Properties And Structure Of The Outer Shells Separating From Starch Granules

Starch,composed of only glucose,has a complex structure and is generally divided into four structural hierarchies:granules,shells,blocklets and molecules.Different from the inner shells,the outer shell of starch is the outermost layer of the granule and has a hard structure,wrapped internal polymers inside.For now few researches was found about outer shell.An unsufficient gelatinization method on separating the outer shells of six starches(potato,sweet potato,cassava,pea,maize and wheat starch)was established using a rapid viscosity analyzer.The separation mechanism of the outer shell was explored,also its properties and molecular structure were analyzed.The main results were as follows:Firstly,an unsufficient gelatinization method was established for separating the outer shells from six starches.The pasting temperatures(PT)were determined by using a standard procedure of rapid viscosity analyzer,PT of six starches were 70,74,72,70,89 and 93℃,respectively.Base on this,the optimum conditions of separating outer shells were determined,the concentration of the starches is 0.5%,and the maximum treatment temperatures are different,ranging from 10℃lower than PT to the gelatinization temperature.The upper and lower layers were separated and freeze dried after settled for 1 h at room temperature.Scanning electron microscopy images showed that the outer shells were largely existed in the lower layer,while the inner blocklets were the major components in the upper layer.The degrees of gelatinization were within the range of 37%-69%for six starches.,ie,incomplete gelatinization occurred.Secondly,by setting different maximum treatment temperatures,the gelatinization process was decomposed.The particle morphology changes at different stages were observed,and the separation mechanism of the outer shells was explored.When gradually heated in water,with the granule swelled,the outer shell expanded and isolated from the inner blocklets and the inner shells disintegrated into dispersed blocklets.As the heating continued,the expansion of the outer shell intensified.The adjacent blocklets were connected by the molecular chains,and there were gaps between them.Some inner blocklets connected to each other and became a bead-like structure,moved toward the outside of the granules through the gaps.When the majority of inner blocklets released from the rupture in the outer shell,the cavity structure was formed.After settled,the suspension was separated to a supernatant layer with a mass of blocklets and another lower layer sedimented with outer shells and their fragments.Thirdly,the properties and structure of the outer shells were studied.The amylose contents of the outer shells of potato,sweet potato,cassava,pea,maize and wheat starch were 18.8%,13.6%,10.9%,22.3%,17.2%and 20.0%,respectively,which were lower than nativ E starches(23.3%-37.6%).The confocal laser scanning microscope images showed that,with the APTS stain,the dyeing ability of the outer shells were weaker than the native starches,indicating that the outer shell were mainly formed by amylopectin.The average particle size of the outer shell of potato starch was the largest,which was 4.8times than that of the native starch(110.6-fold increase in the volume),and the swelling capacity was the strongest.The outer shell of pea starch had the weakest swelling capacity and increased only 1.4 times in diameter(2.7-fold increase in the volume).The average particle size of the outer shells of cassava,sweet potato,corn and wheat starch were 3.5,2.7,2.3 and 1.6 times than that of the native starch,respectively.The X-ray diffraction patterns of the outer shells showed amorphous peaks,the relative crystallinity was between 5.5%and 8.6%,much smaller than native starches(29.5%-41.7%).From the appearance,compared to the native starches,the transparency of the gel formed by the outer shells was improved and the most notable changes were found in potato and tapioca starch.Through texture analysis,reduced hardness and increased cohesiveness were found in the outer shell gels of potato,sweet potato,maize and wheat starch than those in native starches,while the outer shells of cassava and pea starch had weaker ability to form gels.As the starch emulsion systems were not easy to retrograde,their texture properties had not been obtained.Scanning electron microscopy images revealed that the network structures of the outer shell gels from three tuber starches were lamellar structure and similar to their native starches.However,connected by outer shell fragments,the outer shell gels of pea,maize and wheat starch did not form even network structures.By using a HPSEC-MALLS-RI system,it was found that the molecular weight of the outer shells were greater than native starches,and the molecular weights of the amylopectin,middle fraction,and amylose were respectively4.354×10~7-5.193×10~7、1.297×10~7-2.322×10~7 and 6.420×10~6-1.979×10~7 g/mol.The trend of amylopectin chain length distribution of the outer shells,determined by HPSEC and HPAEC,were same as the native starches,but the content of long chains(B2,B3 chain)were increased,indicating that the longer amylopectin side chains participated in the structure of the outer shell.