Cinética de secagem, higroscopicidade e propriedades físicas dos grãos de niger (Guizotia abyssinica (L.f.) Cass.)

Abstract

Agricultural products drying is a fundamental process in post-harvest quality maintenance. The knowledge of the hygroscopic balance and the physical properties, assists in the improvement as in the appropriate dimensioning of processes and equipments used in grain aeration and handling, optimization, control and operation of equipments used in post-harvest stages. The aim of this work was to evaluate and modeling the physical properties, hygroscopy and drying curves of niger grains. To conduct the experiment were used grains of niger (Guizotia abyssinica (L.f.) Cass.), cultivated in Dourados/MS. In order to obtain thin-layer drying curves, the Niger grains were dried in forced air circulation in a fixed-layer experimental dryer in temperatures of 40, 50, 60, 70 and 80 °C. The grains were dried until they reached water content of approximately 0.086 decimal dry basis (d.b). Were adjusted eight mathematical models, frequently used to represent agricultural products drying, to the experimental data of moisture ratio. Also, were determined the effective diffusion coefficient, and the activation energy. To obtain the moisture content balance of niger grains by desorption, the static method was employed under controlled temperature conditions (30, 35 and 40 °C) and relative humidity between 7 and 78.9%. The initial moisture content of the samples were 0.56 decimal (d.b), obtained periodically until reached the moisture content balance, under these conditions. Traditional mathematical models used to describe the hygroscopicity of agricultural products were adjusted to the experimental data. Physical properties were determined for different moisture contents during the drying of: 0.52, 0.43, 0.32, 0.24, 0.16 and 0.09 decimal (d.b). The grains were submitted to oven-drying with forced air circulation, the temperature of 40 °C. During the reduction of moisture content, bulk density, true density, and porosity, as the mass and unit volumetric shrinkage of the product were evaluated. Based of the results found, it is possible to conclude that: the models of Page, Diffusion Approximation and Two Terms are suitable for the representation of the kinetics of thin layer drying of niger; The effective diffusion coefficient increase with the elevation of temperature; In order to evaluate the desorption of niger grains under different temperature conditions and water activities, the models of Chung and Pfost, modified Henderson, Oswin, Sabbah, Sigma Copace, Halsey, Smith and Copace were satisfactory. The physical properties of the niger grains were influenced by the reduction of moisture content mainly the bulk density and the weight volumetric shrinkage.