Rehydration characteristics of freeze-dried tropical fruits

Rehydration behaviours of five freeze-dried fruits, namely, pineapple, mango, guava, acerola and papaya were investigated and related to the structural changes during drying and glass transition temperature. After freeze-drying performed under vacuum conditions (13 kPa) and initial sample temperature near to -30 degrees C, fruits were allowed to rehydrate at 25 degrees C in distilled water for 6 h. The Exponential, Peleg and Weibull equations were applied to fit the experimental data using non-linear regression analysis. The statistical criteria used to evaluate the goodness of fit of each equation were the coefficient of determination (R-2), the standard error of estimate (SEE) and the mean relative percentage deviation modulus (MRD). The Weibull equation was found to be the most adequate for describing the rehydration kinetics, since it gave the higher R2 values and lower SEE and MAD values for all fruits examined in this study. From indices that take into account the quantity of water absorbed and the losses of solutes, it was possible to fully characterize the rehydration process of the freeze-dried fruits. Their water uptake capacity was affected not only by injuries during moisture removal but also by structural collapse induced by the rehydration Process itself, which was explained using the glass transition concept. (C) 2009 Elsevier Ltd. All rights reserved.