Desorption isotherms and thermodynamic properties of prickly pear seeds

Abstract

The objective of this work was to determine the desorption isotherms and thermodynamic properties of prickly pear seeds. These properties are indispensable, particularly in regard to seed drying and storage in order to guarantee its quality prior to its industrial use as an important source of essential oil and meal for medicinal and cosmetic purposes. Desorption isotherms were determined using a static gravimetric method at three temperatures: 40°C, 60°C and 70°C over a relative humidity range of 5–90%. All the curves exhibited type III behavior, according to Brunauer’s classification. Equilibrium moisture contents decreased as the temperature increased. The GAB model fitted well the desorption isotherm data for prickly pear seeds with the monolayer moisture content depending on the temperature and varying between 0.433 and 0.032kg water/kg dry matter. The Clausius–Clapeyron equation was used to evaluate the net isosteric heat of water desorption by using the GAB theoretical model in the studied temperature range. The net isosteric heat and the differential entropy decreased strongly as the moisture content increased, varying from 0.585 to 20.88kJ/mol and from 0.391 to 56.70J/mol/K, respectively, and could be well adjusted by an empirical exponential relationship. The enthalpy–entropy theory proved to be valid since the relationship between enthalpy and entropy was linear. The value of Gibbs free energy was found to be positive and the harmonic temperature (329.18K) differed from the isokinetic temperature (361.32K). Through the entropy–enthalpy theory, it could be concluded that the water desorption isotherm of prickly pear seeds is a non-spontaneous and enthalpy-controlled process.