Roberto J. Aguerre

Swelling and pore structure in starchy materials

Abstract The specific surface areas of different starchy materials were determined by means of the t-method, their values being practically constant in the range of water activities 0·10–0·80. The straightness of the v 1 -t plots observed in all cases indicated the absence of an intrinsic microporous structure. The lack of a pore structure and the high values of the specific surface areas are explained on the basis of the interlamellar expansion of the starch granule structures on uptake of water.

A DIFFUSION MODEL WITH A MOISTURE DEPENDENT DIFFUSION COEFFICIENT FOR PARBOILED RICE

A mathematical model describing moisture migration by diffusion in a solid sphere with variable diffusion coefficient is proposed. An analytical expression for dependence of the diffusion coefficient with moisture content was derived based on the assumption that the activation energy for diffusion varies linearly with the desorption energy. The expression for moisture dependence of diffusion coefficient was used to simulate drying of parboiled rice in the temperature range 50–90°C. The mathematical model shows good agreement between observed and predicted drying rate curves.

A fractal isotherm for multilayer adsorption in foods

Abstract The effect of the fractal surface on physical adsorption has been analysed and a new three-parametric isotherm equation was derived. The model describes the multilayer adsorption on fractal surfaces within the framework of BET theory. It predicts lower buildup of the multilayer coverage on a fractal surface than on a flat surface. The results of the simulation show that increasing the fractal dimension of the surface affects the adsorption isotherms; namely, the upward curvature of the isotherms at high relative pressures is decreased as D increases. For the submonolayer region, the curves become independent of the fractal nature of the absorbent. The isothermal data for water vapor sorption on grains and other starchy materials were used to evaluate the new isotherm equation. The parameters of the isotherms were calculated using computational regression techniques. Fractal dimension ranged from 2.6 to 2.9, approximately, indicating the contribution of the pore structure to the sorption capacity of the products investigated. Results show that the model allows an accurate and simple description of the sorption data for the whole range of water activity.

Modeling Cereal Grain Drying with Variable Diffusivity

ABSTRACT The thin-layer drying or rough rice, maize and sorghum at several temperatures was investigated. Simulation of the experimental data was performed by solving numerically the diffusion equation with variable diffusion coefficient for spherical-shaped grain. To account for this variation, an analytical expression for the dependence of the diffusion coefficient with moisture content was obtained based on considerations of the energy needed to dissociate water molecules from their sites and of resistance to diffusion. When the expression for diffusivity was used for predicting drying behavior of the grains, a good agreement was obtained between experimental data and the theoretical prediction.

Numerical analysis of the diffusion equation for composite solids: Application to grain drying

Abstract The drying of a hydroscopic porous solid consisting of two concentric spheres was simulated by means of Ficks second law of diffusion. The resulting mathematical equation and boundary conditions were numerically solved and the numerical solution was used to simulate the drying of two concentric spheres with different shell thicknesses and relative resistances to moisture flow. The model was used to simulate the drying process of corn. From the comparison between theoretically predicted and observed drying curves it was found that the kernel-to-pericarp diffusivities ratio is about 0·1.

DRYING SIMULATION OF CORN WITH TEMPERING

Abstract This work presents a dirTusion model and experimental investigation or com drying with tempering. The model solves numerically the dirTusion equation for a hygroscopic porous solid consisting of two concentric spheres (starchy endosperm and pericarp) with different shell thickness and relative resistances to moisture flow. Multipass drying was simulated to evaluate the effect of tempering on drying process. Predicted moisture profiles within the kernel were used to evaluate the actual time for complete tempering or tempering index. The effect of the initial moisture for tempering and the number of drying passes on the net drying time were analyzed. A mathematical expression was formulated to estimate the tempering index under different tempering conditions.

MULTILAYER ADSORPTION ON FRACTAL SURFACES: EFFECT OF THE ADSORBATE-ADSORBATE INTERACTIONS ON THE SORPTIONAL EQUILIBRIUM

ABSTRACT An analytical isotherm equation that can be applied to water vapor-biopolymer systems, was developed. The model describes the multilayer adsorption on fractal surfaces with energetic interactions adsorbate-adsorbate (measure in terms of free energy) different from that of bulk water. Assuming explicit mathematical functionalities for the variation of the free energy with the number of adsorbed layers, it is possible to evaluate the effect of the adsorbate-adsorbate interactions on the adsorption capacity of solids of high degree of surface irregularity. For those surfaces with relatively low degree of irregularity (D values slightly larger than 2) it results that the free energy variation with the layer number in the multilayer region affects considerably the sorption capacity of the adsorbent, even for water activities lower than those corresponding to the monolayer moisture content. This effect becomes less marked as the fractal dimension increases (D approaching to 3), being relevant for water...

Unidimensional formulation of fick's second law for prismatic bodies

Abstract Using the concept of similarity numbers, the analytical solution of Ficks second law for diffusion in sphere was conveniently modified to describe a three-dimensional diffusion process. A method to calculate the similarity numbers for prismatic bodies was developed. Concentration values calculated by means of the proposed equation were compared with the corresponding to the analytical solution for prism with satisfactory results.