Papers by Author: M. Vázquez da Silva

Abstract: Osmotic dehydration is a water removal process that consists of placing foods, such as pieces of fruits and vegetables, in a hypertonic solution. Both mass fluxes lead to a decrease of the water activity in the product, increasing its shelf life. It can be used as a single dehydration process or as a pre-treatment of other processes such as drying, pasteurization or freezing. Heat and mass transfer gradients associated to the process produce changes in the chemical, physical and structural characteristics of the vegetable tissue. The knowledge and prediction of these changes are very important because they affect the quality of the final product, process modelling and design of process equipment. This work presents some new experimental data during osmotic dehydration of fruits. Some models were used so as to obtain predicted values during osmotic dehydration.

Abstract: In this work, consideration is given to the problem of dissolution of a buried solid sphere in the liquid flowing uniformly through the packed bed around it. The differential equations describing fluid flow and mass transfer by advection and diffusion in the interstices of the bed are presented and the method for obtaining their numerical solution is indicated.From the surface concentration fields, given by the numerical solution, the concentration boundary layer thickness as a function of the relevant parameters were undertaken. Mathematical expressions that relate the dependence with the Peclet number and d/d₁ ratio of an immersed active sphere are proposed to describe the approximate size of the concentration boundary layer thickness.

Abstract: The transport phenomenon of mass transfers between a moving fluid and a reacting sphere buried in a packed bed, with “uniform velocity”, was analysed numerically, for solute transport by both advection and diffusion to obtain the concentration field and, from it, the dimensionless concentration boundary layer thickness, , for , and . The bed of inert particles is taken to have uniform voidage. For this purpose, numerical solutions of the partial differential equations describing mass concentration of the solute were undertaken to obtain the concentration boundary layer thickness as a function of the relevant parameters. Finally, mathematical expressions that relate the dependence with the Peclet number and inert particle diameter are proposed to describe the approximate size of the concentration boundary layer thickness.

Abstract: In the present work, three different mathematical models that can be applied to simulate the supercritical fluid extraction from herbaceous matrices were studied: Single-Sphere Model I (SS I), Single-Sphere Model II (SS II) and Characteristic Time Model (CTM). It was tested and compared the agreement between models results and available experimental data reported in literature.

Abstract: This paper describes the application of the cylindrical pore model to the study of water vapour adsorption in piles of spherical particles packed in cylindrical containers. The differential mass balance equations were solved numerically using a finite-difference method and the numerical solutions obtained were in good agreement with the experimental data. The experiments were carried out through the measurement of water vapour mass uptake for different piles of silica gel and popcorns (breakfast cereals).

Abstract: In this work the water vapour adsorption was studied, during the initial states of absorption, for different piles of granules held in a cylindrical container. A diffusion model was presented for the description of the physical phenomenon whereby water vapour penetrates along the piles of cylindrical pores. The differential mass balance equation was solved numerically using a finite-difference method. The numerical solutions were tested through the measurement of water vapour mass uptake for different piles of silica gel and breakfast cereals (popcorns), being observed that they are in good agreement with the experimental values.

Abstract: The physical and structural properties of cold-set whey protein isolate gels are largely influenced by the protein concentration and the denaturation conditions, namely temperature and holding time. In this work, we systematically varied the protein concentration, the temperature and holding time of denaturation in order to screen their impact on the resulting heat denatured whey protein isolate (HD-WPI) solution viscosity and gel elasticity. The gelation of the HD-WPI solutions was induced, at room temperature, through the addition of magnesium chloride. Based on the assumption that solution turbidity is associated with light scattered by protein aggregates, an aggregate concentration was computed for the HD-WPI solutions. For all experimental conditions, HD-WPI solution viscosities and gels Young modulus data fall, respectively, on two single curves when plotted against the computed aggregates concentration. Three concentration regimes corresponding to non gelling solutions, weak gels and strong gels could be identified. In this study was verified that cold-set gels produced upon addition of Mg2+ had a large spectrum of elastic properties.

Abstract: In this paper a mathematical model for prediction of moisture gain over storage time for binary mixtures of moisture sensitive products was developed, which considered both the moisture transfer through the polymeric film to the headspace and from the headspace to the product. Moisture content within the product was assumed to be uniform for a given time and to relate to the headspace humidity by the GAB sorption isotherm. The transient period of these experiments was used to estimate the mass transfer coefficients. The model was validated by monitoring the moisture take up by breakfast cereal and chocolate powder mixture in different materials, oriented polypropylene (OPP) and low density polythelene (LDPE), during storage at 25°C and 75% relative humidity (RH). The model showed that for mixes some bias was clear, particularly for the cereal, probably owing to interactions between the products. If the resistance of the product to moisture uptake were not taken into consideration, the model would over-predict the experimental results by up to 20%.

Abstract: This work present a model for prediction of moisture gain over storage time for individually packaged, which considered both the moisture transfer through the polymeric film to the headspace and from the headspace to the product. The transport of moisture between the headspace and each product was assumed to be controlled by external resistance. No interaction between the packaged components was considered. Moisture content within the product was assumed to be uniform for a given time and to relate to the headspace humidity by the GAB sorption isotherm. The isotherm parameters were determined by equilibrating samples at different relative humidity. The transient period of these experiments was used to estimate the mass transfer coefficients. The model was validated by monitoring the moisture take up by breakfast cereal and chocolate powder packed individually in different materials, oriented polypropylene (OPP) and low density polythelene (LDPE), during storage at 25°C and 75% RH (relative humidity). The model provided very good fits for the products packaged individually (with r2 above 0.99).

Abstract: This paper describes a simple experimental technique, easy to set-up in a laboratory, for the measurement of solute solubility in liquids (or gases). Experimental values of solubility were determined for the dissolution of benzoic acid in water and salicylic acid in water, at temperatures between 5°C and 70°C. The solubility experimental values obtained are in good agreement with the theoretical values of solubility and the empirical correlations presented in literature. The results show that it is possible to obtain good results for solubility values, using a simple and inexpensive experimental technique.