Benchtop and synchrotron X-ray micro-tomography to determine the cellular structure of cereal food foams

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Modelling Thermoplastic Filaments’ Sintering by Level Set Method

International audienceThe viscous sintering kinetics of thermoplastic polymers is generally studied by monitoring the evolution of the bonding neck between two particles (spherical, or cylindrical) and using a refined Frenkel-Eshelby's model. Recently, we showed that the entire contour of sintering filaments could be modelled by lemniscates as figure-eight shape curves to assess bonding abilities of a 3D-printable plasticized biopolymer. Using COMSOL Multiphysics ® software, we set up a 2D finite element model of thermoplastic filaments' viscous sintering with flow front tracking by the level set method. This leads to contrasted images of the two phases, i.e. air and polymer, allowing the prediction of the shape of the interface corresponding to the filaments' contour. An image analysis procedure is applied to the simulated sequences and the ones acquired during sintering trials of extruded filaments based on zein, a corn protein plasticized by 20w% glycerol. This method is based on the assessment of the coordinates of sintered filaments' edge pixels and their fitting by lemniscates of Booth. We show that the 2D FEM approach combined with level set method allows simulating the hot melt viscous sintering of a 3D-printable thermoplastic biopolymer as a two-phase flow. Furthermore, the image analysis is successfully applied to simulated and experimental sequences, thanks to the monitoring of the filaments' contour, to assess their bonding kinetics and check its modelling

Determining the Cellular Structure of Two Cereal Food Foams by X-ray Micro-Tomography

The cellular structure of two products, an extruded breakfast cereal and a short dough biscuit, was characterized by two different X-ray micro computed tomographic systems. Acquisitions were made by a compact desktop system Skyscan 1174 (Bruker mu CT, Belgium) and at the European Synchrotron Radiation Facility (ESRF, beamline ID19, France) at different resolutions (voxel size of 6.5 mu m, 7.5 mu m, 16.2 mu m and 25.8 mu m). 3D images were processed for the density, the connectivity index and the granulometry of cells and cell walls. These experiments underlined the importance of the resolution for determination of quantitative measurements such as densities and thicknesses. The median width calculated for the cell walls distribution in the biscuit dropped from 141 to 50 mu m when the voxel size changed from 25.8 to 7.5 mu m. Images well showed that even though the food products had close values of porosity 0.6 and 0.7 for biscuit and extruded breakfast cereal respectively, their cellular structures were very different. The biscuit had small cells (median value of the distribution varied from 125 to 152 mu m, according to resolution) and larger cell walls (50-141 mu m) than the extrudate (32-109 mu m) which, on the contrary, exhibited very large cells (307-400 mu m). Beyond methodological issues, these differences could be clearly attributed to the differences of compositions and processes

Fast solution to determine the elastic behavior of 3D reconstructed biopolymer cellular structures

International audienceThe “fast Fourier transform method”, originally introduced by Moulinec and Suquet (1994), is a breakthrough in numerical algorithms used to estimate the linear and non-linear behavior of heterogeneous media. The method consists of a fixed-point algorithm based on the Lippman-Schwinger equations which is directly applied to microstructure images, thus requiring no meshing. Iterations are carried out in the real space and Fourier domain, where the local behavior of the material and the fields admissibility conditions, respectively, are accounted for. In the classical framework of linear elasticity, the local strain is admissible when it derives from a displacement field, whereas the stress is divergence-free. Eyre and Milton (1998) and Michel et al. (2001) proposed refined algorithms with much better convergence properties in cases of highly (or even infinitely) contrasted media, i.e. porous or rigidly-reinforced composites with arbitrary microstructures, which are used in this work. The Fast Fourier transform method also allows for large-size computations, where the the microstructure is more accurately discretized than in traditional finite element methods. Recently, 3D computations on materials with large representative volume elements have been successfully carried out on systems that contain up to 500^3 discretization points (Willot and Jeulin, 2008).The numerical FFT method is applied to segmented images of a biopolymer cellular material made of starch and containing a large volume fraction of voids (up to 90%). These images are crops of real structures acquired using X-ray tomography (Babin et al, 2007). Under a small deformation assumption, the local behaviour of starch is approximated by an isotropic, linear elastic law, and incompressibility is assumed. Such assumption is in accordance with previously published works dealing with both the cellular and the intrinsic material (Guessasma et al, 2009; Babin et al, 2007). The material is subjected to either hydrostatic or shear strain loading, and its macroscopic as well as local linear elastic response is computed with the FFT method, on images of sizes 200^3 to 300^3, where each voxel has a physical dimension in the range 10–40 µm depending on the sample, more exactly on the cellular structure. As expected, the material behavior is found to be macroscopically compressible due to the presence of voids, and according to the numerical computations, roughly isotropic. This homogenization procedure is compared to experimental data obtained for two sets of microstructures

Granulometry of bread crumb grain: Contributions of 2D and 3D image analysis at different scale

