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