Hygroscopic properties and glass transition of dehydrated mango, apple and banana
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© 2017, Association of Food Scientists & Technologists (India). An undesirable crispiness loss occurs when some dry fruits reach a critical moisture content (Xc) and their glass transition temperature (Tg) matches the storage temperature. Models for sorption isotherms and onset Tg values for dry mango, apple, and banana were used to estimate Xc values at 25 and 32 °C. All models yielded R2 > 0.97 but information theory criteria strongly supported GAB in all but one case (40 °C, mango). The Gordon¿Taylor Tg model (GT) yielded high R2 values for apple and banana but resulted in R2 = 0.834 for mango. As moisture approached zero, mango Tg estimates displayed a downward concavity contrasting with a rapidly increasing trend for apple and banana. The Khalloufi¿Maslouhi¿Ratti (KMR) model for Tg as a function of aw showed a linear behavior. Although the KMR model fitted data with R2 > 0.996, it requires more parameters and when aw approached 0, estimated Tg values increased at a slower rate than for the GT model. In the case of banana and mango, both models predicted approximately the same Xc at 25 °C but not at 32 °C. Finally, all Xc values estimated based on Tg 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.