Impact Of Extrusion Modification On Freeze-Thaw Stability And In Vitro Digestibility Of Rice Starch

Poor freeze-thaw stability and high digestibility are the main problem of frozen rice products and ordinary rice products,respectively.Extrusion technology,as a new processing of rice products,not only can be used to produce the above two products,but also can effectivey use the agricultural by-products such as broken rice.However,the impact of extrusion processing on the freeze-thaw stability and digestibility of rice products are unclear.Therefore,the present thesis based on the extrusion processing,using single extrusion,extrusion combined chemical modification,and extrusion combined enzymatic modification to improve the freeze-thaw stability and digestibility of rice products.It provides theoretical basis for extending the application of extrusion technology and opitimizing the technical solutions for the function and nutrition of staple foods.Focusing on the above objectives,the following parts have been carried out in the present thesis.Firstly,in view of the poor quality of frozen rice products,the only extrusion modification is used to improve the freeze-thaw stability of rice starch,as a new method to improve the freeze-thaw stability of starch.Furthermore,for the digestibilty of ordinary rice products with rice grain products as main component,the in vitro digestion model of rice grains to evaluate the extrusion texturized rice grains which prepared by our laboratory,and found that its digestibilty is slightly lower than the polished rice grains.Then,evaluating the effect of other endogenous components of rice on the digestibility of starch,attempting to explain the lower digestibility of texturize rice,but it is found that the endogenous components reduce the starch digestibility is limited.At last,the extrusion technology combined with citric acid modification and extrusion technology combined with enzymatic modification to reduce starch digestion,aiming to the preparation of a slow digestion extrusion texturize rice,and providing the technical solution to meet the staple food demand of diabetes diet.The following main conclusions were obtained by conducting the above parts of research.The improved extrusion cooking technology?IECT?is used to modify the rice starch.The freeze-thaw stability of IECT-modified rice starch was investigated.Syneresis showed that IECT-modified rice starch could reduce to 27.5%after 1^stt freeze-thaw cycles,while the native rice is 31.4%.In addition,the microstructure of IECT-modified rice had smaller holes and thinner walls,and IECT-modified rice starch had less significant changes in the rheological parameters during the FT cycles than the native starch.These results indicated that IECT modified rice starch had better freeze-thaw stability than native starch.XRD and iodine binding analysis demonstrated that IECT treatment inhibited the association of rice starch.Additionally,the peak at around 20^o was detected in XRD patterns of IECT-modified rice starch,which confirmed the formation of amylose-lipid complex during the IECT treatment.These results suggested that the IECT treatment could improve FT stability of rice starch,which was ascribed to inhibition of starch retrogradation by IECT,especially amylose retrogradation.For ordinary rice products,most of them are usually consumed in the form of rice grains.Based on that,a new method for simulating in vitro digestibility of rice grains was initially established.The digestibility of starch in cooked ordinary rice and cooked texturized rice was compared using an in vitro gastrointestinal tract?GIT?,which involved sequentially exposing samples to simulated oral,gastric,and small intestinal regions.The total extent of starch hydrolysis?%?of texturized rice was slightly lower than that of ordinary rice by the end of the GIT,probably because of its higher resistant starch content.Furthermore,the rate of starch hydrolysis in the small intestine was appreciably slower??2-fold less?for texturized rice than ordinary rice.The texture analysis showed that the extruded texturize rice had a compact structure.SDS-PAGE and microstructure analysis suggested that the proteins in the texturized rice formed a cross-linked network around the starch granules.This network may have retarded the ability of digestive enzymes to access the starch,thereby reducing the hydrolysis rate.On the basis of that starch digestibility is also affected by the food matrix and its compositions.The aim of this work was to investigate the effects of endogenous proteins and lipids on starch digestibility in rice flour,with an emphasis on establishing the underlying physicochemical mechanisms involved.A significant increase in starch digestibility was observed after removal of proteins,lipids,or both.The starch digestibility of the rice flour without lipids was slightly lower than that without proteins,even though the proteins content was about 10-fold higher than the lipids content.Microstructural analysis suggested that the proteins and lipids were normally attached to the surfaces of the starch granules in the native rice flour,thus inhibiting their contact with digestive enzymes.Moreover,the proteins and lipids restricted the swelling of the starch granules,which may have decreased their digestion by reducing their surface areas.In addition,amylose-lipid complex was detected in the rice flour,which is also known to slow down starch digestion.Although the study found that endogenous components can reduce starch digestibility,but its effect is still limited,therefore this thesis continued to reduce the starch digestibility by extruded extrusion modification combined chemical modification and extrusion combined enzymatic modification.Firstly,citric acid-esterified rice starch was synthesized using a one-step reactive extrusion method.The digestibility,structure,and correlated physicochemical properties of the esterified starch were then measured.An in vitro digestion study indicated that esterification significantly increased the level of resistant starch?RS?in the samples.Fourier transform infrared spectroscopy?FT-IR?confirmed esterification had occurred between the citric acid and starch,with a new peak appearing at around 1730 cm^-1.X-ray diffraction indicated that esterification caused a decrease in the degree of crystallinity in the starch.The swelling power and solubility of starch citrate were both lower than unmodified starch.Turbidity measurements suggested there was a slower rate of reassociation in starch citrates during storage.These results suggest that citric acid esterification promoted cross-linking within the starch that significantly reduced its digestibility.Then,extrusion combined with enzyme modification to reduce rice starch digestibility was investigated.?-Amylase is an exo-hydrolase that specifically cleaves?-1,4 glycosidic linkages of gelatinized starches.In this study,starch granules were disrupted by extrusion under different feed moisture conditions,and then subjected to?-amylolysis.The degree of starch gelatinization increased with increasing feed moisture content during extrusion,leading to faster?-amylolysis.The hydrolysis of invitro starch digestion study was reduced for extruded samples treated with?-amylase,which was attributed to an increase in resistant starch?RS?after?-amylase treatment.Indeed,X-ray diffraction?XRD?indicated that the crystalline structure in the extruded starch was either partially or fully lost after?-amylase treatment.Similarly,Fourier transform infrared?FTIR?analysis indicated there was a higher level of amorphous regions in the starch after?-amylase treatment.Overall,our results suggest that enzymatic treatment of extruded starch with?-amylolysis reduces the ratio of crystalline-to-amorphous regions,which increases the level of resistant starch,thereby slowing down digestion.These results have important implications for the development of healthier starch-based foods.To sum up,extrusion technology can not only improve the freeze-thaw stability of starch,but also can combine other modification techniques to reduce the digestibility of starch.By using extrusion technology to change the structure and texture of the food components,it is possible to produce slowly digestible texturize rice.This paper is of great significance for improving product quality,improving product nutrition characteristics and developing starchy foods with lower glycemic index.