Water sorption behavior and thermal analysis of freeze-dried, Refractance Window-dried and hot-air dried açaí (Euterpe oleracea Martius) juice

Açaí, a dark purple berry native to the Amazonian region, has been recognized for its high antioxidant capacity. However, açaí is very perishable and processing is essential to preserve its bioactive compounds. In this work, three drying methods - freeze-drying, Refractance Window-drying (RW), and hot-air drying - were applied to dehydrate açaí juice. Working moisture sorption isotherms and thermal analysis of the powders were performed immediately after drying. Moisture content and water activity of the dried samples were evaluated after drying and during three-month storage at room temperature. The glass transition behavior of the dried açaí was studied by differential scanning calorimetry (DSC). The isotherms were fit to the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de-Boer (GAB) models. Moisture contents of the dried samples were all below the monolayer values. The water activity values were also low, indicating relatively good product stability. All isotherms were sigmoidal in shape. The BET and GAB models showed a good fit to the isotherm data. The DSC thermograms of the dried powders and oil fractions revealed that the lipids present in the açaí powders are liquid at room temperature. The DSC thermograms of the solids fraction suggested a subtle glass transition between 50 and 60 °C.