M. Vázquez da Silva

Impact of MG2+ and tara gum concentrations on flow and textural properties of WPI solutions and cold-set gels.

Cold-set gels of whey protein isolate (WPI) and of WPI plus tara gum (TG) were produced through the addition of magnesium chloride to heat-denatured (HD) WPI and WPI+TG solutions. The flow behaviour of the WPI, TG, and WPI+TG samples was analyzed: WPI solutions exhibit Newtonian behavior while the behavior of the TG and WPI+TG solutions can be described through the Cross and Carreau models. The mechanical characterization of the cold-set gels was done through puncture tests, and the Youngs modulus for each cold-set gel was obtained. Statistical analysis of the mechanical data was made according to the Marquadt-Levenberg algorithm.

Cold-Set Whey Protein Isolate Gels Prepared with Mg2+: Denaturing Conditions and Phase Diagram

Cold-set gels of whey protein isolate (WPI) were produced. The cold gelation was induced through the addition of magnesium chloride whose cation is an alternative to the most common cations used in this kind of gelation. A phase diagram for denatured WPI in magnesium was determined by varying protein and salt concentrations. In this diagram the protein and cation concentrations varied from 0.3 to 8.0 wt % and 1 to 75 mM, respectively. The viscoelastic behaviour of the WPI was analysed, whereas gels elastic properties were measured by means of texture tests. Mathematical manipulation of the experimental data relative to the puncture tests was made in order to obtain the Young’s modulus for each cold-set gel.

Dynamic and Equilibrium Swelling of Biodegradable Starch-Based Superabsorbent Polymers

A series of biodegradable superabsorbent polymers (SAP’s) based on starch grafted with acrylic acid (AA) and crosslinked with N,N’-methylenebisacrylamide (MBIS) were obtained. The parameters that define SAP’s properties, such as starch type, amount of initiator, acrylic acid and crosslinker concentrations, and degree of neutralization were varied. The dynamic swelling was interpreted in terms of both a simple power of time equation and a more detailed model based on a coupled diffusion–relaxation mechanism. The diffusivity coefficients calculated from diffusion–relaxation model were compared with the microimages obtained by optical microscopic.

Water Sorption Isotherms and Textural Properties of Biodegradable Starch-based Superabsorbent Polymers

A series of biodegradable superabsorbent polymers (SAP) based on starch grafted with acrylic acid (AA) and crosslinked with N,N’-methylenebisacrylamide (MBIS) were obtained. The parameters that define SAPs properties, such as starch type, amount of initiator, acrylic acid and crosslinker concentrations, and degree of neutralization were varied according to an experimental design. The swelling rate, the degree of swelling, the water sorption isotherms and the texture profile of the new materials were determined. A new method was used to measure the swelling rate. The water sorption results were correlated using various isothermal models as the BET (Brunauer-Emmett-Teller) and GAB (Guggenheim-Andersson-DeBoer) models. Hydrogels obtained from regular cornstarch presented better absorption capacities than those from waxy cornstarch. Maximum swelling was observed for 70% neutralization of carboxylic acid groups. More rigid SAPs presented lower swelling rates, but not necessarily lower swelling degrees.

Flow Past a Sphere Buried in a Porous Media: Concentration Boundary Layer Thickness

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.

Numerical Analysis of Mass Transfer Around a Sphere Buried in Porous Media: Concentration Contours and Boundary Layer Thickness

The mass transfer process between a moving fluid and a slightly soluble or fast reacting sphere buried in a packed bed, with “uniform velocity”, was obtained numerically, for solute transport by both advection and diffusion. Fluid flow in the granular bed around the sphere was assumed to follow Darcy’s law and the elliptic Partial Differential Equations (PDE), resulting from a differential material balance on the solute in an elementary control volume, was studied and solved numerically over the “whole range” of values of the relevant parameters (Peclet number and Schmidt number). The numerical solutions gave the concentration contour plots and concentration boundary layer thickness as a function of the relevant parameters. For each concentration level, the width and downstream length of the corresponding contour surface were determined. General expressions are presented to predict contaminant “plume” size downstream of the polluting source. An important feature of this work is the detailed discussion of the finite differences method adopted, with emphasis on the High-Resolution Schemes (HRS) used in the discretization of the convection term of the PDE.

Performance and Modelling of Water Vapour Adsorption in Piles of Granules Using a Cylindrical Pore Model

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).

A Mathematical Model to Predict Moisture Uptake of Dry Products Packaged Individually in Flexible Films

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).

Experimental Values of Solubility of Organic Compounds in Water for a Wide Range of Temperature Values. A New Experimental Technique

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.

High-solid systems of partially hydrolysed waxy maize starch: rheological and structural characterisations

The rheological behaviour of some high-solid mixed dispersions of partially hydrolysed waxy maize starch (HWS), in the presence of whey protein concentrate (WPC) and urea, was followed by small-amplitude oscillatory measurements in the linear domain, under heating to 80 °C and cooling to 25 °C, at pH 7.5. The non-gelling HWS sample was obtained by sonication of previously gelatinised waxy maize starch. Steady-shear flow and microstructure were investigated after cooling to 25 °C. Dispersions were prepared at 1:1 (m/m) WPC/urea ratio, with increasing HWS concentrations. Typical solid- and liquid-like rheological behaviours were observed after quenching the mixtures to low temperature, depending on HWS/WPC ratio. At the lowest HWS/WPC ratio, a thixotropic structure was characterised under steady-shear flow, and visualised by light microscopy at low magnification as a continuous network. With increasing HWS/WPC ratio within a certain range, gelling and non-gelling mixtures resulted in phase-separated structures, which hindered viscous flow. At the highest HWS/WPC ratio, a shear-thinning dispersion was formed, consistent with the liquid-like small deformation properties and the homogeneous image obtained by light microscopy.