3. Results and discussion
3.1. Moisture sorption isotherms
The water adsorption and desorption data of fresh unripe banana at studied temperatures (25, 40, 50 and 60) C are shown in Fig. 1. It is clearly observed that the isotherms of unripe banana describe multilayer adsorption and can be identified as type II, showing asymptotic behavior with two different regions. Each point is the average of three experimental determinations. All obtained standard deviation values were less than ±2.7 g/100 g and ±0.024 for moisture content and aw data, respectively. It was also observed a hysteresis region in the 0.8e0.9 range aw. For aw ranging from 0.85 to 0.95, the curves showed a steep rise. It was observed an inversion of the effect of temperature at water activity higher than 0.7. According to Rahman (1995) the water activity shifted by temperature is mainly due the change in water binding, dissociation of water or increase the solubility of solute in water. At water activity values above 0.7, the moisture content decreased when temperature increased, whereas at water activity values less than 0.7 a different behavior was observed, the curves crossed over for the adsorption data and for the desorption data. The latter means that the unripe banana becomes less hygroscopic, which can be explained, the kinetic energy associated with water molecules present increases with increase in temperature. As a consequence, there are a decrease of the attractive forces and escape of water molecules (Figura and Teixeira, 2007).
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