International audienceSix bread crumbs were prepared from three different recipes and three baking procedures. Images of crumb were acquired in 2D at a macroscopic scale by using a flat bed scanner (resolution 85 [mu]m) and in 3D at a local scale by X-ray tomography (resolution 10 [mu]m). The cellular structure was assessed by mathematical morphology. 2D image analysis was completed by principal component analysis. The first principal component was found to reflect crumb fineness, in agreement with the mean cell size determined in 3D at a local scale. 3D mean cell wall size were about 220 [mu]m and were not significantly different. The second principal component was linked to the 2D macroscopic heterogeneity of the crumb and to the macroscopic cell wall thickness. 2D images can be applied to the rapid control of crumb grain and could be used to quantify the cellular structure for the calculation of mechanical properties

Bread collapse. Causes of the technological defect and impact of depanning time on bread quality

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Fragmentation of two soft cereal products during oral processing in the elderly: Impact of product properties and oral health status

International audienceThis study investigated the mechanisms of fragmentation leading to bolus formation during chewing in the elderly population for two cereal foods of different compositions and cellular structure: sponge-cake (SC) and brioche (B). For both products, mechanical properties were characterized by uniaxial compression and 3D cellular structure was determined using x-ray micro-tomography. Stress-strain curves showed two distinct ductile-like behaviors: product B underwent plastic deformation, whereas product SC displayed a hyper-elastic behavior. Twenty subjects aged 65 years and over with two different oral health conditions (poor vs satisfactory dental status, variable stimulated salivary flow rate) were asked to consume both products. Bolus particle size was determined at three different chewing stages through image analysis, and the resulting particle size distribution (PSD) curves were fitted by Gompertz model. The model parameters were related to bolus particle heterogeneity and fragmentation, thanks to their correlations with median particle size diameter D-50 and interquartile ratio (D-75/D-25), directly extracted from PSD curves. The use of model parameters allowed discriminating between chewing sequences for both products and revealed different fragmentation patterns: while SC boli exhibited a continuous particle size reduction during chewing, B displayed a combination of fragmentation and agglomeration. In addition, results showed that subjects with a satisfactory dental status produced significantly more degraded boli than those with a poor dental status. These results highlight distinct fragmentation mechanisms for these two soft products that were interpreted in relation to their differences in composition, structure and mechanical behavior

On the representative elementary size concept to evaluate the compatibilisation of a plasticised biopolymer blend

International audienceThis study combines experimental and numerical approaches to investigate the microstructure and mechanical behaviour of non-miscible plasticised starchi/zein blends. The concept of Representative Elementary Size (RES) is used to rank the effect of five different plasticisers (cholinium acetate, glycerol, butyl methyl imidazolium chloride, glycerol-choline chloride, urea-choline chloride) inducing microstructural and mechanical changes in the blends. Microstructural and mechanical RESs are derived from microscopy image analysis and Finite Element Modelling of elasticity behaviour of studied blends. Compared to the usual consideration of ultimate mechanical properties (elongation and stress at break), the RES-based approach allows detecting the presence of perfect or imperfect interface between starch and zein particles depending on the nature of plasticiser

Fusion Bonding Behavior of Plasticized Corn Proteins in Fused Deposition Modeling Process

International audienceThe processing of natural biopolymers by Fused Deposition Modeling (FDM) opens perspectives for applications in food and health domains by taking advantages of their edibility, biocompatibility and bioresorbability. Glycerol plasticized zeins (proteins from maize kernels) present thermomechanical properties matching the extrusion step requirements of FDM (at T-printing= 130 degrees C for 20% of glycerol). The present work focuses on the fusion-bonding step of the process between adjacent filaments. Mechanisms at the root of the thermal bonding of amorphous polymers at T>T-g are governed by melt's surface tension (Gamma, driving force) and viscosity (eta, limiting force). In addition, healing phenomenon, assessed by the degree of healing, Dh, increases with time as Dh proportional to t(1/4). It originates from the diffusion of polymer chains across the interface in accordance with the reptation theory. Dynamic rheological properties of molten extruded filaments of plasticized zeins were determined in a pre-heated oscillatory rheometer at 130 degrees C, with |eta*|(gamma)over dot=1.6(s-1) ranging from 0.6 to 0.8kPa.s. Gamma was estimated from the evolution of the fusion-bonding neck growth between two extrudates (polymer sintering model). The 0.1mm.s(-1) bonding rate observed at 130 degrees C allowed estimating a melt surface tension of 30-40mN.m(-1). Concurrently surface energy measurements were conducted on solid plasticized zein at 20 degrees C using the sessile drop method: By varying liquids deposited on zein-based surface and following Owens and Wendt's approach, gamma(SV) was found to amount to 39.2 +/- 1.6mN.m(-1), with the dispersive component gamma(d)(SV) = 4.2 +/- 0.4mN.m(-1) and the polar one gamma(p)(SV) = 35.0 +/- 1.2mN.m(-1). These values were used to extrapolate a melt surface tension using the typical surface tension dependence d gamma/dT approximate to-0.05mN.m(-1).K-1 like for synthetic polymers following the Eotvos' law. The extrapolated values at 130 degrees C were in agreement with those obtained from fusion-bonding experiments

BREAD – PAN INTERFACE. IMPACT OF CRUST STRUCTURE ON BREAD DEMOLDING

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