Differential Scanning Calorimetry (DSC) Analysis
The thermal properties of the protein in both heating treated (steaming and roasting) and untreated walnut kernel samples are carried out by DSC. The denaturation peak temperature (Tp), onset temperature (To), endset temperature (Te) and denaturation enthalpy (ΔH) of protein in walnut kernel before and after heating treatment are summarized in Table 3. Tp, To, Te and ΔH play the important roles in the thermal properties of proteins, which is attributed to being closely connected with the protein conformation. Tp is the temperature at which protein denatures, To represents the initial temperature of the protein denaturation, Te is the temperature of at which protein denaturation ends, while ΔH represents the energy required during protein denaturation [21,37]. As shown Table 3, there was a significant difference for the Te and Tp of protein in walnut kernel before and after heating treatment (P< 0.05). The results showed that the heating processing increased the Te and Tp values of protein, the highest Tp values were observed after 15 min of roasting and 30 min of steaming, which was 102.83 °C and 102.22 °C, respectively, the highest Te values were found after 15 min of roasting and 20 min of steaming, which was 116.04 °C and 114.50 °C, respectively. While Tonset in all samples slightly changed. With increasing steaming and roasting time, there is no significant change in Tp (P > 0.05), and To and Te of other samples except roasting for 15 min also had not the significant difference (P> 0.05). It was also found that the denaturation enthalpy (ΔH) of protein in walnut kernel increased after steaming and roasting treatment (Table 3). The increase of random coil and ɑ-helix proportions in protein can reflect the changes of ΔH [38]. As can be shown in Table 3, there are an increase in the ΔH values of roasted and steamed walnut kernel compared to that of untreated sample, which suggested that the heating treatment could increase the unfolding molecular structure of protein and change the stable structure of protein [39]. The value of ΔH was the highest in roasting and steaming for 20 min, which was 7.07 and 8.69 J/g, respectively. The result indicated that heating (roasting and steaming) treatment at 95 °C improved the denaturation temperature and ΔH values of protein in walnut kernel. However, it was obvious that ΔH was lower in dry-heated (roasting) samples compared to the moist–heat (steaming) samples as a result of both different thermal processing methods.