Glass transition of pomegranate skin, as analyzed by thermal, mechanical, and nuclear magnetic resonance methods

Glass transition of pomegranate skin was measured by thermal, mechanical, and magnetic resonance techniques. Differential scanning calorimetry thermogram showed a shift (i.e., onset glass transition at 20 degrees C) followed by an endothermic peak (i.e., solids-melting peak at 165 degrees C and enthalpy 140 kJ/kg). Overlapping of the glass transition and melting was observed in the differential scanning calorimetry thermogram; however, more sensitive modulated differential scanning calorimetry allowed to separate two transitions (i.e., glass transition from reversible and melting from non-reversible thermograms). The onset of mechanical glass-rubber transition from differential mechanical thermal analysis was observed at 122 degrees C with a shift in the storage modulus (E); however, the onset of liquid-like or entangled-reaction dominating transition was observed at 70 degrees C (i.e., onset peak in loss modulus, E) and peak at 184 degrees C. In addition onset peak in tan was observed at 113 degrees C and peak at 201 degrees C. Spin-spin (T-2 relaxation) and spin-lattice (T-1 relaxation) relaxations in time domain nuclear magnetic resonance was modeled by two-exponential relaxation curve (i.e., rigid and flexible domains). T-2 relaxation showed maximum peak with an onset at 40 degrees C with maximum peak at 150 degrees C. Rigid domain of T-1 relaxation showed a minimum peak onset at 40 degrees C and a minimum peak at 120 degrees C, whereas flexible component showed an onset at 20 degrees C and a minimum peak at 160 degrees C.