Sorption equilibrium and thermodynamic characteristics of soya bean

Desorption equilibrium moisture content and water activity data for soya bean (Glycine max) TGX 1440-1E were determined using the static gravimetric method. Measurements were taken in the water activity range of 0.07-0.98 at four temperatures (40, 50, 60 and 70°C). A non-linear regression programme was used to fit five moisture sorption isotherm models [modified Henderson, modified Chung-Pfost, modified Halsey, modified Oswin and modified Guggenheim-Anderson-de Boer (GAB)] to the experimental data. The models were compared using the standard error of estimate, mean relative percent deviation and residual plots. The modified Oswin model, which gave the least standard error of 1.91% and least mean relative percent deviation of 10.15 among the five models, when equilibrium moisture content was taken as the dependent variable, was considered to be the best for predicting the equilibrium moisture content of soya bean. The moisture sorption isotherms of soya bean were sigmoidal in shape, of the type II and were markedly affected by temperature. The modified Halsey model, which gave the least standard error of 0.07 and least means relative percent deviation of 16.67, when water activity was taken as the dependent variable, was considered the best for predicting the water activity of soya bean. The desorption equilibrium moisture content and water activity relationships as expressed by the moisture sorption isotherm models that best predicted the experimental data, were used to determine the thermodynamic characteristics of soya bean. The heat of vaporisation of moisture in soya bean decreased with increase in moisture content and approached the latent heat of pure water at a free water point of between 20 and 22% moisture content (d.b.). The surface potential of moisture in the seed increased with increase in water activity, but was not significantly affected by temperature. Net integral enthalpy decreased from a value of 550 J g-1 as the moisture content increased from 5.20% (d.b.). The trend became asymptotic at a value of 80 J g-1 as the moisture content of 13% (d.b.) was approached. Net integral entropy decreased with increase in moisture content to a minimum value of -0.38 J g-1 K-1 at the moisture content of 11% (d.b.), and thereafter, increased with further increase in moisture content.