| Starch molecular conformation,determined by hydrogen bonding,Van der Waals and electrostatic interaction,means the arrangement of atoms or groups in space.And it plays an important role in the change of starch structure and properties.In this study,molecular dynamics simulation was used to explore the effects of temperature,pressure and ion concentration on starch molecular conformation,molecular stability and secondary interactions.Compared with temperature and pressure,the effects of salts on starch are more complex.Especially the effect rules and gelatinization process of ion on starch is not fully clarified.Thus,the effect rules and gelatinization process were analyzed firstly by using SEM,RVA,Zeta potentiometers etc.And the main results and conclusions are as follows:(1)The effect rules of salts on potato starch were analyzed.The results showed that,with the increase of MgCl2 and NaCl concentrations from 0 to 4 mol/L,the viscosity,setback and breakdown values of potato starch showed the trends of falling first and then rising.While the gelatinization temperature,relative crystallinity and sedimentation rate showed the trends of rising first and then falling.And the inflection points were all at the concentration of 0.5 mol/L.Then,further analysis was performed to explain this phenomenon.The results showed that,when salt concentrations were lower than 0.5 mol/L,ions will exudate from starch granules.While salt concentrations were higher than 0.5 mol/L,starch granules will absorb external ions.It indicates that the ion absorption and ion exudation have the opposite effect trends on starch.And the higher the degree of either absorption or exudation is,the greater the effects on starch properties will be.(2)The gelatinization process and mechanism of potato starch induced by 4 mol/L MgCl2 at room temperature were analyzed.From the perspective of granule morphology:At the beginning of gelatinization,the granules expanded slightly(1.5 times of the original granules),and cracks appeared on the surface;then,the swelling degree increased to 3 times of the original starch,cracks and holes appeared on the surface,which provided conditions for the entry of external water molecules and ions and the leaching of internal molecules;after that,the swelling degree of granules continued to increase to 8 times of the original,the shell was torn,and the internal molecules were completely exposed;finally,the gelatinization of starch was completed and gel formed.From the perspective of viscosity:The viscosity did not increase significantly within 0.5 h;during 0.5 to 6 h,the rising speed of viscosity increased first and then decreased,and reached the maximum at 2 h(8 cp/min);after 6 h,the viscosity gradually stabilized(1106 cp),and gelatinization ended.From the perspective of crystallization:The original crystallization peak of potato starch disappeared within 0.5 h,which indicated that gelatinization took place simultaneously from the inside and surface of the granules.In addition,MgCl2 can combine with starch molecules to form a new crystal structure(2θ=15.5 ° and 25.5°).From the perspective of Raman spectra:During the gelatinization process,the hydration of starch increased gradually with no new chemical groups formed.(3)The effects of MgCl2 concentration on starch molecular conformation and secondary interaction were analyzed.The results showed that,on the one hand,with increase of MgCl2 concentration from 0 to 4 mol/L,the electrostatic interaction of starch molecules increased by 23.4 times,and the residues of starch molecules would becme close to each other under the strong electrostatic interaction.Then,more compact conformations would be formed:the pitch,interchain distance,Rg and surface area of starch molecules in 4 mol/L MgCl2 respectively decreased to 80.1%,48.1%,83.6%and 81.3%of that in pure water.This may be the key reason for the appearance of cracks and holes on the granule surface at the beginning of starch gelatinization.On the other hand,with the increase of MgCl2 concentration from 0 to 4 mol/L,the 4C1 chair conformation decreased from 98.1%to 90.1%,which means that high concentration of MgCl2 can significantly increase the free energy of starch molecules and make its structure more unstable.Meanwhile,compared with the pure water environment,the hydration ability of starch molecules in 4 mol/L MgCl2 environment increased by 65.6 times.The two factors should be the key reason of the starch gelatinization caused by MgCl2.(4)The effects of temperature on starch molecular conformation and secondary interaction were analyzed.The results showed that,firstly,high temperature bent the linear starch molecules.Secondly,high temperature increased the interchain distances of double-amylose and amylopectin.Especially for the double-amylose,the interchain distance at 100℃ has reached 1.07 nm,which means double-amylose will be untwisted at 100℃,indicating that high temperature can directly act on the crystallization region.Thirdly,high temperature increased the potential energies,free energies and movements of starch molecules,and made starch molecules more unstable,which should be an important factor of starch gelatinization caused by high temperature.Fourthly,the hydrogen bond between starch and water at 100℃ was only 0.05 times of that at 4℃.It indicates high temperature can destroy the formation of hydrogen bond between starch and water,then reduce the restriction of starch to water,and make water molecules more easily enter into granules,which should be one of the important reasons for the high expansion during heating gelatinization.(5)The complexation process details and mechanism between starch molecules and triglyceride were analyzed.The results showed that,firstly,not only amylose and amylopectin,but double-amylose can also complex with triglyceride,indicating that the interaction between lipids and starch can take place in the crystalline region.Secondly,starch residues of one turn helix(8 residues)are enough to bind a triglyceride molecule firmly.Thirdly,the complex is maintained by Van der Waals and electrostatic interaction.And the total energy of VDW and electrostatic interaction of each turn helix can reach 80 KJ/mol.Fourthly,starch molecules will form a V-type helix after the complexation,showing a denser structure than before.Fifthly,the average proportion of 4C1 chair conformation of glucose residue after complexation decreased from 98.1%to 93.6%,indicating that the residues complexed with triglyceride become more stable than the former or the free residues.(6)The effects of pressure on starch molecular conformation and secondary interaction were analyzed.The results showed that,firstly,high pressure decreased the potential energies,free energies and movements,and made starch molecules more stable.When the pressure increased to 600 MPa,all the glucose residues turned to 4C1 chair conformation.And the movements of starch molecules at 900 MPa was only 71.2%of that at 0.1 MPa,which is contrast to thermal effects.This may be the key reason for that high-pressure gelatinization have less amylose leaching than heating gelatinization.Secondly,the starch-water and intramolecular hydrogen bonds at 900 MPa respectively became to 61.5%and 154.5%of that at 0.1 MPa,which is consistent with starch gelatinization process.Thirdly,combined with the above results,the mechanism of high pressure gelatinization could be deduced:The strong hydration ability of starch molecules and weak movements of water molecules make water difficult to enter the inside of starch granule.When water molecules reach the surface of starch granule,it will be bound by starch molecules through hydration bonding.Thus,a dense water layer will appear on the surface of starch,and a confined space can be formed in the starch granule.Finally,starch granules collapse due to the differential pressure of intra/extra.In conclusion,high salt concentration can make starch molecules more compact,and it can significantly increase the electrostatic interaction and hydration ability of starch molecules.High temperature can bent the linear starch molecules and make double-amylose untwist.Meanwhile,high temperature can significantly increase the molecular potential energy,free energy and movement of starch.High pressure can destroy the intramolecular hydrogen bonds,and promote the formation of hydrogen bond between starch and water.Besides,the complexation process details and mechanism between starch molecules and triglyceride were analyzed.Furthermore,the effect rules of salts on starch properties were analyzed from the perspective of ion absorption and exudation.Meanwhile,the process of starch gelatinization induced by high salt concentration was clarified. |
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