Regresar

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

Abstract:

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 T2 relaxation parameters included the relative intensity (A2i) and the population of T2i component (M2i). Three proton populations focusing on approximately 0.93–4.72, 25–49, and 402–505 ms were identified as T2b, T21, and T22, respectively. The generated ice crystals damaged the hydration layer of the soybean protein and T2b 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 T2b fraction. The appearance of exchangeable hydrophilic group protons caused an increase in A2b from 2 to 6 h. The subsequent increases in A2b and M2b 6 h later were due to access to the unfrozen free water. The water molecules (T21 fraction) changed into ice crystals, reducing A21 and M21. The disappearance of the T22 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 T2 relaxation parameters and color parameters. LF-NMR has great potential as a reliable tool for the study of tofu.