Proton relaxation in freeze-dried broccoli as measured by low-frequency nuclear magnetic resonance (LF-NMR) and its relationship with the thermal glass transition

Low-frequency nuclear magnetic resonance (LF-NMR) was used to identify different proton pools and their mobility in freeze-dried broccoli (moisture 0.01 to 0.25 g g sample⁻¹ and temperature 193 to 443 K) containing un-freezable water. Three pools of protons were determined from transversal T₂ (i.e., spin–spin) relaxation times. These were T_2b (i.e., tightly bound pools of protons associated with macromolecules), T₂₁ (i.e., protons in the strongly bound water with the solids) and T₂₂ (i.e., protons in the weakly bound or capillary water). Two critical temperatures and one peak temperature from the plot of T_2b, T₂₁ and T₂₂ versus temperature were identified and related to the moisture content. The critical temperatures determined from T_2b and T₂₁ increased with the increase in moisture up to BET-monolayer followed by an exponential decrease. However, the first critical temperature from T₂₂ increased and reached to a plateau, while the second and third critical temperatures increased linearly with the increase in moisture. The critical temperature from T_2b was determined from the intersection of the first and second segments and compared with the onset glass as measured earlier by differential scanning calorimetry (DSC). At moisture 0.01 g g sample⁻¹, this critical temperature showed lower than the thermal glass transition temperature. The opposite trend was observed at or above moisture 0.05 g g sample⁻¹, and the difference between critical temperature and glass transition temperature increased with the increase in moisture content. It was also observed that peak temperatures were close to the solids-melting temperature as measured by thermal analysis. The peak indicated the disruption of the macromolecules and creating interlinked melted or compacted solid mass and caused to decrease the proton mobility.