Abstract
The aim of this work was to investigate the role of microwave parameters and moisture content on the free volume (FV) changes of rice starch by positron annihilation lifetime spectroscopy analysis (PALS) and to explore the potential relationship between the changes of FV and physicochemical properties of rice starch. Microwave heating and water molecules lead to the increasing of FV of starch. However, this result is largely influenced by the plasticization of water molecule. The anti-plasticization caused by water evaporation resulting in a decrease in the size and concentration of FV during microwave heating. Significant decrease (p < 0.05) in the thickness of amorphous region of microwave-heated rice starch was found by small angle X-ray scattering (SAXS), and the glass transition temperature (Tg) and gelatinization temperature significantly increase (p < 0.05) after microwave heating. According to correlation analysis, the power intensity and heating time were correlated negatively with the lifetime of o-Ps. In addition, the changes of amorphous region and Tg of rice starch were strongly related to FV changes. These results provided a theoretical basis for further research on the directional regulation of FV and improvement the quality of starch-based food by using microwave treatment.
Highlights
Free volume (FV), the concept of which was first proposed by Fox and Flory with the motivation for illustrating the glass transition process, was defined as the unoccupied volume composed by interspace of molecular chain or imperfections in random arrangement of molecular[1,2]
The increasing of moisture content resulted in higher τ3 and I3 values of starch samples, which indicated that the free volume (FV) in the starch was expanded by water adsorption
The peak of lifetime distribution shifted to the right and was reduced in width with increase of moisture content, which indicated that the water molecules resulted in the FV of rice starch increased
Summary
Free volume (FV), the concept of which was first proposed by Fox and Flory with the motivation for illustrating the glass transition process, was defined as the unoccupied volume composed by interspace of molecular chain or imperfections in random arrangement of molecular[1,2]. There are no available publications regarding the effects of microwave heating on the FV holes of starch it can manipulate FV of other polymer materials[13]. Moisture content, impacting the hydration process of starch, www.nature.com/scientificreports/. Www.nature.com/scientificreports is an important factor in the process of microwave heating starch as we previously reported[15]. It is still not clear, microwave heating and water molecule, which plays a more important roles in the FV changes of starch. The first objective of this study was to evaluate the effect of moisture content and microwave parameters on the FV changes of rice starch during microwave heating. The second objective was to provide insights into the relationship between the changes of FV and physicochemical properties of rice starch upon the microwave processing
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