Morphology And Gelatinization Of Corn Starches With Different Amylose/amylopectin Content Under Shearless And Shear Conditions

Starches, the primary source of stored energy in most plants, have been widely used in food and non-food industrials. Many food and non-food industrials using starch rely on the colloidal properties of its two structurally distinct a-D-glucan components, amylose and amylopectin. These components are not single molecular entities, but rather each constitutes a family of related molecular species with diversity in morphology, fine structure and function. The ratio of amylose to amylopectin within a given type of starch is a vital important point to be considered with respect to the starch functionality in processing. Starch displays a unique thermal behavior, when heated in water, known as gelatinization. None of the theories documented in literature adequately explain the clear mechanisms of structural changes that occur in starch granules during gelatinization process. In this study, the morphologies and gelatinizations of starch granules of four native cornstarches with different amylose/amylopectin contents (waxy:0/100; maize:23/77; Gelose 50:50/50; Gelose 80: 80/20) were used as modal materials and systematically studied under shearless and shear conditions using different methods. Some confused and arguable points reported previously were carefully retested and confirmed using some latest equipments and designed experiments. The results showed as following:The morphologies and microstructures of cornstarches with different mylose/amylopectin ratios were firstly systematically studied by microscopy with ordinary and polarized light, scanning electron microscopy (SEM) and X-ray diffraction (XRD), nuclear magnetic resonance (NMR). It was found that, generally, the granules of the amylopectin-rich starches were more regular in shape than those of the amylose-rich starches, but the surfaces of the amylose-rich starches were smoother than those of the amylopectin-rich starches. The birefringence and particle size of the granules were waxy> normal maize> G50> G80, which also corresponds with the trend of molecular weight. Waxy and normal maize starches showed typical A-type patterns, while G50 and G80 show B-type patterns. Crystallinity increases with increasing the amylopectin content. The results of 13C CP/MAS spectra show that the double helical content was found to be increased with the amylopectin content increase.Confocal laser scanning microscopy (CLSM), SEM, XRD and acidic hydrolysis were used to study the internal structural characteristics of cornstarches. CLSM provides a good approach to exploring the internal structure of the granules. Acid hybridization of starch by HCl was used to enhance the difference between amorphous and crystalline ranges. It was firstly found that the high-amylose starches (G50 and G80) were brighter than those of low-amylopectin starches (waxy and maize) under confocal laser light, and the average (decreasing) fluorescence intensity sequence of the granules was G80> G50> maize> waxy. Waxy and maize starches showed clear internal cavities and channels, whilst G50 and G80 had bright cores. Compare the native to acid hydrolyzed starch, the sharpness and intensity of the diffraction patterns are slight improved (waxy and maize) and significantly with high-amylose (G50 and G80). Sharp growth ring structure can clearly be observed for low-amylose starches (waxy and maize) after acid hydrolysis. No growth rings were found in high amylose (G50 and G80) in this experiment. The X-ray patterns of acid hydrolyzed starches retain the same diffraction type as that of the parent starch.A microscope with a hot-stage and CLSM were used to observe gelatinization under shearless conditions during heating and isothermally at different temperatures. An increase in starch granule diameter, disappearance of birefringence and granule disappearing were used to describe the gelatinization. The diameter growth rate and final accretion ratio sequence of starch granules during heating under shearless conditions correspond with amylose/amylopectin ratio. The growth rate of granular diameter under shear conditions was controlled by two factors:swelling and dissolving. Increased amylose content led to a lower granule diameter growth rate and accretion ratio, and a higher temperature before birefringence disappeared. The temperature-induced changes of the cornstarch granules in excess of water were firstly studied under CLSM. Gelatinization of all starches starts at the hilum and the adjacent of the channels, and spreads rapidly to the periphery. This work is the first time that three-dimensional images of partly gelatinized granules have been constructed and presented from different confocal images, which allows further exploration of the mechanisms of gelatinization.The heterogeneity and complexity of starch structure within granules influence the particle structure changes of starch granules. The physical model of starch granule gelatinization under shearless condition was established through systematically studied the morphological and microstructural changes during gelatinization. The Water molecules primarily diffused from the surface of starch granules, passed through the channels to the cavity and lateral diffused from cavity and channels throughout the granule matrix. From the diffusion process, the matrix of native corn starch granules was supposed to be divided into three regions:the granule matrix surrounding cavity and channels (D1), dense packed layer beneath the surface (D3), and the intermediate organized area (D2). The possible pathway of starch granule gelatinization in starch granules during gelatinization started from the central cavity along the channel to spread around, the order of D1, D2, D3.Gelatinization under shear conditions was firstly investigated using a rheoscope. It is first time that viscosity and microstructure variations have been recorded at same time. A direct relationship between viscosity and variation of starch granules for starch gelatinisation under shear stress was established. There are two steps of starch gelatinisation in a shear field with respect to the physical transition of starch granular-moisture ingress and leaching of the amylopectin or amylose, and the whole granule dissolving. In the first step the viscosity increases are minor, and the size increase of starch granule by swelling during gelatinisation is minor or negative. In the second step, the starch granule disappears in a short time and viscosity increases sharply. This increase in viscosity is likely to be dominated by the propensity of the swollen granules to interact, especially at high concentrations, increasing the viscosity of the dispersion. In this step, the mean granule size increases markedly, the granule, becomes more transparent until finally it bursts releasing the solubilized material inside into the solution.