THERMODYNAMIC PROPERTIES OF WHITE BEAN GRAINS FOR DIFFERENT EQUILIBRIUM WATER CONTENTS

Abstract

ABSTRACT In drying processes and in the storage of agricultural products, the study of thermodynamic properties is indispensable in the search for solutions to issues of stability and optimization of the conditions of agro-industrial processes. The aim of this work was to determine hygroscopic equilibrium isotherms and study the thermodynamic properties of white bean grains submitted to different equilibrium temperatures and relative humidity levels. Phaseolus vulgaris L. grains were used, with an initial water content of 0.142 (dry basis), temperature variations ranging between 18 and 50 °C, and relative humidity between 19% and 78%. Fifteen mathematical models were tested to determine the hygroscopic equilibrium isotherms, and the Sigma model was the one that best suited the experimental data. The thermodynamic properties of the beans were determined from Sigma-Copace model. The integral isosteric heat of desorption and the differential entropy increased with the reduction of the water content, and the phenomenon of water desorption of white bean grains was a non-spontaneous process controlled by enthalpy.

Keywords

• hygroscopic balance
• isosteric heat
• enthalpy
• entropy
• Gibbs free energy