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

Abstract

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 T2b (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 T2b, T21 and T22 versus temperature were identified and related to the moisture content. The critical temperatures determined from T2b 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 T2b 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.