State diagram and water adsorption isotherm of raspberry (Rubus idaeus)

Thermal transitions of freeze-dried raspberry powder (Rubus idaeus) were analyzed by using differential scanning calorimetry. Freeze-dried raspberry powders containing unfreezable and freezable water were examined to develop the state diagram of raspberry. The state diagram of freeze-dried raspberry powders included the glass line: glass transition temperature versus solids content, freezing curve; initial freezing point versus solids content; end point of freezing T'(m), corresponding solids content X'(s) characteristic glass transition T'(g), and corresponding solids contents X-s '' of maximally-freeze-concentrated raspberry. The conditions of the maximal-freeze-concentrate obtained from freezing curve corresponded to T'(m) = -38 degrees C and X'(s)= 0.78 kg solids/kg raspberry and T'(g) = -47 degrees C and X ''(s) = 0.82 kg solids/kg raspberry. The T'(g) was determined by extending the freezing curve to glass line. The quantities Of Unfreezable water identified from enthalpy data and the freezing curve were comparable. Adsorption isotherms of freeze-dried raspberries were determined at room temperature by the isopiestic method and the data was modeled with BET and GAB equations. The BET and GAB monolayer moisture contents were observed to be 0.045 and 0.074 kg water/kg dry raspberry solids, respectively. The state diagram and water sorption properties of raspberries are useful in optimizing the retention of anthocyanins, phenolics concentrations and antioxidant activities in freeze-dried and frozen raspberries during storage. (c) 2008 Elsevier Ltd. All rights reserved.