Moisture sorption isotherms and chemical composition of Omani dates

The chemical composition and water sorption isotherms of two Omani date varieties, Fard and Khalas, were determined. Compositional analysis showed that Khalas contained more glucose (44.69±0.04%) than Fard (43.55±0.70%), but less non-starch polysaccharide (NSP). Khalas had 3.95±0.04% and Fard had 4.44±0.03%. Moisture sorption isotherms conducted at three temperatures displayed a crossing effect due to the dissolution of crystalline sugars at higher temperatures and moisture contents. Modeling with the GAB equation predicted monolayer moisture contents (X_m) of 13.9% for Fard and 14.4% for Khalas. Isosteric heat of sorption, for both varieties, varied from 9.4 to -1.6 kJ/mol as the moisture content changed from 5.0 to 40.0%. Differences in moisture sorption behavior of the two varieties were attributed to compositional differences. Compositional data alone, could not act as a universal moisture sorption model since other complicating factors, such as crystalline sugar dissolution, were involved.