Adsorption isotherm and heat of sorption of fresh- and osmo-oven dried plantain slices

Firm ripe plantain was transversely cut into 10-mm slices and osmotically pretreated in 52 °, 60 ° and 68 °B sucrose solutions, for 12 h at 25 °C. Fresh- and osmosed plantain slices were air-dried in a cross flow forced draught oven at 60 °C for 72 h. Adsorption isotherms of the products were determined at 20 °C and 40 °C, using a gravimetric-static method. Adsorption data were fitted into eight isotherm models. Isosteric heat of sorption was calculated using the Clausius-Clapeyron equation. Adsorption isotherms of fresh- and osmo-oven dried plantain slices followed type I (J-shaped) isotherms, characteristic of high sugar products. Inversion of 20 C and 40 C isotherms occurred between a_w 0.65 and 0.70. Among the models tested, the Guggenhein Anderson deBoer (GAB) gave the best fit. Isosteric heat of sorption increased with decreasing moisture contents. Negative isosteric heat of sorption occurred at high moisture content. Moreover, isosteric heat of sorption increased with increase in sucrose solution concentration during the osmotic dehydration prior to oven drying.