In Vitro Protein Digestibility (IVPD)
The vitro protein digestion rate of untreated, steaming and roasting walnut kernel was evaluated. The results of in vitro protein digestibility of walnut kernel before and after heating are shown in Fig. 3. For all samples, the rate of digestion in the intestine were higher than those in the stomach because of the higher number of catalytic sites on trypsin, which was supported by described finding of Fang et al.40 Compared with untreated walnut kernel, the IVPD of protein increased after heating, which indicated that the heat-processing could enhance IVPD to some extent, which was agreement with the results of Chinma et al. [46]. The reason might be that heating treatment could improve the hydrolysis activity of intrinsic protease and the solubility of protein in walnut kernel, and remove the protease inhibitors, tannins, and phytic acid [46,47]. This also indicated that the heat-processing could potentially improve the digestibility of protein in food systems [48]. The digestion of protein is also related to its secondary structure [49]. The reference reported that the α-helix content of proteins with heating treatment increased and the β-sheet content decreased, which would lead to an increase of the food digestibility [50]. The results in this study were agreement with the finding of Carbonaro et al. [50]. R30 and S30 presented the highest percentage of protein digestibility (gastric digestion: 15.42% and 17.83%, respectively; intestinal digestion: 22.37% and 27.31%, respectively), the lowest IVPD was identified in untreated sample (gastric digestion: 4.18%; intestinal digestion: 7.58%). It was attributed to the unfolding of protein structure during heating treatment, that was that the more hydrophobic groups were exposed, which would result in proteinase to produce the additional binding sites. Then, the protein skeleton was decomposed and the IVPD increased at last [21]. It was interesting that the digestibility of samples by steaming treatment was significantly higher than that of roasting treatment (P < 0.05). One of the main contributing factors to this phenomenon might be the water in steaming [30]. Protein application value can be reflected by IVPD, which also influences the quality of protein absorbed by the body. In general, the high-quality proteins have high IVPD [21]. As more amino acids from the protein backbone are released by proteolysis, which can be better digested and absorbed by the body [21]. Consequently, these results proved that the proper heat treatment could influence the structural c haracteristic (tertiary and secondary structure) of protein, which would be helpful to enhancing the occupating coefficient of protein in walnut kernel and expanding the applications in food products.