Measurement of Moisture Migration Kinetics in Environmental Scanning Electron Microscopy
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Measurement of Moisture Migration Kinetics in Environmental Scanning Electron Microscopy Xiaohu Tang and Mario de Rooij Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, Netherlands
ABSTRACT Volume change of cereal materials caused by moisture migration is studied by Environmental Scanning Electron Microscopy (ESEM) using videomicroscopy and stereoscopy in combination with image processing. It is shown that the in situ volume change can be monitored by following the change of projection area, assuming isotropic volume change. Also the volume change measured by stereoscopy matches well with the measurement of the projection area change by videomicroscopy (after normalization). It is concluded that more volume changes in a higher relative humidity (RH) region with more active kinetic behavior. The exact volume change in between two relative humidities can be obtained by interpolating the equilibrium volumes. All the sorption/desorption curves can be well described by the ‘parallel exponential kinetics’ model (PEK model), which described two independent, parallel processes. INTRODUCTION Moisture migration is one of the important factors that affect the shelf life of food products. Improvement of manufacturing and storage processes can only be developed if moisture transport and moisture-matrix interaction are well understood on a fundamental level. It is known that moisture migration into food products is often accompanied by volume changes. Starch is the main component of many food products and consists of two polysaccharide polymers: linear amylose and highly branched amylopectine. When water molecules enter the starch granules they disrupt the glucan chains and elongate the interchain hydrogen bonds, which is accompanied by a volume change. Volume change measurements therefore are critical to understand the mechanism of moisture migration processes. Much work has been done on water uptake and swelling of starch-based food such as biscuit, including determination of weight and density changes by use of isotherm curves [1, 2], evaluating the expansion ratio of length or thickness based on the assumption of isotropic swelling [3, 4], or stereoscopic imaging combined with 3D reconstruction. In this study we describe a procedure that provides a rapid way of indirectly measuring in situ volume changes by the combination of ESEM, stereoscopy, and videomicroscopy techniques. EXPERIMENT Biscuit particles, provided by Unilever R&D (Vlaardingen, the Netherlands) (main components: ~80% starch, ~10% protein, and ~10% oil), are irradiated by 10keV beam energy
Figure 1 Principle of stereoscopy in ESEM (left) and 3-D view of a biscuit particle (right) in 66%RH (above) and 99%RH (below) respectively
inside a FEI XL-30 ESEM (Eindhoven, the Netherlands) under low vacuum mode. The RH value inside ESEM chamber can be dynamically controlled by adjusting temperature and/or water vapor pressure. A Gaseous Secondary Electron Detector (GSED) is used to take images during both sor
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