Hygroscopic properties and glass transition of dehydrated mango, apple and banana

An undesirable crispiness loss occurs when some dry fruits reach a critical moisture content (X (c) ) and their glass transition temperature (T (g) ) matches the storage temperature. Models for sorption isotherms and onset T (g) values for dry mango, apple, and banana were used to estimate X (c) values at 25 and 32 A degrees C. All models yielded R (2) > 0.97 but information theory criteria strongly supported GAB in all but one case (40 A degrees C, mango). The Gordon-Taylor T (g) model (GT) yielded high R (2) values for apple and banana but resulted in R (2) = 0.834 for mango. As moisture approached zero, mango T (g) estimates displayed a downward concavity contrasting with a rapidly increasing trend for apple and banana. The Khalloufi-Maslouhi-Ratti (KMR) model for T (g) as a function of a (w) showed a linear behavior. Although the KMR model fitted data with R (2) > 0.996, it requires more parameters and when a (w) approached 0, estimated T (g) values increased at a slower rate than for the GT model. In the case of banana and mango, both models predicted approximately the same X (c) at 25 A degrees C but not at 32 A degrees C. Finally, all X (c) values estimated based on T (g) were lower than the monolayer values obtained with the GAB (apple and banana) and BET (mango) models. These results indicate that the glass transition induced by moisture uptake dominates the quality degradation of these dry fruits.