Effect Of AMIMCl/Water Mixtures On Phase Transition And Property Of Starch During Processing

Starch, as a natural polymer, has been widely used in the industrial fields, such as food, medicine and chemical engineering. Due to the unique super molecular structure, starch usually needs physical and chemical modification before some industrial applications. and processing properties. Starch is insoluble in most solvents, which restricts the processing and development of the kind of natural polymer. Ionic liquids(ILs), as “green solvents”, reaction are non-volatile and non-oxidized, have high thermal stability and good solubility of starch. ILs has been employed as solvents, plasticizers and catalyzes in starch processing. Due to the high price of ILs, therefore it is reasonably expected to design and develop a new blending solvent system with water to replace a proportion of the more expensive ILs.The objective of the present work is to provide a fundamental understanding of the effect of IL on the morphology and phase transition of starch, which in turn would afford useful information about starch structure at the molecular level, thereby contributing to our understanding of the corresponding mechanisms. In the current study, the more common and inexpensive 1-allyl-3-methlimidazolium chloride(AMIMCl) was employed and diluted with water to creat a solvent mixture with limited AMIMCl content. In addition, corn sarches with different amylose content(amylose/amylopectin:80/20, 50/50, 23/77 and 0/100) were also employed to compare and interpret the relative results. Results showed that:In the blending system of AMIMCl/water, the morphologies and internal structure of starch were significantly influenced by the ratios of AMIMCl/water and the amylose content.When the AMIMCl/water ratio was lower than 50/50, during heating process, both low amylose corn starch(waxy and normal starch) and high amylose corn starch(G50 and G80) showed similar change trends: the phase transition temperature, observed by both differential scanning calorimeter and microscopy, moved to high temperature with the increasing AMIMCl content. It is due to the high concentration of water in the system, which caused the ion pairs of [AMIM]+Cl- easily dissociated. In this case, one Cl- anion interacted locally with the hydrogen of one OH group of starch, while the cation [AMIM]+ interacted weakly with the oxygen atom of another OH group. In such a situation, due to the limited cations and anions of AMIMCl, the two OH groups of starch that were involved were still able to form hydrogen bonds with other OH groups of starch or water, which bridged the starch helices into a network. At the ratio of 50/50, the phase transition occurred in the highest temperature region, and the range became narrow, implying that starch helices were effectively rearranged and therefore more homogeneous and stable. However, using such mixture solvent to process starch at room temperature, no noticeable difference was observed on the granule appearance and internal structure of starch.When the AMIMCl/water ratio was higher than 50/50, during heating process, the peak of phase transition shifted to lower temperature with an increase in AMIMCl content. In such environment, AMIMCl concentration was high enough to provide large amounts of ion pairs; as a result, more free Cl- were available to interact with the hydrogen of the remaining OH groups, which damaged the hydrogen-bonding force in starch and decoupled the helices. During this process, a small amount of water may also compete with IL to interact with the OH groups of starch. Therefore, starch dissolution and gelatinization simultaneously occurred and overlapped. As the ratio was 80/20, the phase transition temperature occurred in the lowest temperature region. It needed to point out that, starch granules were apparently changed by this blending solvent at room temperature, companying with an enlargement of holes and caves, and an appearance of fine cracks. And the relative crystallinity(RC) of treated samples decreased.In pure AMIMCl, the phase transition was caused by starch dissolution, and the temperature of high amylose starch was higher than that of low amylose starch. After treated by pure IL, the birefringences of low amylose starch samples gradually disappeared by IOD observation with the increasing storage time, while high amylose starch still kept granule intergrity with a decrease in relative crystallinity.