| In the food and biological field,the study of the free volume of materials can help to explain the change of macroscopic properties.As a typical multihydroxyl material,starch has the intrinsic advantage of enhancement of the dielectric response and rearrangement of the interfacial polarization which is caused by the induced change of segments mediated by microwave in the presence of water.The effect of microwave on the lamellar structure of starch has been investigated.However,the change of free volume of starch in microwave field is not clear.Therefore,the corresponding relationship between microwave heating properties and microstructure of starch may be reflected after the observation of starch free volume changes and the analysis of the macroscopic properties and submicrostructure under microwave irradiation.In this study,the effects of water contents on the dielectric properties,free volume and glass transition temperature of rice starch were investigated.The influence of microwave parameters and moisture content on the free volume was examined.The difference of free volume change after microwave and conduction heating was analyzed.The relationship between the free volume change and the properties and structure of starch was investigated.Firstly,the dielectric properties and microwave absorbing property of rice starch with different water content were measured by vector network analysis.It was found that the dielectric properties of starch varied significantly with water content.There was a linear relationship between the dielectric constant and the water content,which indicated that starch and water didn't interfered the absorption ability of microwave with each other.With the increase of moisture content,the microwave absorbing property of the starch system was enhanced,and the microwave absorption of starch follows the heating mechanism of dipole polarization.Then,the effects of water on the free volume and glass transition temperature(Tg)of rice starch were investigated by DSC and positron annihilation lifetime spectroscopy(PALS).Tg of rice starch decreased from 124? to 23? and the free volume increased from 77.98 ?3 to 105.40 ?3 with water content from 10% to 35%.The effects of microwave power,treatment time and moisture content on the free volume of rice starch were studied.With the increase of microwave heating time and power,the positron annihilation lifetime and the intensity values were reduced.The changes of free volume size and concentration were different with microwave heating and conventional heating method.The effects of microwave power and treatment time on the structure and thermal properties of rice starch were studied,which were compared with free volume studies.The crystallinity of starch was not significantly changed after microwave treatment,and microwave treatment did not significantly change the long-periodic size of the starch,but the size of the crystalline area was slightly larger,and the amorphous area became slightly smaller after SAXS analysis.Glass transition temperature Tg increased with the increasing intensity of microwave,and gelatinization temperature of starch after microwave was higher.The thermal stability of starch and the glass transition temperature of freeze-concentrated gelatinized starch-water systems were not significantly changed after microwave treatment.Above all,the activity of starch molecules could be decreased by reduction of starch free volume after being heated in microwave field and furthermore,the glass transition and pasting properties of starch could be influenced which provide a new insight for the study of starch macroscopic properties from free volume aspect according to this study. |
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