Characterization of Osmotic Dehydrated Apple by NMR and DSC

Osmotic dehydration of apple in concentrated sucrose solutions was followed by low-resolution nuclear magnetic resonance. Spin-spin and spin-lattice relaxation times were measured as a function of degree Brix index of the fruits during the process. A rapid decrease of both relaxation times was found between 0 and 3 h of soaking. After 3 h of osmotic dehydration, the decay of relaxation times tended to be less dramatic. Water mobility changes were analyzed inside the fruits by population of distributed exponentials. The spin-lattice relaxation time distribution showed a reduction of mobility of the water molecules in the cells due to greater sucrose concentration and interaction. The spin-spin relaxation time distribution revealed a shrinkage phenomenon associated with the removal of water in the cytoplasm. Differential scanning calorimetry data showed changes of the freezing point of the dehydrated fruits and the associated enthalpy of crystallization as a function of the amount of sugar present in the fruit. Measurements of the temperature of the maximally freeze-concentrated matrix were in accordance with literature data on similar model sucrose-water solutions. Unfrozen water fractions were also estimated by differential scanning calorimetry from the melting endotherms.