Constantino Suarez

Analysis of the sorptional characteristics of amaranth starch

The water vapor adsorption isotherms of amaranth starch were determined for the temperature range 25‐50∞C and water activity 0.04‐0.90. Comparison with adsorption isotherms of wheat and corn native starches showed that the three starches have similar adsorption capacities at low aw values. Amaranth starch showed higher sorption capacity than corn and wheat starches at high aw values. Two BET-like isotherm equations were used to fit the equilibrium data: the GAB isotherm and a fractal isotherm. Although the best fit was obtained with fractal isotherm, the values of C and mm obtained from both equations were practically coincident. The fractal dimension D was in average 2.77 revealing the high degree of surface roughness of the starch. The eAect of temperature on the equilibrium data was satisfactorily modeled by means of a simple two-parameter equation, covering the whole aw range investigated. An analytical expression was used to estimate the variation of the isosteric heat with moisture. The isosteric heat reached its maximum value of 30 kJ/mol at moisture levels of about 5% (db). ” 1999 Elsevier Science Ltd. All rights reserved.

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.

Modeling Starch Gelatinization Kinetics of Milled Rice Flour

ABSTRACT Milled long-grain rice samples were evaluated by differential scanning calorimetry (DSC) to determine the kinetics of starch gelatinization. The experiments were conducted with milled rice flour with a 10.6% degree of milling. DSC thermograms were obtained from 35 to 110°C using heating rates between 1°C/min and 15°C/min. The rate constants were evaluated, and two activation energies were found for different temperature ranges. At <70.1°C gelatinization was not completed. It was assumed that at <70.1°C gelatinization would only affect the amorphous regions. During the subsequent phase the crystalline regions destabilized by the amorphous component begin to gelatinize. For moisture content of 70%, wet basis, and a heating rate of 12°C/min, the enthalpy of gelatinization reaches a constant value of 7.3 J/g.

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.

Corn steeping: influence of time and lactic acid on isolation and thermal properties of starch

Abstract A semi-dent corn hybrid was laboratory batch-steeped at 52°C in a sulfur dioxide solution (2500 ppm) and sulfur dioxide solution with lactic acid (2500 ppm SO2+0.5% v/v lactic acid) for various time intervals. Starch was laboratory wet milled and gelatinization characteristics were analyzed by differential scanning calorimeter (DSC). Starch isolated from corn steeped in SO2 solution alone had higher peak temperatures and narrower gelatinization range as the steeping time increased. This demonstrated that the wet milling process anneals corn starch during the isolation procedure. Such an effect was less pronounced when starch was milled from corn steeped with lactic acid. Higher starch yields were obtained from corn steeped in sulfur dioxide solution with lactic acid.

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.