Moisture sorption isotherms and thermodynamic properties of dried litchi pulp

Abstract: In order to reveal the complex relationship among the moisture content, the water activity and the temperature, and to provide technical data for preservation of dried 'Wuye' litchi (Litchi Chinensis Sonn. cv. Wuye), the static gravimetric method was used to determine moisture sorption isotherms of dried 'Wuye' litchi pulp at three temperatures (20, 30, and 40°C) over a range of water activities from 0.112 to 0.976 based on the adsorption theory. The experimental data of dried 'Wuye' litchi pulp were fitted by eight models (BET, Halsey, Henderson, Peleg, Smith, GAB, Modified Henderson, Oswin) available in the literature. The optimal model used to describe the sorption isotherms of dried 'Wuye' litchi pulp was determined by comparing the models' coefficient of determination (R2) and root mean square error. Thermodynamic properties such as net isosteric heat, enthalpy changes, entropy changes, and free energy that provide a deeper understanding of the properties of water and energy requirements associated with sorption process were determined from moisture adsorption isotherm data of dried 'Wuye' litchi pulp at different temperatures. The results showed that the moisture sorption isotherms of dried 'Wuye' litchi pulp exhibited type Ⅲ sigmoid shape. At low and intermediate water activity, the equilibrium moisture content increased slowly with water activity (0.1-0.4), while at high water activity (>0.4) equilibrium moisture content increased rapidly with water activity. The equilibrium moisture content decreased with the increasing temperature at constant water activity. The Peleg model was found to be the best for describing the relationship between the moisture content, the water activity and temperature. The agreement between experiment and predicted values of this model was excellent (R2 ranging from 0.9950-0.9979，root mean square error ranging from 1.9431-2.7102). The thermodynamic properties of dried 'Wuye' litchi pulp showed that net isosteric heat of adsorption decreased with an increasing in moisture constant, and approached to zero at high moisture content. enthalpy changes had the same values as the net isosteric heat ranged from 0.95 to 186.98 kJ/mol. entropy changes values ranged from 1.11 to 587.24 J/(mol·K) at moisture levels ranging from 8%-50% for dried 'Wuye' litchi pulp. These values decreased as moisture content increased and did not show temperature dependence. Free energy ranged from 0.60 to 18.58 kJ/mol for dried 'Wuye' litchi pulp. Free energy became smaller as the moisture content and the temperature increased. The plot of the enthalpy versus entropy showed a linear relationship for dried 'Wuye' litchi pulp with R2 equal to 0.9999, which indicated the existence of compensation. The values of isokinetic temperature and harmonic mean temperature were 319.34 K and 302.93 K respectively. Therefore, the enthalpy-entropy compensation theory was suitable for water adsorption of dried 'Wuye' litchi pulp because isokinetic temperature was significantly different from the value of harmonic mean temperature. Since isokinetic temperature > harmonic mean temperature, the mechanism of the adsorption process of dried 'Wuye' litchi pulp was enthalpy driven. The results provide a theoretical basis for the processing and storage stability of dried 'Wuye' litchi pulp.