Modelling of moisture diffusion in pores of banana foam mat using a 2-D stochastic pore network: Determination of moisture diffusion coefficient during adsorption process Preeda Prakotmak a, * , Somchart Soponronnarit a , Somkiat Prachayawarakorn b a School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi126 Pracha u-tid Road, Bangkok 10140, Thailand b Department of Chemical Engineering King Mongkut’s University of Technology Thonburi126 Pracha u-tid Road, Bangkok 10140, Thailand article info Article history: Received 23 November 2008 Received in revised form 1 July 2009 Accepted 6 July 2009 Available online 31 July 2009 Keywords: Adsorption kinetics Banana foam mat Pore diffusivity Pore network abstract The purpose of this research was to determine the diffusion coefficient of moisture in the pores of banana foam mat using stochastic pore network. A 2-D pore network was used to represent the pore voids inside the banana foam sample and the moisture movement inside the individual pore segments was described by Fick’s law. To determine the moisture diffusion coefficient, the adsorption experiments were carried out with standard static method using saturated salt solutions. Two banana foam densities of 0.21 and 0.26 g/cm 3 were used to adsorb the water vapour. The interactions between moisture and pore structure were illustrated using a 3-D pictorial representation of network concentration gradients in spaces with colour representing the moisture content. The network model described the experimental results rela- tively well. The diffusion coefficient of moisture in pores was in order of 10 9 m 2 /s which was nine times higher than the effective diffusion coefficient calculated from the continuum model. The value of mois- ture diffusion coefficient was dependent on the temperature and independent of the foam densities and the relative humidity, except for the diffusivity determined from the condition at higher relative humid- ity of 70%. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Moisture migration during storage is of great importance to food quality, in particular for dried crispy products such as biscuits, ready-to-eat cereals and snacks. An increase in the product mois- ture content, which in turn leads to the loss of crispness, generally occurs by migration of water vapor from the ambient into the product. The rate of moisture adsorption into porous food is gov- erned by the environmental conditions, i.e., temperature and rela- tive humidity. The rate also relies upon the porous structure of the food (Guillard et al., 2003; Roca et al., 2006). Therefore, an ability to model water migration in porous food is of great interest. To date, two different approaches have been developed for studying mois- ture movement in porous foods. The first modeling is based on a continuum model. In this type of model, porous spaces are considered as a continuum, consistent with its appearance on the macroscopic scale. In this case, the effective moisture diffusivity includes in itself all microscopic com- plexities of the porous structure (e.g., sizes of pores and their con- nectedness) as well as mechanisms of mass transfer, which may occur by molecular diffusion or capillary flow, etc. (Efremov and Kudra, 2004; Roca et al., 2006). When a dried porous food is subject to humid air, water vapor transports from the air to the sample surface and is then assumed to diffuse into the internal area of the sample via the pores by assuming that the solid act as an impermeable surface. This adsorption phenomenon is described by the Fick’s second law of diffusion. Under isothermal condition, moisture transport in a porous food with an infinite slab geometry (Chen, 2007) can be described by: @M @t ¼ rðD eff rMÞ ð1Þ where M is the moisture content (kg/kg d.b.), t the time (s), D eff the effective moisture diffusivity (m 2 /s), which can be expressed by: D eff ¼ eD p s ð2Þ where e is the porosity of the material (dimensionless), D p the ac- tual diffusivity in the pore voids (m 2 /s) and s the tortuosity factor (dimensionless). Tortuosity factor accounts for the fact that the pore 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.07.004 * Corresponding author. Tel.: +662 470 8695; fax: +662 470 8663. E-mail address: preeda_list@hotmail.com (P. Prakotmak). Journal of Food Engineering 96 (2010) 119–126 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng