Transactions of the ASABE Vol. 53(1): 191-197 E 2010 American Society of Agricultural and Biological Engineers ISSN 2151-0032 191 W ATER DESORPTION AND THERMODYNAMIC PROPERTIES OF OKRA SEEDS A. L. D. Goneli, P. C. Corrêa, G. H. H. Oliveira, F. M. Botelho ABSTRACT. The objective of the present work was to determine desorption isotherms of okra seeds for diverse conditions of temperature (10°C to 50°C) and relative humidity (0.11 to 0.96) of the air, as well as determine the values of isosteric heat of desorption, differential entropy, and Gibbs free energy. To obtain the equilibrium moisture content, a static method was used. Experimental data were adjusted to frequently used mathematical models for representation of hygroscopicity of agricultural products. Calculation of the net isosteric heat of desorption was performed based on the Clausius-Clapeyron thermodynamic relation. From the obtained results, it was concluded that equilibrium moisture content of the okra seeds decreased with an increase in temperature at the same relative humidity, as is the case of the majority of agricultural products. Based on statistical parameters, the Chung-Pfost model best represented the hygroscopicity of the okra seeds. Values of the integral isosteric heat of desorption as well as the differential entropy and Gibbs free energy for the okra seeds decreased with an increase in moisture content. Keywords. Abelmoschus esculentus L., Desorption isotherms, Entropy, Gibbs, Isosteric heat. kra (Abelmoschus esculentus L. Moench) is a flowering plant in the mallow family cultivated in Africa, Asia, the U.S., Australia, and Brazil (Du‐ zyaman, 1997). In Brazil, excellent growing con‐ ditions are encountered, especially in terms of climate, and okra is commonly grown in the northeast and southeast re‐ gions of the country. The plant presents desired characteris‐ tics, such as rapid cycle, economically viable production costs, resistance to diseases, and high nutritional value (Mota et al., 2000). Okra seeds have an average germinative dura‐ tion of five years when well conserved. Beside its use for growth of new plants, in countries such as Nigeria, okra seed has also been used for oil extraction due to its high protein concentration (Oyelade et al., 2003). Ac‐ cording to Çalişir et al. (2005), in Turkey some okra seed va‐ rieties are roasted and grounded to be used as a coffee substitute. During storage, physical, chemical, and microbiological changes can occur in okra seeds that, depending on interac‐ tion between these factors and the environment, can cause quality losses. Therefore, one needs to understand the exist‐ ing relationships between the product and the temperature and relative humidity of the air in order to minimize these po‐ tential alterations. Submitted for review in June 2009 as manuscript number FPE 8080; approved for publication by the Food & Process Engineering Institute Division of ASABE in January 2010. The authors are André L. D. Goneli, Postdoctoral Fellow, Department of Agricultural Engineering, Grande Dourados Federal University, Dourados, Brazil; Paulo C. Corrêa, Doctoral Fellow, and Gabriel H. H. Oliveira, Graduate Student, Department of Agricultural Engineering, Federal University of Viçosa, Brazil; and Fernando M. Botelho, Professor, Instituto Federal de Educação, Ciência e Tecnologia de Mato Grosso, Campus Juína, Brazil. Corresponding author: Gabriel H. H. Oliveira, Federal University of Viçosa, P.O. Box 270, 36570-000 Viçosa, MG, Brazil; phone: +55-031-3899-2030; fax: +55-031-3891-1943; e-mail: gabriel_ufv@yahoo.com.br. As in various other agricultural products, seeds have the capacity of relinquishing or absorbing moisture from the en‐ vironment. If their moisture content increases, the risk of fun‐ gal emergence also increases, severely compromising the germinative strength and vigor of the seeds. On the other hand, reduction in moisture content promotes economic losses due to loss of product mass (Yazdani et al., 2006). These alterations in moisture content continue until the prod‐ uct enters equilibrium with the surrounding air conditions. The equilibrium moisture content is reached when the partial pressure of water vapor in the product is equal to the partial pressure of water vapor of the surrounding air. The relationship between the moisture content of a partic‐ ular product and the relative humidity of equilibrium for a specific temperature can be expressed by mathematical equa‐ tions, which are known as isotherms or hygroscopic equilibri‐ um curves (Kaymak-Ertekin and Gedik, 2004). Currently, more than 200 proposed equations exist in the literature for representation of the hygroscopic equilibrium phenomena of agricultural products. These models differ from each other in their theoretical or empirical basis and in the quality of in‐ volved parameters (Mulet et al., 2002). According to Ayranci and Duman (2005), isotherms are important for definition of the dehydration limits of products, estimates of changes in the moisture content at determined temperature and relative humidity conditions, and definition of the levels of adequate moisture for initiation of microor‐ ganism activity, which can promote deterioration of the prod‐ uct. By means of the desorption isotherms, the amount of energy necessary for the drying process can also be obtained, represented by the isosteric heat of sorption (Wang and Bren‐ nan, 1991; Mulet et al., 1999). According to Wang and Brennan (1991), for water remov‐ al from hygroscopic materials, the required energy is greater than that required to vaporize the same amount of free water, at the same temperature and pressure conditions. Because of the strong bonds between water and the surface of the adsor‐ O