Pistachio nut is one of the most important agricultural exporting commodities in Iran that is recognized as the best in the world in terms of taste and flavour. It is important to improve pistachio processing, drying, storing and packaging, to further win the trust of foreign consumers. Drying of pistachio is the most important stage in its processing chain. The equilibrium moisture content (EMC), that is defined as the moisture content of a hygroscopic material in equilibrium with a particular environmental condition (temperature and relative humidity), is a vital parameter in studying the drying process. In the present study the equilibrium moisture contents were determined for three important caltivars of Iranian pistachio (Ohadi, Kalleghoochi and Abdollahi) using gravimetric method at 25, 37 and 50°C over a range of relative humidities from 0.11 to 0.85 at four replications. Experiments were conducted using the thin-layer drying method in the controlled and sealed environmental chambers designed and fabricated for this purpose. Thermodynamical conditions inside the chambers were kept constant. The temperature was thermostatically controlled and relative humidity of the air surrounding the samples was regulated using the saturated salt solutions. Desorption isotherms for these three caltivars were determined. The EMC data were plotted against relative humidities (RH) at a constant temperature for all caltivars. The curves were 5- shaped generally rising sharply at high relative humidities. Five mathematical models (Chung-Pfost, Halsey, Henderson, Oswin and Smith) were applied in analysing the experimental data. The results indicated the analysis showed that Oswin, Smith and Chung- Pfost models were most suitable for describing and predicting the EMC vs RH relationship for Ohadi, Kalleghoochi and Abdollahi, caltivars respectively. The results also indicated that the EMC in all caltivars, decreased with an increase in temperature at constant relative humidity. Caltivar as well as temperature had significant effects on the values of EMC.
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