Study On The Effects Of Amino Acids On Aggregation Structures And Morphologies And Phase Transformation Of Starches

Starches and proteins are main components of starchy foods.The molecular interactions between starch and protein have a great effect on the texture,processing,and storage properties of starchy foods.However,the complicated multi-level structures of protein macromolecules have complicated the direct study on their interactions with starch.The study on the interactions among starch and amino acid,as the polymer unit of proteins,could laid a solid foundation to the study of the interactions of starch and protein.Additionally,as additives and nutrient enhancer,amino acids have long been introduced into the food processing area.Therefore,it is of great significance to study the interactions between starch and amino acid molecules,which could provide a guideline to the developing of food system enhanced by the addition of amino acids.In this work,corn starch and three representative amino acids,namely neutral glycine,alkaline lysine,and acid glutamic acid were used as model materials.The effects of the type and content of amino acid on the aggregation structures and morphologies and heat-induced phase transformation of starches were systematically studied and the relationships among starch,amino acid and water molecules were explored with a combination of the traditional characterization techniques and molecular dynamics simulation.This work would provide a theoretical guideline to the developing of starchy foods.The main results are as follows:(1)The results showed that the structure morphology of starch varies with the type and content of amino acids.With the increased content of alkaline lysine and acid glutamic acid,the SEM characterization results showed that the morphologies of granular starches increasingly became rough,with some sunken holes.Comparably,the starches treated with neutral glycine did not show any significant differences.The results of polar microscopy and powder X-ray diffraction characterizations showed the A-type crystalline structure of starch were not changed after adding amino acids.However,the crystalline content of starches gradually increased with the increased content of lysine and glutamic acid.The strong acidic or alkaline condition of non-neutral amino acid solutions might damage the amorphous areas of starch granules,which resulted in an increased in relatively crystalline content.Additionally,the characterization from the deconvolution of FTIR revealed that the hydrogen bond interaction between the three amino acids and starch molecules was the main reason to reduce the organized structure of starches.(2)The results showed that the type and content of amino acids had significant effects on the gelatinization and retrogradation properties of starch.A hot stage equipped on a microscope to allow a program-elevated temperature was used to collect pictures of dynamic gelatinization behaviors of starch granules.And the Canny edge detection was combined with the mathematical morphology operation of particle image segmentation technology to monitor the kinetics of phase transition of starch gelatinization behaviors without an involvement of stress.We also compared the results of this technique with that of an in-situ thermal analysis using differential scanning calorimetry.The results of image analysis and in-situ thermal analysis characterization simultaneously revealed that the gelatinization transformation behavior of starch molecules was inhibited by these three amino acids,namely a reduced swelling kinetics of starch granules and an increased characteristic temperature of gelatinization.With an increased addition of amino acids,the inhibitory effect initially increased and decreasing later.And the gelatinization peak temperature of starch was increased by 3.71 ?,6.00 ? and 5.58 ?by glycine,lysine and glutamic acid as the content is about 10%.In a Brabender viscometer system with shearing force,the characteristic viscosities of starch pasting gradually decreased with the increased content of amino acid,indicating that amino acids can effectively plasticize starch paste solution.Additionally,the retrogradation of starch were significantly inhibited by amino acids and glutamic acid showed a largest effect,followed lysine and glycine.The above results indicated that the thermal phase transition behavior of starch was closely related to the strong physical molecular interactions,including electrostatic and hydrogen bonding interactions between amino acid and starch or water molecules.(3)Molecular dynamics simulation technique was adopted and the conformational changes of starch molecule in different concentration of glycine,lysine and glutamic acid aqueous solution systems were simulated.In addition,several statistical analysis methods,including the root meant square deviation(RMSD),spiral dihedral angle distribution,radial distribution function(RDF)of starch molecules and water molecules,the hydrogen bonding interaction and energy between solvent molecules,were used to explore the molecular dynamics mechanism.The analysis results showed that the conformational stability of starch molecules and the interaction between solvent molecules were affected by the type and content of amino acids.Among the three amino acids,lysine and glutamic acid had greater effects than glycine.With an increase of amino acid amount,the root mean square deviation of the starch molecule gradually decreased,the speed of change of the helix dihedral angle slowed down,and the distribution density of water molecules around starch molecules decreases gradually,indicating that the conformational stability of starch molecules was improved.Hydrogen bonding interaction and interaction energy analysis showed that the number of hydrogen bonds between starch and water molecules greatly decreased,but the number of hydrogen bonds between water and amino acid molecules increased significantly with the increase of content of amino acid,indicating that the solvation effect on starch was restrained by amino acid via interacting with water molecules,which further enhanced the conformation stability of starch and caused starch gelatinization lag.Through molecular dynamics simulation,the interaction among amino acid,starch and water molecules that influenced the macroscopic properties of starch was revealed at the micro level,which is of great significance for the development and production of amino acid and starch compound food.