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

Abstract

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°C) followed by an endothermic peak (i.e., solids-melting peak at 165°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°C with a shift in the storage modulus (E′); however, the onset of liquid-like or entangled-reaction dominating transition was observed at 70°C (i.e., onset peak in loss modulus, E′′) and peak at 184°C. In addition onset peak in tan δ was observed at 113°C and peak at 201°C. Spin–spin (T2 relaxation) and spin-lattice (T1 relaxation) relaxations in time domain nuclear magnetic resonance was modeled by two-exponential relaxation curve (i.e., rigid and flexible domains). T2 relaxation showed maximum peak with an onset at 40°C with maximum peak at 150°C. Rigid domain of T1 relaxation showed a minimum peak onset at 40°C and a minimum peak at 120°C, whereas flexible component showed an onset at 20°C and a minimum peak at 160°C.