Structural Evolution And Phase Transition Of Starch Under Thermal And Shear Condition

Starch is one kind of natural polymer which is synthesized by plant photosynthesis.Starch is also the mainly components in food.Starch is widely used in food,construction,paper and textiles industries by physical,chemical and biological modification.The physic-chemical properties of starch are determined by its own complex structure.Therefore,understanding the evolution of starch multi-scale structure during typical physical processing is an important field in starch science.Starches with different structural were chosen as the model materials in this dissertation.The size exclusion chromatography(SEC),X-ray diffraction(XRD),small-angle X-ray scattering(SAXS)and scanning electron microscopy(SEM)were to use to study the change of starch multi-scale structure.The rapid viscosity analyzer(RVA 4800)was used to establish the thermal-shear force coupling environment,the effect of heating temperature,rheological and texture properties of starch or starch paste/gel were studied.The milling process with hammer milling were chosen as shear environment,the structure evolution e of rice flour during the milling process,and the influence of the viscosity,rheology and texture of the rice flour paste/gel were also studied.Those results will provide a theoretical basis for the physical processing of starch.The main findings are as follows:(1)Starch with different structural characteristics were chosen as model materials,the SEC,XRD,SAXS and SEM used to study the starch multi-scale and the method were established to get the starch structure parameters such as starch molecular size,crystalline type,crystallinity,lamellar structure and granular morphology.Those parameters were analysis by principal element analysis(PCA)and cluster analysis,results show that and the crystallinity type,amylose content and varieties were the mainly index to raw starch classification.(2)RVA 4800 were used to establish a thermal-shear coupling environment to study the viscosity changes of corn starches with different amylose/amylopectin.The results show that the gelatinization temperatures of waxy and normal were about 76°C and 80°C,respectively.The peak viscosity has no obvious change at different heating temperature,while the trough viscosity,final viscosity and setback decrease with the increasing heating temperature.The gelatinization temperature of high amylose corn starch(G50 and G80)was about 117°C and 124°C,respectively.The peak viscosity increases with the increasing heating temperature.The trend of trough viscosity,final viscosity and setback is not consistent with the increasing heating temperature.(3)The starches with different crystalline type such as wheat,tapioca,potato,mung and pea,were used for the represent A,B and C type starch,the RVA results show the peak viscosity of starch did not change significantly with different heating temperature,while the trough viscosity and final viscosity decreased gradually with the increasing heating temperature,and the storage modulus decreased.The starch paste/gel after high temperature treatment also keep elastic properties.(4)The hammer mill was used to establish the high-shear environment and the rice were chosen as raw material.Rice was grinded for different times under the dry and semi-dry conditions.The results showed that amylose and amylopectin in rice flour degraded rapidly under dry conditions,and the change of crystallinity was higher,however,the lamellar structure of starch still existed,but the amorphous thickness in lamellar structure decreased.In the semi-dry process,amylose and amylopectin degraded slowly,the changes of crystallinity and amorphous thickness were smaller,which indicated that water would reduce the degradation degree of starch in rice flour.Compared with waxy rice flour,amylose rich rice flour degraded faster.