Caracterização física de sementes de feijão-caupi durante o processo de secagem

Abstract

Information obtained through determined physical properties and drying curves in thin layer are important for studies of mass and heat transference and of air moving in grain masses, silos measurement, post-harvest equipments project and improvement, as well as for analyzing commercial viability of drying systems. In this sense, the aim of the present study was to determine the variation of moisture content and physical characteristics of cowpea seeds during drying process. The study was conducted at the Laboratory of Pre-Processing and Agricultural Products Storage, belonging to the Faculty of Agricultural Sciences, Federal University of Grande Dourados, Dourados- MS, the second half of 2015. Cowpea seeds, BRS Guariba cultivar, harvested with a moisture content of, approximately, 0.47 decimal d.b. (dry basis), were used. For the analysis of physical properties, seeds were submitted to drying in a stove with forced air circulation, at a temperature of 40°C, until reach moisture content of, about, 0.11 decimal d.b. During moisture content reduction of product, bulk and true density, porosity, thousand-seed weight, characteristic dimensions, geometric diameter, unitary volume, circularity, sphericity, surface and projected area, surface/volume ratio and surface/mass ratio, as well as volumetric shrinkage of mass and unitary were evaluated. The study of drying kinetics was realized in an experimental drier of fixed-bed, using the combination among five temperatures (40, 50, 60, 70 and 80°C) and three drying air velocities (0.4, 1.0 and 1.6 m s-1). The product was dried until reach moisture content of, approximately, 0,103 decimal d.b. Mathematical models traditionally used to represent drying of agricultural products were adjusted to experimental data of moisture ratio. Also, water reduction rate, effective diffusion coefficient, activation energy and main thermodynamic properties were determined. Results obtained allow concluding that all physical properties of cowpea seeds are influenced by its moisture content reduction. Also, it concluded that, among others, the Page model satisfactorily represents the drying curves of cowpea seeds, which are more influenced by temperature than drying air velocity. Elevation of temperature and drying air velocity results in increased of the water reduction rate and in the values of effective diffusion coefficient. Drying air velocity elevation promotes increases in the magnitudes of activation energy. Thermodynamic properties, enthalpy and entropy specific and Gibbs free energy are influenced by variations of temperature and drying air velocity.