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Starch is a semi-crystalline polysaccharide organized into granules composed of amylose and amylopectin, whose hierarchical structure governs its physicochemical behavior. It is a widely available, renewable biopolymer used in numerous applications ranging from food processing to bio-based materials. Once transformed, starch forms a 3D solid network, the mechanical and transport properties of which are influenced by its interaction with water. A key feature of hydrated starch is that water does not exist as a single homogeneous phase. Instead, it is distributed between water with high mobility, which we call free water, and water strongly confined within the polymer network at the nanometric scale, which we call bound water. While starch–water interactions have been extensively studied during hydration and gelatinization [1–3], the reverse process, i.e., drying, has received little attention from a physical perspective. Drying is a key step in almost all starch-based processes. water transport during starch drying is investigated using low-field ¹H NMR relaxometry and MRI, which provide non-invasive, time-resolved measurements and have been successfully applied to controlled nanoporous materials [4] as well as bio-based hygroscopic media [5-6]. The experiments revealed that drying dynamics are strongly dependent on both the initial state of starch (i.e., native or transformed) and the imposed drying conditions. These parameters control not only the overall drying kinetics but also the dominant transport mechanisms and associated microstructural evolution. The results revealed two successive drying regimes: an initial constant-rate period dominated by the drying of free water and associated with the homogeneous shrinkage of the material. This regime is followed by a falling-rate period associated with heterogeneous shrinkage. A spatially resolved analysis revealed that starch drying can be described within a two-region diffusion framework separated by a moving interface. After the initial stage (i.e., constant rate drying), a drying front appears and progressively propagates inward. This interface marks the local disappearance of free water and separates an outer region containing only bound water, where transport proceeds via diffusion toward the surface. In both regions, moisture transport is governed by diffusion of bound water through the solid matrix. Drying therefore evolves toward a falling-rate regime controlled by confined water, as observed in other hygroscopic porous materials such as wood [5], despite the deformable nature of starch.
| References | [1] H.-R. Tang, J. Godward, B. Hills, The distribution of water in native starch granules—a multinuclear NMR study, Carbohydrate Polymers 43 (2000) 375–387. https://doi.org/10.1016/s0144-8617(00)00183-1 [2] K. Tananuwong, D.S. Reid, DSC and NMR relaxation studies of starch–water interactions during gelatinization, Carbohydrate Polymers 58 (2004) 345–358. https://doi.org/10.1016/j.carbpol.2004.08.003. [3] Yao, J., Chen, Y., Tian, S. et al. Investigating Morphology of Food Systems and Water-biopolymer Interactions in Food Using 1H NMR Relaxometry. Food Biophysics 17, 150–164 (2022). https://doi.org/10.1007/s11483-021-09712-9 [4] B. Maillet, P. Huber, G. Dittrich, and P. Coussot, “Diffusionlike Drying of a Nanoporous Solid as Revealed by Magnetic Resonance Imaging,” Phys. Rev. Applied, vol. 18, no. 5, Nov. 2022, https://doi.org/10.1103/PhysRevApplied.18.054027 [5] M. Cocusse et al. Two-step diffusion in cellular hygroscopic (vascular plant-like) materials.Sci. Adv.8, eabm7830(2022). https://doi.org/10.1126/sciadv.abm7830 [6] Y. Zou, B. Maillet, L. Brochard, and P. Coussot, “Fast transport diffusion of bound water in cellulose fiber network,” Cellulose, vol. 30, no. 12, pp. 7463–7478, July 2023, https://doi.org/10.1007/s10570-023-05369-4 |
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| Country | France |
| Green Housing & Porous Media Focused Abstracts | This abstract is related to Green Housing |
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