Moisture Sorption Isotherm and Isosteric Heat of Sorption of Four-finger Threadfin (Eleutheronema tetradactylum)

Equilibrium moisture content (EMC) is a crucial factor in the design of drying systems and fish storage, particularly in traditional solar drying methods used by rural households and community enterprise groups. The sorption isotherms of four-finger threadfin were determined using the conventional static gravimetric method at temperatures of 40, 50, 60, 70, and 80 °C with relative humidity (RH) ranging from 11.1 ± 0.4 to 95.4 ± 0.6%. The isotherms were fitted to twenty-two different equations, including Adam-Shove, BET, Cauri, Chung-Pfost, Freundlich, GAB, Halsey, Henderson, Henderson-Thompson, Kuhn, Oswin, Peleg, Smith, modified BET, modified Chung-Pfost, modified GAB, modified Halsey, modified Henderson, modified Mujica, modified Oswin, modified Ratti, and modified Smith. The results indicated that EMC decreased when the temperature increased at constant RH. Model performance was assessed using various statistical indices and residual plots. The experimental sorption data, which exhibited a Type III isotherm, were best described by the modified Ratti model for both desorption and adsorption isotherms at 40, 50, 60, and 70 °C. However, at 80 °C, the Peleg model provided the best fit for both desorption and adsorption isotherms. Furthermore, the net isosteric heat of sorption (Q_stn) for four-finger threadfin decreases exponentially with increasing EMC, ranging from 263.1–306.7 kJ/mol. Adsorption values were found to be higher than desorption values, as calculated using the Clausius–Clapeyron equation.