Moisture sorption isotherms of cowpea (Vigna unguiculata L. Walp) and its protein isolate at 10, 20 and 30 °C Erol Ayranci * , Osman Duman Department of Chemistry, Akdeniz University, Antalya, Turkey Received 7 May 2003; received in revised form 4 August 2004; accepted 19 August 2004 Abstract Moisture sorption isotherms of cowpea (Vigna unguiculata L. Walp), powdered cowpea and protein isolate of cowpea at 10, 20 and 30 °C were derived. The sorption data were treated according to many well-known sorption isotherm equations. The goodness of fit was evaluated on the basis of criteria such as the mean relative deviation modulus, coefficient of determination and standard errors of models. It was found that the GAB model was the most satisfactory model for representation of the sorption data. Mono- layer moisture contents (M 0 ) were determined from the GAB and the BET equations. The temperature dependence of M 0 was exam- ined. Heats of sorption data were evaluated using the Clausius–Clapeyron equation. An attempt was made to determine the moisture contribution of the protein component of cowpea to the total moisture content. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Moisture sorption isotherm; Cowpea; Protein isolate; Heat of sorption; Additivity of moisture content 1. Introduction Cowpea (Vigna unguiculata L. Walp) is one of the most widely adopted grain legumes. Dry grain for human consumption is the principal product of the cow- pea plant. Protein content of cowpea ranges from 23% to 33% on a dry basis (Ehlers & Hall, 1997). Foods con- taining considerable amount of protein such as cowpea may be utilized either as they are or by preparing their protein concentrates and isolates. Protein concentrates or isolates may constitute one of the nutritious additives to other foods. Cowpea is also used in the production of value-added foods such as cowpea flour, extruded snack items, cowpea rice and cowpea couscous. Since cowpea is produced in huge quantities, especially in hot regions of the world, its proper packaging and storage either as raw material or as protein isolate gain importance and expand its utilization. The control of moisture content of foods during processing and storage is very important as water has many roles in food reactions and food quality. In this re- spect the moisture sorption isotherm is an extremely important tool in food science because it can be used to predict changes in food stability and to select appro- priate packaging materials and ingredients. There are very many works on moisture sorption isotherms of foods over the last two decades. Some of these works are related to the determination of moisture sorption isotherms and some are related to the mathematical for- mulations to represent the moisture sorption isotherms (Ayranci, 1995; Debnath, Hemavathy, & Bhat, 2002; Maskan & Gogus, 1997; Menkov, 2000). In a recent paper by Al-Muhtaseb, McMinn, and Magee (2002), moisture sorption characteristics of food products were reviewed and applicabilities of various mathemati- cal models were discussed. Chen (2003) reported the moisture sorption isotherms of pea seeds with different 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.08.044 * Corresponding author. Tel.: +90 242 310 2315; fax: +90 242 227 8911. E-mail address: eayranci@akdeniz.edu.tr (E. Ayranci). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 70 (2005) 83–91