Evolutionary Behavior Of Structure And Properties Of Waxy Maize Starch During The Growth Period

Starch plays an important role in food industry since it is commonly used as raw material and it is a major component in food products.With the development of science and technology,the demands for nutritional and healthy food are increasing urgently,the application and nutritional functions of starch are receiving more and more attention.The green and safe modification technology of starch has become a hotspot of research in the field of starch science and food nutrition,due to the deficiencies of starch,including the unstable viscosity,easy aging property,weak processing tolerance,and the risk of causing the insufficiency of chronic metabolic diseases such as obesity,diabetes,and hyperlipidemia.Based on the research of key enzymes and related gene expressions on starch synthesis in biosynthetic processes,this thesis proposed that the gene expression at different growth stages of waxy maize affects the activity of key enzymes,which results in multi-scale structural evolution of waxy maize starch and further affects its functional properties.The relationship between the regulation and influences was discussed,and it is of great scientific and academic value to lay a foundation for the regulation of starch application and nutrient function through biological genetic modification from the source.RNA-Seq sequencing technology was used to sequence the high-throughput group of waxy maize kernels in different growth stages,and the gene differential expression profiles of key enzymes involved in the starch synthesis of waxy maize were obtained at each grain growth stages.The screening of key differential genes of pathological pathways for starch and sucrose metabolism revealed 39 high-expressed genes.Among them,GRMZM2G129451 and GRMZM2G48551 genes modulated the changes in the activity of SS?and SS?enzymes,affecting the A,B₁,B₂ and B₃ segments of amylopectin;GRMZM2G032628 and GRMZM2G088753 genes regulated the activity of BE?b and BE?enzymes,changed the average number of branched chains on the starch cluster structure,and also participated in the synthesis of A,B₁,B₂ and B₃ segments;GRMZM2G138060 gene regulated the activity of DBE enzyme and the long-chain debranching behavior of amylopectin.The change in size and distribution of molecular weight is related to the number of A,B,and C chains and the density of branch points.The double helix structure content depends on the branch point density,the amount of B₁ segment synthesis and the debranching behavior.The chain structure evolution of waxy maize starch in different growth stages was thus:First of all,as the growth period prolonged,the shortest chain-A?DP6-12?increased continuously,B₁?DP13-24?and long chain B₃?DP>36?decreased continuously,and B₂?DP25-36?chain increased gradually;secondly,the average molecular weight?M_w?and number-average molecular weight?M_n?of waxy maize starch first decreased and then increased,and the molecular weight distribution first converged to the low molecular region and then shifted back to the macromolecular region;thirdly,the double helix structure in waxy maize starch showed a decreasing trend in its amount,while the amount of amorphous structure continued to increase;furthermore,the degree of short-range ordering of molecules on the surface of particles decreased continuously.The studies have shown that the evolution of the starch aggregate structure of waxy maize depends on the changes of its chain structureat different growth stages.The number of double helix molecules is positively related to its relative crystallinity.The ordered structure is positively related to the crystalline structure and the regular and dense local domains of the segments.The disordered structure is mainly composed of A short chain,B₃ long chain,the segments that do not participate in crystallization and ordering structure,the branch points and amylose.Crystallization,ordering,and disordered structures affect the structure and thickness of the crystalline and amorphous layers in the layered structure,thereby affecting the growth ring of starch.Therefore,the evolution of starch aggregation structure of waxy maize at different growth stages was obtained with the following observations:1)with the prolongation of the grain growth,the particle of waxy maize starch gradually became plump and formed a smooth surface and the average particle size of the particles also increased;2)the growth ring structure became cleargradually,and the pores extending from the nucleus of the granule to the edge were formedcontinuously;3)the degree of ordering in the amorphous background region and crystalline region in the structure increased steadily,which gave rise to the thickening of the crystalline layer of the lamellar structure,and the thickness of semi-crystalline layered structure was between 9.33 and 9.64nm;4)the relative crystallinity of waxy maize starch decreased gradually,and all of them were typical A-type crystal structures.The analysis results demonstrated that the multi-scale structure evolution of waxy maize starch at different growth stages resulted in the changes in its paste properties,rheological properties and digestibility.With the prolongation of the growth period,the A segment increasedand the B₁ segment decreased,and the short-range ordering degree of the particles on the surface of the particles decreased continuously.Therefore,the gelatinization temperature and the viscosity of the starch of the waxy maize reduced constantly,while the disintegration value and the peak viscosity both increased steadily.Due to the continuous increase of B₂ segments,the degree of entanglement between the molecular chains in the paste liquid system increased,resulting in the increase of gelation,apparent viscosity,intrinsic viscosity,thixotropic ring area,and shear thinning phenomenon.Because the amount of B₂segment increased and the molecular weight distribution converged to the low molecular region first and then shifted back toward the macromolecular region,the strain-stiffening rate S of the nonlinear rheological system decreased first and then increased,and the shear thickening and thinning behavior also evolved along with the change of the chain structure.As the double helix structure and relative crystallinity decreased and the amorphous structure increased,the RDS content of waxy maize starch gradually increased,while the SDS and RS content gradually decreased.In this thesis,strain-controlled rheometer was introduced into the field of starch research.The large amplitude oscillatory shearing method was used to obtain the nonlinear rheological properties and its chain structure changes of waxy maize starch at different growth stages.These research methods provide more comprehensive information on rheological properties for starch applications.This thesis clearly clarifies that gene expressions affect the activity of enzymes involved in the starch synthesis from the gene and molecular level.By modifying the multi-scale structure of starch through the biosynthesis and regulation of its functional properties,the relationship between gene expression,key enzyme activity,multi-scale structure and functional properties was investigated.It is expected to establish a theoretical basis for the application of biotechnology in the regulation the functional properties of starch,and also to provide new research ideas and methods to obtain the nutritional and healthy waxy maize starch products of high-quality.