Pascual E. Viollaz

Equilibrium sorption isotherms and thermodynamic properties of starch and gluten

Abstract A modification of the GAB isotherm (Guggenheim–Anderson–De Boer) is proposed in order to correlate the sorption data for water activities higher than 0.9. The proposed isotherm retains the desirable properties of the GAB isotherm, i.e. good fitting in the range of a w between 0.05 and 0.80, and also provides a noticeable improvement in the fitting quality for high values of a w , by introducing a new term with an additional constant. Sorption data for native potato starch and gluten at different temperatures (2°C, 20°C, 40°C and 67°C) were well correlated for the whole a w range. For starch at 67°C, the values of the constants of the proposed equation do not follow the same tendency obtained for lower temperatures, suggesting that the structure of the material could be changed due to the high temperature. The proposed isotherm can be of interest in the area of drying given that there are few isotherms that accurately represent sorption data at different temperatures in the zone of high a w . Also, it can be useful to predict other thermodynamic functions. In addition, a new procedure is proposed to determine the isosteric heat by using a second order polynomial for representing the variation of moisture as a function of temperature at a fixed water activity.

Drying of foods: Evaluation of a drying model

Abstract The drying of potato slabs in air flow is analysed by the simultaneous numerical solution of the differential heat balance and diffusion equations for a shrinking body. A uniform temperature profile in the solid and temperature dependent diffusion coefficient is assumed. Comparison between predicted and experimental mean moisture content and temperature variations in the sample provides a satisfactory agreement for the moisture range from approximately 5.0 to 0.4 kg water/ kg dry solid. For this moisture range it can be assumed that the diffusion coefficient is not a function of moisture content.

Shrinkage effect on drying behavior of potato slabs

Abstract The rate of drying of potato slabs at different initial moisture contents and thicknesses was investigated. The variation of surface area with time was measured in order to consider the degree of shrinking of the samples during drying. Experimental drying curves were interpreted in terms of Ficks law for shrinking bodies and by means of the classical diffusional model, without shrinking. It was found that for relatively short drying time Ficks model with shrinking correlates adequately with the experimental results, with a diffusion coefficient independent of moisture content. Howerver, in terms of the classical diffusional model the experimental data predict a strong dependence of the diffusion coefficient on moisture content. Such dependence was attributed to a shortcoming of the classical ‘Fickian’ model and not to physical reasons.

A mathematical model to predict the temperature of maize kernels during drying

Abstract A model representing the temperature changes in maize grain during drying has been developed. The model has the principal advantage of having an analytical solution, requiring only the drying curve and the physical properties of the grain. The validity of the model was tested experimentally, and the agreement between calculated and experimental temperature profiles was satisfactory.

Heats and Entropies of Sorption of Cereal Grains : A Comparison Between Integral and Differential Quantities

ABSTRACT Several thermodynamic properties for maize, rough rice and wheat has been calculated using desorption isotherms available in the literature. It was obtained an analytical expression to predict the differential heat (isosteric) as function of moisture content using a three parameters equilibrium model, based on enthalpy-entropy compensation effect, which takes into account the effet of temperature. Two integral heats of sorption were calculated, the first from the slopes of the iso-spreading pressure lines and the second from integration of the differential heat. This last one was used to estimate the energy requirement to remove water from initial moisture content to different final moisture levels, which is useful in drying area. Finally it was calculated the differential and integral entropies of sorption as function of moisture content.

Effect of temperature and SO2 on the rates of water absorption of three maize hybrids

Abstract The rates of water absorption of three maizes, dent (PIONEER 3379), semident (ICI RF67) and flint (CARGILL T-42), in plain water and 0.25% SO2 aqueous solution were studied. The steeping temperatures were 45, 55 and 65 °C. The absorption kinetics in plain water and SO2 solution follows a ‘Fickian’ behavior (moisture content of the kernels varies with the square root of time) during the first hours of steeping. The rates of absorption in SO2 solution were slightly higher than in plain water at all temperatures, even though at the onset of steeping, the SO2 seemed to retard the rate of absorption. This increase was relatively higher for flint and semident types than for dent maize. The activation energies for water sorption in SO2 solution were lower than for plain water, this effect being more marked for flint and semident types.

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 drying model for three-dimensional shrinking bodies

Abstract A mathematical model to predict the drying kinetics of shrinking bodies was proposed, assuming unidirectional drying and three-dimensional shrinkage. The differential equation for mass transfer in a finite slab was solved, using boundary conditions of the first kind. The model was numerically solved by finite differences, taking into account a convective term in the mass balance equation, which appears as a consequence of non-unidirectional shrinkage. This additional term significantly changed the value of the predicted diffusion coefficient. Differences when compared with previous solutions are significant for drying of solids with high initial moisture content. Numerical solutions were obtained for absorption as well as for desorption processes. The predicted apparent diffusion coefficients, calculated from the slopes of the semilog drying kinetics curves, were different in both cases.

Analysis of moisture profiles, mass Biot number and driving forces during drying of potato slabs

The drying of shrinking bodies is analysed by numerically solving a drying model in order to obtain a deeper insight into the drying process, calculating moisture profiles and various forms of Biot number. Moisture profiles shows great differences when different units of concentration are used. Calculated Biot numbers do not satisfactorily explain the drying kinetic curves for solids with a high initial moisture content, because the equilibrium curve is not a straight line. Kinetics drying curves are satisfactorily analysed in terms of variation of total driving force with time, which differs markedly from the corresponding variation of the total driving force in terms of temperature, for a pure heat transfer process.

Drying Simulation of a Solid Slab with Three Dimensional Shrinkage

ABSTRACT A mathematical model is developed to simulate the drying of a hygroscopic porous solid. The model, based on the gradient of moisture concentration per unit volume as driving force, takes into account the migration of water within the solid by diffusion and the evaporation at the interface. A mathematical equation for diffusion in a slab with three dimensional shrinkage has been derived, assuming that the magnitude of shrinkage is equal to the volume of water evaporated. The resulting diffusion equation and the heat balance eauation for infinite thermal conductivitv were solved n;merically with temperature dependent diffusion coefficient and convective boundary conditions. The deDendence of the desorption isotherm with temperature is-also considered. corndination of all these factors in a single model provides a tool that is effective in predictinq dryinq behavior and also useful in exploring and understanding the impact of important variables on the drying process.