Sorption equilibrium and thermodynamic properties of cowpea ( Vigna unguiculata)

Abstract

Equilibrium relative humidity (ERH)–moisture content data for cowpea ( Vigna unguiculata) at five different temperatures (40, 50, 60, 70 and 80 °C) and moisture content values ranging from 4.5% to 22.2% (dry basis) were determined using the vapour pressure manometric method. A non-linear least squares regression programme was used to evaluate four moisture sorption models. The modified Henderson model gave the least standard error of estimate of 5.8% for ERH and 1.3% for equilibrium moisture content (EMC), as a result, this model was considered to be the best for predicting the ERH and EMC of cowpea. The experimental data was also used to determine the thermodynamic functions of cowpea. Latent heat of vaporization decreased with increase in moisture content and approached the latent heat of pure water at the moisture content of 24% (dry basis). The spreading pressure increased with increase in relative humidity but decreased with increase in temperature more remarkably in the high range of relative humidity (RH>0.65). Net integral enthalpy decreased with moisture content and the trend became asymptotic as the moisture content of 18% was approached. Net integral entropy was negative in value and it increased with increase in moisture content in the moisture range of 4–6.4% dry basis. It thereafter, remained constant until the moisture content of 14.2% was reached, and then it continued to increase with increase in moisture content.