Molecular Structure Of Intermediate Material Of Maize Starch

Starch is an important raw material for the food industry and the molecular structure of starch restricts the performance of starch-based products.It is generally accepted that starch comprises mainly highly branched amylopectin(AP),predominantly linear amylose(AM),and the presence of an additional fraction referred to as intermediate materials(IM,its properties apparently intermediate to AP and AM)in starches has been suggested.IM has been considered as one of the major factors affecting the starch properties,but its molecular structure is not clear,so that it cannot fully understand the structure and properties of starch.In present study,amylopectin,intermediate material,and amylose from four kinds of typical maize starch were obtained using differential alcohol precipitation using n-butanol and isoamyl alcohol.The chain structure,fine structure,and physicochemical property of IM were investigated use the HPSEC-MALLS-RI、1H-NMR、HPAEC-PAD and other analytical techniques.The purpose of this study was to lay a theoretical foundation for the selection of raw materials for the processing of starchy food.The main results are as follows:1.The molecular weight and chain conformation of IMMaize starch contained 5%一 9%IM.IM had the structural properties between linear AM and highly branched AP(degree of branching is 3.65%～4.23%),was a mild branched polymers(degree of branching is 0.41%～0.54%)with slightly larger Mw than AM but far less than AP.IM mainly composed of two types of sub-fractions.IM1 represented the transitional component whose molecular structure was similar to that of AP but with substantially longer average chains length and smaller branching degree,whereas IM2 was a primarily linear fraction like AM,but had slightly branched linear glucan chains.The contents of IM1 and IM2 ranged from 3.36%to 14.85%,85.15%to 96.64%,respectively.The weight-average molecular weight(Mw)of IM1 and IM2 was 2.32×107-8.02×107 g/mol,103×106～3.93×106 g/mol,z-root mean square radius of gyration(Rz)was 116.11～171.74 nm,97.58-139.22 nm,respectively.In DMSO-LiBr solution,the molecular chain of IM1 was more stretched,showed a branched flexible globular chain conformation,while the molecular extension of IM2 was relatively poor.2.The fine structure of IMThe branch chains of IM were mainly long chains and super-long chains,the relative content was 31.05%～35.88%,22.77%～36.67%,respectively.The fine structure of IM branched chain was characterized by unit chain structure and internal chain structure.Unit chain composition showed that the short chains(fa,degree of polymerization,DP=6～12)content of IM was 16.89%～25.25%,short B1 chain(fbl,DP=13～24)was 53.26%～57.14%,long B2 chain(fb2,DP=25～36)was 12.82%～19.37%,long B3 chain(fb3,DP>36)was 5.19%～7.36%,and the average chain length was 18.53～20.75.Compared with AP,IM contained less of the short chain,more of the long B2 chain and the long B3 chain,and then its average chain length of branch chains was longer.Internal chains composition results show that the content of external chains(A-chains)of internal chains segment in IM was 49.45%～50.04%,and the content of B-chain(chains connected the A-chains or the other B-chain)was 49.96%～50.55%.Compared with AP,IM possessed less of the A-chains,more of the B-chains.The difference of A-chains between IM and AP was mainly reflected in the Afp-chains(fingerprint region),and the difference of B-chains distribution is mainly reflected in the BSmajor-chain(the major chain segment)and BL-chains(long B-chains).Compared with AP,IM had longer average total internal chain length,average internal chain length and average external chain length,but smaller molecular size,which can be considered to that the branching structure of IM is more sparse,and the distance between the adjacent two α-(1,6)glycosides bonds is longer.3.The physicochemical properties of IMThe color of IM complex iodine was similar to that of AM,but its iodine binding capacity was much closer to that of AP.Compared with the native starch,IM and AP had stronger swelling and dissolving ability,more easily and rapidly hydrolyzed by acid or enzyme,while AM showed the relative properties of acid hydrolysis because of its swelling and dissolving ability is poor.The physicochemical properties of starch were mostly determined by the composition ratio,the molecular weight,the multi dispersion of fractions in starch and the branched chain segment composition of IM and AP.The more long branch chains of IM,the higher determination of amylose content,the lower degree of branching,the weaker swelling or dissolution ability and the stronger acid enzyme hydrolysis of starch.4.The variety difference of IMMaize starch with the higher apparent amylose content(AAC)usually had more IM,and the relative content of the corresponding sub-fraction IM2 was also higher.In addition,its IM showed much lower Mw,possessed significant fewer short branch chains,more long B2-chains and long B3-chains,and then the average chain length was longer.Compared with the low AAC maize starch(NM),the maize starch(S4)with higher AAC contained less A chain(especially cluster A-chains,Acltr-chains)as well as more BSmajor-chains and BL-chain).