Eur Food Res Technol (2006) 223: 577–584 DOI 10.1007/s00217-006-0256-6 REVIEW ARTICLE M. Yazdani · P. Sazandehchi · M. Azizi · P. Ghobadi Moisture sorption isotherms and isosteric heat for pistachio Received: 24 July 2005 / Revised: 24 December 2005 / Accepted: 4 January 2006 / Published online: 9 February 2006 C  Springer-Verlag 2006 Abstract The equilibrium moisture contents (EMC) of pistachio were determined using the standard static- gravimetric method at 15, 25, 35 and 40 ◦ C for pistachio powder at 15, 35 ◦ C for pistachio kernel and pistachio nut for water activity (a w ) ranging from 0.11 to 0.9. At a given water activity, the results show that the moisture con- tent decreases with increasing temperature. The experimen- tal sorption curves are then described by the BET, GAB, Henderson, Oswin, Smith and Halsey models. A nonlin- ear regression analysis method was used to evaluate the constants of the sorption equations. The Smith model was found to be suitable for describing the sorption curves. The isosteric heat of adsorption of water was determined as a function of moisture content from the equilibrium data at different temperatures using the Clasius–Clapeyron equa- tion. Keywords Equilibrium moisture content (EMC) . Pistachio . Adsorption . Desorption . Hysteresis . Isosteric heat . Water activity (a w ) Introduction The pistachio nut (Pistacia vera L.) is one of the popular tree nuts of the world. The kernels are a good source of fat (50–60%) and contain unsaturated fatty acids (linolenic, linolenic and oleic acid). Due to its high nutritional value and split shell, pistachio is an increasingly important nut crop consumed as raw, salted or roasted. There has been a dramatic increase in production of pistachio in different re- gions of the world during the past 20 years. Pistachio is cul- tivated in the Middle East, United States and Mediterranean countries. Iran is one of the biggest producer and exporter M. Yazdani () · P. Sazandehchi · M. Azizi · P. Ghobadi Chemical Engineering Department, Tarbiat Modarres University, P.O. Box 15815-1633, Tehran, Iran e-mail: manouchehr yazdani@behranoil.com Tel.: +98-9125042346, 22277674 Fax: +98-21-22277746 of pistachio nuts. It has produced more than 250,000 tonnes in 2003 and exported 115,335 tonnes to different countries in 2002 (FAO, 2003). A right processing after harvest is very important on pistachio quality. When pistachios arrive at the processing plant, the following procedures are con- ducted: (a) dehulling, to separate soft hull from nuts; (b) trash and blank separation, to remove blank pistachios and trashes such as small branches, remaining shells and leaves; (c) unpeeled pistachios separation, to remove unpeeled and unripe nuts; (d) washing, which involves spraying water at high pressures on the pistachios to clean the nuts; (e) dry- ing, to decrease moisture content of pistachios from 37 to 40% to the appropriate level; (h) roasting; and (i) packaging [1, 2]. Moisture sorption isotherms describe the relationship be- tween the equilibrium moisture content and the water ac- tivity at constant temperatures and pressures. For food ma- terials these isotherms give information about the sorption mechanism and the interaction of food biopolymers with water [3]. The moisture sorption isotherms are extremely important in modelling the drying process, in design and optimization of drying equipment, in predicting shelf-life stability, in calculating moisture changes which may occur during storage and in selecting appropriate packaging mate- rial [3]. Because of the seasonal nature of pistachio harvest- ing, a large amount of it is stored during the months follow- ing the harvest. During the storage period, if the surround- ing moisture conditions in the cold-storage room are not rigorously maintained, the hygroscopic characteristics of pistachios promote a considerable change in their moisture content. If the moisture content increases, the risk of the ap- pearance of fungi is greater. On the other hand, if the mois- ture content decreases, the problems would be economical because of the loss of weight, therefore it is necessary to know the equilibrium moisture content of the pistachios for various water activities and temperatures that would allow to correctly specify the storage conditions for the pista- chios. The quality of a dried food product and its cost are greatly influenced by the drying operation. Products that have been dried at lower temperatures exhibit good storage stability, but require longer processing times. Low water ac-