Water sorption and glass transition temperatures in red raspberry (Rubus idaeus)

Water sorption isotherms and glass transition temperatures of raspberries were determined to understand interactions between water and biopolymers. Water adsorption and desorption isotherms of raspberries were determined with an isopiestic method. Thermal transitions of raspberries equilibrated at selected water concentrations using adsorption and desorption were determined by differential scanning calorimetry (DSC). The sorption isotherm data were modeled by BET and GAB equations, while the plasticizing influence of water on glass transition was modeled by the Gordon–Taylor equation. Equilibrium water concentrations varied at equivalent water activities during adsorption and desorption indicating occurrence of hysteresis and irreversibility of thermodynamic processes. The monolayer water concentrations of 0.099 and 0.108 kg water/kg dry raspberry solids obtained by BET and GAB models during desorption were larger than those during adsorption (0.059 and 0.074 kg water/kg dry raspberry solids). The glass transition temperature of raspberries decreased with increasing water concentrations. The Gordon–Taylor parameters Tgs and k obtained for raspberries during adsorption were 42.6 °C and 4.73 and during desorption were 44.9 °C and 5.03, respectively. The characteristic glass transition temperature of the maximally freeze concentrated solution T′gT′g was −63.1 ± 5 °C and the onset of ice crystal melting temperature T′mT′m was −32.3 ± 0.4 °C. Although the water activity differed significantly at equivalent water concentrations obtained using absorption or desorption, the glass transition temperatures of raspberries were dependent on the concentration of water present not the method of equilibration.