Moisture sorption isotherm characteristics of plantain (Musa, AAB)

Abstract

Moisture sorption isotherms of fresh, pre-treated and freeze-dried plantain, at 40°C, 50°C and 60°C, were constructed using the static gravimetric method. The pre-treatment involved blanching and infusion in 40° Brix sucrose at 4°C for 16 h. The isotherms of the fresh plantain were of BET type II, while those of the freeze-dried samples were BET type I. Hysteresis was observed between the adsorption isotherm of the freeze-dried plantain and the desorption isotherm of the fresh samples. At low values of aw, below 0.40, the moisture contents of the pre-treated samples were lower than that of the fresh plantain, at corresponding values of aw. However, above aw 0.50 the reverse applied. In both desorption and adsorption an increase in temperature resulted in lower equilibrium moisture contents at corresponding values of aw. The monolayer moisture contents of the moist-infused and freeze-dried samples were lower than those of the fresh material. Of the five models tested the Henderson, S. M. (1952, Agricultural Engineering 33, 29± 32) and GAB (1981) versions fitted best to the isotherms of the fresh material. The Iglesias & Chirife model (1978) was the best fit to the isotherm of the infused material. The isosteric heat of sorption for the fresh plantain, ranged from 47.4 kJ/g mol at 18% moisture, dry solids basis, to 73.2 kJ/g mol at 2% moisture, dry solids basis. The comparable range for the freeze-dried material was 42.2±635 kJ /g. mol