Freezing-induced proton dynamics in tofu evaluated by low-field nuclear magnetic resonance

The main purpose of the study was to understand and interpret the effects of freezing times on the proton dynamics and chrominance of tofu. Low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) were used to monitor real-time changes in microstructure and water distribution at different freezing times. The T-2 relaxation parameters included the relative intensity (A(2i)) and the population of T-2i component (M-2i). Three proton populations focusing on approximately 0.93-4.72, 25-49, and 402-505 ms were identified as T-2b, T-21, and T-22, respectively. The generated ice crystals damaged the hydration layer of the soybean protein and T-2b increased over the 2 h. The side chains of the soybean protein then began to unite owing to the protein's reduced affinity for water, making the protons of the hydrophilic groups exchangeable, and resulting in decreased mobility of the T-2b fraction. The appearance of exchangeable hydrophilic group protons caused an increase in A(2b) from 2 to 6 h. The subsequent increases in A(2b) and M-2b 6 h later were due to access to the unfrozen free water. The water molecules ( T-21 fraction) changed into ice crystals, reducing A(21) and M-21. The disappearance of the T-22 fraction peak 6 h later was attributed to residual unfrozen water molecules, with the minor component turning into ice with a fast relaxation time. The MRI results showed that the outline of the sample was blurred at 2 h and could not be detected 6 h later. Significant correlations were further detected between T-2 relaxation parameters and color parameters. LF-NMR has great potential as a reliable tool for the study of tofu.