| Rice starch is the main component of rice,whose properties affect the quality and nutritional functions of rice and rice-based products.However,rice starch is commonly known as food with nutritional deficiencies including the higher content of rapidly digestible starch(RDS),and the risk of causing the unsufficiency of chronic metabolic diseases such as obesity,diabetes,and hyperlipidemia.Moreover,the unstable viscosity,easy aging property and weak processing tolerance of rice starch are also unable to meet the requirements of modern processing technology,which constraints on the sustainable development of the rice processing industry.Based on these shortcomings,a method combined extrusion processing(EP)with addition of polyphenols to modulate the digestibility and paste properties of rice starch was developed and the relationship among EP conditions,multi-scale structure,digestibility/paste properties of rice stach were established.Furthermore,the related molecular mechanism for the interaction between polyphenols and rice starch molecules under EP and its effect on the starch digestibility and paste properties are proposed in this thesis.The details and main conclusions are shown as followings:The effect of moisture content and EP conditions on the digestibility of rice starch was investigated through single-factor experiments and the optimal EP conditions were futher obtained by response surface methodology:the system moisture content was 40%,the extrusion temperature was 80°C,and the screw speed was 150 r/min.It turned out that the content of anti-enzymatic component(SDS+RS)was increased from 5.88%to 19.05%.Based on these results,a new method was established to control the digestibility of rice starch by EP and polyphenol molecular interaction.The results showed that both caffeic acid and chlorogenic acid could significantly reduce the RDS content of rice starch.However,by comparing the effect of these two polyphenols on the RS and SDS content of rice starch,it was found that caffeic acid mainly increased the RS content of rice starch(from 10.35%to34.45%)and had little effect on the SDS content(from 8.70%to 10.98%)while chlorogenic acid not only increased the RS content(reached 23.81%)but also increased the SDS content(increased more than 18%).The synergistic effects EP and polyphenols molecular interaction on modulating gelatinization,rheology and retrogradation properties of rice starch were systematically investigated.The results showed that EP increased the gelatinization temperature,hot paste stability and cold paste stability of rice starch,reduced the peak viscosity,disintegration value,retrogradation,apparent viscosity,shear thinning,thixotropy and gel strength of the paste,increased the retrogradation enthalpy value in short term as well as reduced the retrogradation degree in long term.Besides,the peak viscosity,retrogradation,apparent viscosity,shear thinning,thixotropy and gel structure strength of rice starch paste were reduced by the interaction of caffeic acid/chlorogenic acid during EP.What’s more,the short-term,long-term retrogradation enthalp of rice starch were also significantly reduced,which effectively inhibited the retrogradation of rice starch during the whole storage period.Mmeanwhile,the synergistic effect of EP and polyphenols molecular interaction on multi-scale structure in rice starch were systemically explored by using modern analytic technologies.The results showed that,with EP,M_Wand R_Zwere decreased,the double helix structure,crystalline structure,semi-crystal layer structure and granular structure were destroyed to some extent,and the amorphous structure increased,together with the new formation of single helix structure,V-type crystalline structure,short-range ordered structure,nano aggregate ordered structure and granular structure.The interaction of caffeic acid/chlorogenic acid with starch molecules further promoted the changes of multi-scale structure of starch,especially increased the single helix structure,V-type crystalline structure,crystallinity,surface short-range order and nano aggregate compactness,which resulted in a higher degree of ordering compared to extruded rice starch.Based on the above results,the related physical models of the relationship among EP conditions,multi-scale structure,digestibility/past properties of rice starch were established.It was revealed that,on the one hand,the synergistic effect of mechanical force,thermal energy and water molecules during EP could not only break the starch molecular chains but also interated with water molecules by forming hydrogen bond,which resulted in the destruction of the original hydrogen bond structure of starch molecules and the depolymerization of the molecular chain,transformation from long-range ordered structure to disordered one,thus weakened the degree of entanglement of molecular chains and motion resistance in starch paste system.On the other hand,it could also promote the movement and rearrangement of molecular chains,which facilitated the formation of new V-type crystalline structure and short-range order structure,eventually modulated the digestibility and past properties of rice starch.Therefore,different EP conditions induced different degree of changes in multi-scale structure of rice starch,so suitable EP conditions could be selected to modulate the digestibility and paste properties of starch from the structural level.Moreover,the related physical models of the relationship among EP,structure and addition of polyphenols,multi-scale structure,digestibility/past properties of rice starch were further established at molecular level.The interaction of caffeic acid/chlorogenic acid with starch molecules would further affect the multi-scale structure of rice starch.The highly active polyhydroxy and carboxyl groups of caffeic acid/chlorogenic acid could form strong intermolecular hydrogen bond structure with the hydroxyl groups on the starch molecular chains,which promoted the depolymerization or re-arrangement of starch molecular chain.At the same time,some caffeic acid or chlorogenic acid molecules entered into amylose molecular cavity and formed single helix complexs and V-type crystalline structure,which resulted in the increasing ordered degree of multi-scale structure.Besides,it also hindered the proximity,winding and aggregation of the molecular chains in the starch paste system,and reducing the flow resistance in the recrystallization and paste system,thus affect the digestibility and the paste properties of rice starch.With the increasing addition of caffeic acid/chlorogenic acid,the above trend became more obvious.However,the effect of caffeic acid on the digestibility and paste properties of rice starch was stronger due to its smaller molecular volume and steric hindrance compared to chlorogenic acid.The effective method,regular pattern and molecular mechanism for EP with polyphenols molecular interaction on modulating digestibility and paste properties of rice starch were investigated in this work,which had important academic value and practical significance.This results will be beneficial to give rice starch excellent paste properties and nutritional functions and also provide theoretical basis and technical support for the development of rice-based products with high quality and nutritional health. |
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