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.