divided into two parts:DP 6-24 and DP 25-89.The proportions of DP 6-24of starch blocklets from potato,sweet potato,cassava,pea,maize and wheat were 74.5%,74.3%,71.6%,73.5%,73.4%and 71.3%,respectively.The proportions of DP 6-24 of starch blocklets were higher than that of native starches,which were more than 70%.Consequently,molecular weight distribution,gyration radii and chain length distribution of starch blocklets were significantly different from those of native starches.