Thermal Effect Of Microwave On Structure Of Rice Starch

The application of microwave for heat processing or reheating of starchy food fits the current trend ofthe development of new technology of physical field. As a carbohydrate polymer, the structure,composition, and morphology of starch are very complex. Up to now the research on starchy materials andfoods in the microwave field mainly focus on the behavior related to gelatinization and its application, andthe lack of research on the influence of electromagnetic properties on the microwave heating effectfundamentally hinder the cognition of the structural change of starch in the microwave field, fail to makeobjective safety evaluation, and limit the development of microwave technology in the field of starchy food.In this study we selected stable rice starch as the object, examined the fundamental electromagneticproperties of the rice starch based on the mechanism of the microwave heating process, determined the lossmechanism of the material in the microwave field, established and evaluated the control heating model,investigated the effect of microwave on dielectric properties and absorbing properties of rice starch.Moreover, we investigated the effect of microwave on the structure, morphology, thermal properties andwater distribution of starch at all levels, elucidated the major mechanism of microwave on starchy material,and conducted research on the temperature distribution and heat transfer model of rice starchy foods duringthe microwave reheating process. The major results of the current study are as follows:（1）Preparation of rice starch and electromagnetic response characteristicsWith the basic components, pasting properties, particle morphology and particle size distribution asindicators, the effect of alkaline, alcalase and neutrase on the extraction of rice starch was investigated todetermine the optimal preparation process, and the electrical properties and the magnetic properties wereexamined to establish the loss type and inspect the absorbing properties of the water system. The resultsshowed that neutrase had minor influence on the starch granule. In the dry state, for the complexpermittivity of both the native and completely gelatinized starch, ε′was2.7±0.1, and ε″lied in the range of0～0.2, with tanδε below0.1, indicating that starch is a weak absorbing material in microwave. For thecomplex permeability, μ′was1.00±0.10，and μ″and tanδμ were both0.19±0.02，showing weak magneticinduction. Transmission line theory is suitable for the examination of absorbing characteristics of complexliquid materials, and the results matched the resonance model of the electromagnetic wavelength. The ε′ofthe starch emulsion decreased with the increase of the concentration with linear relationship, and ε″showedlittle change with the increase of the concentration. Under2.45GHz, when the concentration of the nativestarch increased from1%to30%, the reflection loss decreased from-5.1dB to-1.2dB，and theelectromagnetic absorbability decreased from69.1％to24.1％. Native starch significantly interfere withthe microwave absorbing characteristics of water in the mixed system, and the gelatinized starch showedlittle influence on the microwave absorbing characteristics, whereas water dominated the microwaveabsorbing characteristics of the pre-gelatinized starch system.（2）Effect of microwave heating on the dielectric and absorbing properties of rice starchBased on the heating law of the microwave and the conduction heating, the microwave heating modelwas established with the slow heating conduction as the control. The effect of the microwave heatingprocess on starch, the intervention based on the Debye polarization mechanism of the complex permittivityof the system, and the absorbing behavior of the system were investigated. The results showed thatmicrowave heating promoted the rearrangement of the starch emulsion under the low temperatureconditions, so that the system exhibited a relatively stable dielectric response. After microwave heating thesamples showed prominent dielectric characteristics, and ε ’, ε "and tanδε values were higher than those atthe rapid and slow conduction methods in the heating range of45~65℃. Microwave heating processshowed significant influence on the simplified Cole-Cole plot curves of starch emulsion with a significantvariation of the curves from those of the other two methods at2.45±0.05GHz. The microwave heatedsample changed the original polarization law at2.45GHz±0.01GHz, and showed a dual polarizationphenomenon. Microwave processing of starch showed a slight decrease in the absorbing performancebefore gelatinization temperature, which was in line with the conduction heating process afterwards.Although microwave processing at low temperature changed the polarization characteristics of the materialin the vicinity of the frequency of2.45GHz, after processing the microwave absorption characteristics ofthe material were not enhanced.（3）Effect of microwave heating on structural characteristics of rice starchThe FTIR, InVia Reflex Raman microscope sys-tem,13C CP/MAS NMR, wide-angle and smallangle X-ray diffraction techniques were applied to investigate the effect of microwave heating process onthe molecular structure such as molecule structure, the relative degree of crystallinity, the ordered structureand disordered structure contents of rice starch. The FTIR results showed that microwave heating does notproduce new chemical bonds and the chemical group in starch, and the vibration energy of the disorderedand ordered structure slight changed, indicating the rapid heating effect was the major factor for thechanges in the chemical bonds in starch under microwave. The Raman spectroscopy showed an inhibitoryeffect of microwave on the increase in the skeleton of the CH group vibration near the gelatinization temperature during the rapid heating process, and microwave heating of starch molecules showed nosignificant effect on the vibration intensity of the glycosidic linkage, pyranose ring, and-C-O, C-O-Hgroups on the skeleton. The results of13C CP/MAS NMR showed that the rapid heating effect ofmicrowave is the major factor for the differences in the amorphous starch granules, double-helix structureand V-type single helix structure contents, and the effect of microwave on the ordered internal structure ofstarch granules was not significantly. Small angle X-ray diffraction indicated that the rapid heating effect ofthe microwave resulted in tighter arrangement of the double helix structure of the amylopectin molecules inthe crystalline layer and compression of the amorphous layer. Electromagnetic effect is obviously resultedin irregular alternation of the layered structure, and destructive effects on the rapid heating. When thetemperature was above60°C，the molecular vibration mechanism of microwave accelerated the destructionof the lamellar repeats architecture of starch. The results of the wide angle X-ray diffraction showed thatthe rapid heating effect of microwave caused the decrease of the degree of crystallinity of starch below thegelatinization temperature.（4） Effect of microwave heating on the morphology, thermal properties and water status of ricestarchThe particle size distribution, polarizing microscope, SEM, TG, DSC, the1H NMR were applied tostudy the destruction of birefringence and granule morphology of rice starch by microwave, and theinfluence on the activation energy and enthalpy. The IR and CPMG results indicated that the rapid heatingeffect is the main reason for the disappearance of birefringence and change in granule morphology, andmicrowave did not procure the asynchronous nature of the disappearance of birefringence and change ingranule morphology, and did not affect the imaging effect of the radial arrangement of amylopectin chains.The rapid heating effect of the microwave prompted the apparent peak of activation energy at50℃, andthe rapid heating effect of the microwave resulted in the enthalpy value first increased and then decreasedwith the increases of the temperature, different from the slow heating treatment, indicating that the effect ofmicrowave on thermal stability of the pre-gelatinized starch mainly depended on the rapid heating effect ofthe microwave. Microwave heating affected the internal water distribution and dynamics of starch granules,and the rapid heating effects limited the damaging effects of hydrogen bonds between starch molecules andstarch with water. However the molecular vibration mechanism of microwave accelerated the destructionof the hydrogen bonds, and the vibration mechanism showed stronger effect then the rapid heating effect.（5）Temperature distribution and heat transfer model of rice starchy food under microwave reheatingThe physical parameters and dielectric properties of rice components under microwave heating wasinspected with cylindrical material, and the influence of the sample features on the temperature distributionin the microwave reheating process was investigated through the heating uniformity analysis. The optimalsize of the rice was selected, and the heat transfer model was built according to the law of conservation ofenergy, the Fourier equation. Lambert law was used to calculate the microwave absorption power, togetherwith the initial and boundary conditions of the rice and the finite difference method (FDM) numericalalgorithm, MATLAB was applied for the numerical prediction of the temperature distribution in rice undermicrowave heating process. The results showed that temperature showed a significant impact on thespecific heat and dielectric loss on rice, with little effect on the thermal conductivity and the dielectricconstant. Polynomials can show a good reflection on the variation of the rice dielectric loss withtemperature（R2=0.9785），and the thermal conductivity averaged at0.760W/(m℃) in the range of4~80℃.At the same temperature, the rice permittivity is greater at a frequency of0.915GHz than at the frequencyof2.45GHz, whereas the dielectric loss factor is the opposite. The temperature distribution pattern ofcylindrical rice revealed that temperature distribution of the sample in the axial direction distributedsymmetrically, showing increased corner and surface heating trend with the increase in the height/diameter ratio, and L/D had a significant impact on the internal temperature distribution of the sample.With the temperature coefficient of variation (COV) as the evaluation indicator of the temperatureuniformity of, the L/D and the radius for the COV was obtained to determine the optimal cylindrical-typerice with uniformity of microwave heating (a radius of4cm, height4cm), with high fitness of thepredictive model（R2between0.996and0.999）.Microwave showed impact on starch and starch emulsion throughout the entire process. Themicrowave electromagnetic effects were only limited in molecule polarization mechanism, skeletontightness, and water distribution of the layered structure at the micro level intervention. Rapid microwaveheating effect was the principal factors for macro differences in physical and chemical properties of starch.The microwave heating process of starchy foods can be described based on the thermal properties and thedielectric characteristics of the material.