FTIR Analysis of Walnut Kernel During Thermal Treatment
The untreated and heated walnut kernel samples were analyzed by FTIR spectroscopy to study the change in amide I frequency ((1600–1700 cm-1), which chiefly implies the C = O stretch vibration of the amide group [28]. The corresponding secondary structure of protein includes α-helix (1648 and 1664 cm-1), β-sheet (1615–1637 cm-1 and 1682–1700 cm-1), β-turn (1664–1681 cm-1) and random coil (1637 to 1648 cm-1) [32]. Table 2 shows the percentage content of the secondary structure of protein in walnut kernel treated with different time of steaming and roasting. With the processing time continued to increase, the change in the β-sheet and β-turns structures gradually slowed down, while the α-helix and random coil gradually rose up. The percentage of β-sheet structure (47.16%) was the highest in untreated sample, and the random coil occupied the lowest percentage (12.86%). It was interesting that the protein secondary structure in walnut kernel significantly changed after the steaming and roasting treatment. As shown Table 2, the relative contents of β-sheet (39.91% and 35.67%) and β-turns (19.98% and 19.19%) of the samples by steaming and roasting treatment for 30 min decreased, respectively, which decreased by 18.17% and 36.79%,28.61% and 41.79% compared with the native, respectively. Furthermore, the relative contents of α-helix (24.44% and 25.92%) and random coil (14.47% and 16.85%) of samples by steaming and roasting treatment for 30 min increased, respectively, which increased by 50.91% and 54.44%, 11.13% and 23.68% compared with the untreated, respectively. The results exhibited that the β-sheet and β-turns structure of protein in heat-treated walnut kernel were transformed to α-helix and random coil structures. It was supported by the investigation of Vanga et al. [33], which found that the α-helix and random coil structures of protein in peanut with an increase in the heat treatment time increased. The reason for this phenomenon was that the original rigid structure of the protein was destroyed to a certain extent. Therefore, the flexibility of protein increased and the conformation transformed from order to disorder [28,34]. On the other hand, the hydrogen bonds of protein might be destroyed by heat, which led to a greater degree of random distribution of molecular free energy [28,35]. As a consequence, it was concluded that a set of reactions in walnut kernels after steaming and roasting treatment occurred a set of reactions, such as protein denaturation and aggregation, which would cause the interconversion between α-helix, β-sheet, β-turn and random coil structures [28].
At the same time, it was also found that the different secondary structures of protein was presented in steaming and roasting walnut kernels due to two distinct thermal processing (moisture and dry heating method) (Table 2). It was obvious that the content of β-sheets was higher in walnut kernel by roasting treatment for 30 min compared to the steaming treatment (36.67%), which accounted for 39.91% of the whole secondary structure (Table 2). On the contrary, a low content of the ɑ-helix and random coil was determined for walnut kernel by roasting treatment for 30 min, which was 24.44% and 14.47%, respectively, while that of walnut kernel by steaming treatment was higher (25.92% and 16.85%, respectively). It could be inferred that the steaming and roasting heating would cause the structural changes of protein in walnut kernel, which was attributed to the denaturation, cross-linking and aggregation of protein through heating treatment.36