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Unripe acerola powders obtained from freeze-drying without (AV) and with polydextrose (AP) were characterized by using differential scanning calorimetry (DSC) and vapor sorption isotherms at (25, 35, and 45) °C in water activity (a w) range of 0.10–0.90. State diagrams built from DSC results, such as the glass transition (T g) and freezing (T m) points, were compared with those of the sorption isotherms. The Gordon-Taylor equation presented a good fit for T g , in which the solids glass transition increased and the k parameter decreased in the presence of polydextrose. The isotherms obtained were Brunauer-Emmett-Teller (BET) type III, and the Guggenheim-Anderson-de Boer (GAB) equation fitted to the experimental data showed slightly higher monolayer water content for AP, and not affected by temperature. Although there was a divergence in the stability when comparing the criteria of a w and T g , polydextrose increased the a w and T g of freeze-dried unripe acerola and conferred a stabler powder. • The stability criterion between water activity and glass transition was studied. • State diagrams of unripe acerola with polydextrose were built. • Polydextrose used as a drying aid increased the glass transition temperature. • Sorption isotherms of unripe acerola powders were BET Type III. [ABSTRACT FROM AUTHOR]