Elsevier

Food and Bioproducts Processing

Volume 98, April 2016, Pages 161-172
Food and Bioproducts Processing

Anisotropic diffusion during osmotic dehydration of celery stalks in salt solution

https://doi.org/10.1016/j.fbp.2016.01.005Get rights and content

Highlights

  • Experimental investigation of anisotropic mass transfer during osmotic dehydration of celery.

  • Mathematical modeling of the process, considering two geometries: cubic and cylindrical.

  • Estimation of equilibrium moisture loss and solid gain using a two parameter method.

  • Estimation of moisture and salt effective diffusivities in x, y, z and r directions in the celery.

  • Developing correlations for equilibrium moisture loss and solid gain and effective moisture and salt effective diffusivities in celery.

Abstract

In this study, mass transfer during osmotic dehydration of bulk of celery stalks in NaCl salt solution was investigated. Experiments were carried out in the three initial solution concentrations of 10%, 18% and 25% (w/w) and at the three temperatures of 35 °C, 45 °C and 55 °C. The fruit to solution volume ratios were considered 1:3. Due to the asymmetric structure of the celery stalks, two different geometries of cylindrical and cubical were considered for mathematical modeling of the dehydration process. A two-parameter model was used to evaluate the equilibrium values of moisture loss and solute gain by the samples. Water and salt effective diffusivities were obtained using the first six terms of the series solution of analytical solution of Fick's second law in the cubical and cylindrical geometries. Two different groups of celery stalks were used for estimation of moisture and salt effective diffusivities in different directions. Predictions of the mathematical model in both geometries were in agreement with the experimental data. The water and salt effective diffusivities in z direction (ranged from 0.972 × 10−9 to 3.663 × 10−9 (m2/s)), were always much higher than those in x, y and r directions (ranged from 1.031 × 10−10 to 6.919 × 10−10 (m2/s)). The values of water and salt effective diffusivities in z direction were close to each other in both geometries. The water and salt effective diffusivities were increased with increasing the initial solution concentrations and temperatures.

Introduction

Osmotic dehydration is widely used for partial removal of water from plant tissues by immersion in a hypertonic (osmotic) solution. The difference between the osmotic pressure in the material and solution gives rise to simultaneous counter-current water diffusion from the food to the solution and solute diffusion into the food. Osmotic dehydration has many advantages such as: improving the organoleptic food properties such as color, flavor, or aroma and texture; increasing the product storage time; energy saving compared to other drying techniques; simple process equipment and reducing the processing time due to the absence of phase change. This process is considered to be as a pre-process for conventional drying systems (Lewicki and Lenart, 2015).

The rate of mass transfer in osmotic dehydration is affected by concentration and temperature of the osmotic solution (Herman Lara et al., 2013, Rastogi and Raghavarao, 1994, Rastogi and Raghavarao, 1997), agitation (Mavroudis et al., 1998), food to osmotic solution volume ratio (Da Conceicao Silva et al., 2012), food structure (porosity, etc.), shape and size (Ruiz Lopez et al., 2010, Sirousazar et al., 2009, Van Nieuwenhuijzen et al., 2001), nature and molecular weight of the osmotic solute (Lenart and Flink, 1984, Tsamoa et al., 2005) and pressure (Fito, 1994, Rastogi and Niranjan, 1998, Rastogi and Raghavarao, 1996). Fickian diffusion model is usually used for modeling of mass transfer during drying and osmotic dehydration of solids. In some researches, the dehydration process was carried out in high solution to material volume ratio in one dimensional (Abbasi-Souraki et al., 2012, Abbasi-Souraki et al., 2013a, Abbasi-Souraki et al., 2013b) and multidimensional systems (Abbasi-Souraki and Mowla, 2008, Da Silva et al., 2014, Ruiz Lopez et al., 2010). Some researches were also carried out in low solution to material volume ratio and thus varying solution concentration in one dimensional (Singh et al., 2007, Telis et al., 2003) and multidimensional systems (Khin et al., 2006, Ruiz Lopez et al., 2010). Solutions of the Fickian diffusion equation with different boundary conditions have been developed, comprehensively, by Crank (1975).

In some agricultural products, internal textures could be different from the longitudinal direction compared with the lateral direction of growth (Fernando et al., 2011), and considering the same properties for all directions of the product (isotropic structure), could potentially lead to misleading predictions. In this regard, different methods have been used by various authors for taking into account the anisotropic nature of the food products. Rossello et al. (1997) and Abbasi-Souraki and Mowla (2008) studied the drying behavior of cylindrical green beans. In their research, the anisotropic structure of green bean samples has been taken account, by defining different diffusivities in the radial and axial directions. Fernando et al. (2011) estimated the axial and radial moisture diffusivities of anisotropic banana, cassava and pumpkin by using the drying curves of samples with different thicknesses. Zhang et al. (2011) calculated the salt diffusivities, in x and y directions, for anisotropic grass carp muscles by measuring the salt content of muscles in different points. Pacheco Aguirre et al. (2014) estimated the axial, radial and angular effective diffusivities of anisotropic cylindrical carrot samples. At first, the axial and radial diffusivities were obtained using the effect of increasing the length of samples on the drying curves, and then the angular diffusivity was obtained using the drying curves of samples with equal length and different cut angles.

The objectives of this research were to study the dehydration behavior of celery stalks during osmotic dehydration in salt solution and developing a mathematical model for explaining the anisotropic mass transfer in this foodstuff during dehydration. Experiments were carried out by dehydration of different cuts of celery stalks in salt solutions of different concentrations and temperatures in a batch osmo-reactor. Due to the asymmetric structure of the celery stalks, two types of geometries, cylindrical and cubical, were considered for mathematical modeling of the dehydration process. By fitting the experimental data to the models, effective diffusivities in different directions were estimated.

Section snippets

Materials

In this work, fresh and green celery from Rasht, situated in the north of Iran, was used as raw material. Celeries were purchased from the same supplier to maximize reproducibility of results. Because of homogeneity of the samples, care was taken when selecting the samples, to take celeries with approximately the same shape, color and degree of ripening. Ends and leafy tops of bunches of celeries were removed and only the middle portions, were used to produce the required samples. Then celeries

Mathematical modeling

A number of celery stalks are immersed in a limited volume of solution, and the solution is well stirred. If the volume of the solution in which the bodies are immersed is not large enough to neglect the amount of solute taken up or moisture extracted from the solid, then the concentration in the liquid will change as diffusion proceeds. By stirring the solution well, time would be the only parameter affecting the concentration of the solute in the fluid. The unsteady-state one-dimensional mass

Estimation of the equilibrium water loss and solid gain

Equilibrium values of water loss (WL) and solid gain (SG) were predicted by fitting the linear form of Azuara model to the experimental data, according to Eqs. (22), (23), respectively. Table 2, Table 3 show the equilibrium water loss (WL) and solid gain (SG) values as well as the regression coefficients R2 and RMSE for two cuts of celery stalks at nine combinations of three concentrations and temperatures. The high values of regression coefficient and low values of RMSE confirm the

Conclusions

Mass transfer during osmotic dehydration of celery stalks in a batch osmo-reactor was investigated. The two-parameter model developed by Azuara et al. (1992) was used to evaluate the equilibrium values of moisture loss and solute gain by the samples. Due to the asymmetric structures of celery stalks, cubical and cylindrical geometries were selected for mathematical modeling of mass transfer during osmotic dehydration of the samples. Effective diffusivities in x or y direction in the cubic

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