Effects Of Constant Power And Uniform Microwave On Multiscale Structure And Digestibility Of Rice Starch

Linear amylose and highly branched amylopectin form the multi-scale structure of starch through different arrangement and combination ways,and the structural changes of each scale will affect the overall properties of starch.Microwave processing has become an important physical means of starch modification because of its high efficiency,and it has been proved that microwave treatment can significantly change the structure and properties of starch.However,domestic microwave ovens used in the most researches have the defects of uneven heating,resulting in poor reproducibility in the obtained data,which is not conducive to study mechanism of microwave modified starch.Besides,most current studies on microwave modified starch have ignored the influence of heating rate,water content and energy in the heating process,resulting in a large difference in the research results.Therefore,this paper firstly optimized and confirmed the power constancy and heating uniformity of microwave equipment,and obtained the optimal microwave operating condition,then used Indica rice and waxy rice starch as raw material,studying the influence of molecular structure,helix structure,short-range ordered structure,crystalline structure,lamellar structure and morphology and digestibility of rice starch under the best microwave operating condition,and explaining the mechanism of microwave effect on starch digestibility.Meanwhile,By comparing the effects of microwave rapid heating(300 W)and microwave slow heating(100 W)on the multi-scale structure of starch,the effects of heating rate and water change in the heating process on rice starch were discussed,and the potential mechanism of microwave modified starch was explored.The main conclusions showed as follow:1)In terms of microwave equipment,constant power microwave equipment ensured constant power by applying solid state microwave source to output microwave and improved the heating uniform by waveguide heating that reduced microwave reflection.Besides,the heating uniformity and repeatability of constant power microwave equipment had been verified by means of HFSS electromagnetic field simulation and the monitoring of temperature change in the actual experiment.2)In terms of starch molecular structure,constant power and uniform microwave could not change the chemical bond composition in the starch molecules,but broken the ?-1,4-glycoside bond and ?-1,6-glycoside bond of Indica and waxy rice starch,reducing the branching degree of these two kinds of rice starch.Meanwhile,through comparing the changes of branching degree of rice starch under different heating rates,it was found that the rapid microwave heating(300 W)caused a greater damage to the starch chain structure of rice starch.This may be because the friction and collision between polar molecules in the starch granules are more intense at 300 W power,resulting in an increase in the number of hydrogen bond breaks between and within the starch molecules.3)In terms of starch short-range ordered structure,constant power and uniform microwave treatment decreased the contents of helix structure and short-range ordered structure,and the higher the microwave heating temperature and the faster the heating rate,the destruction of starch short-range ordered structures more significantly,suggesting that both thermal effects and rapid heating effect can influence the short-range ordered structure of the rice starch.Microwave thermal effect could destroy the intermolecular hydrogen bond of starch,unscrew the double helix structure,resulting in the decrease of the ordered structure.The microwave rapid heating effects caused rapid migration of water in starch,reducing the stability of the ordered structure of starch.4)In terms of starch crystallization and lamellar structure,the constant power and uniform microwave did not change the crystal pattern of Indica rice and waxy rice starch,but destroyed the crystalline structure of rice starch,and the higher the microwave heating temperature and the faster the heating rate,the more severely damage found in crystalline structure of starch.There are two reasons for the different changes of relative crystallinity of rice starch under the power of 100 W and 300 W,on the one hand,the energy input at a higher density at a power of 300 W made the water molecules in the starch boil and evaporate quickly without time to move out,which caused extremely high internal vapor pressure in the starch.The pressure and the rapid migration of water may lead to the destruction of the crystalline structure more easily.;On the other hand,long-term high temperature and low humidity environment may recover the loose starch chain and cause recrystallization under the condition of microwave slow heating mode,thereby the relative crystallinity of rice starch in slow heating mode was not reduced as much as that in fast heating mode.The change of the parameters of starch lamellar size also explained the destruction of starch crystallinity.After constant power microwave,the thickness of the crystalline layer of and waxy rice starch decreased,the thickness of amorphous layer increased,and the thickness of semi-crystalline layer increased.Moreover,these parameters of rice starch changed more obviously with microwave rapid heating processing.5)In terms of morphology structure of starch,constant power microwave could destroy the granule morphology of Indica and waxy rice starch,With the increase of temperature,the morphology of rice starch showed different degrees of roughness and fragmentation,and the higher the final temperature of the microwave,the greater the degree of fragmentation.Meanwhile,the damage caused by microwave slow heating inferior to that caused by the microwave rapid heating.6)In terms of starch digestibility,constant power and uniform microwave increased the digestibility of Indica and waxy rice starch,suggesting increase in RDS and SDS,decrease in RS.The influence of microwave on the starch digestibility increased with the increase of the temperature.Microwave destroyed the structure of starch molecular chain,helix structure,crystal structure and morphology of rice starch,which expose more enzyme binding sites,thereby increasing the digestibility of rice starch.