Amino Acid Composition Analysis
Amino acid (AA) composition is an important chemical property of
protein. The reason is that it can determine the nutritional value of
protein [26]. AA composition of the untreated and heat-processed
samples of walnut kernel is
exhibited in Table 1. The data of experiments revealed that the protein
in different WKF samples included 16 amino acids, including 7 essential
amino acids (EAA) and 9 nonessential amino acids (NEAA) (Table 1). No
significant difference was found in the total content of AA between
untreated, roasting and steaming samples for 15 and 20 min (P> 0.05). The AA content of the sample was significantly
higher than that of other samples when it was steamed for 30 min
(P < 00.05). From the Table 1, it was observed that the
amount of acidic AA in samples was higher than alkaline AA, which
suggested that the WKF had the acidic characteristic [27]. These
findings were in agreement with those previously reported for walnut
meal flour [27]. Glutamic (Glu), aspartic (Asp), and Arginine (Arg)
in all WKF were relatively abundant (Table 1), Asp contents in
untreated, S15-S30 and R15-30 were 7.13, 5.77-7.24 and 6.04-5.10 g
100g-1, respectively; Glu contents were 19.15,
15.83-15.50 and18.60-17.66 g 100g -1, respectively;
Arg contents were13.26, 13.48-15.18 and 13.90-13.31 g 100g-1, respectively.
In addition, EAA content could reflect the nutritional value of protein
[28]. All samples contained EAA contained (except Tryptophan). The
total EAA contents in untreated, S15, S20, S30, R15, R20 and R30 were
26.72, 26.93, 27.89, 30.19, 27.07, 26.11 and 25.74 g 100g-1, respectively, which occupied 30.42, 33.33, 32.12,
33.23, 31.08, 31.12 and 31.2 %of the total AA content, respectively.
Therefore, the walnut kernel obtained by steam heating could be regarded
as high quality of natural proteins due to being composed of the most of
the EAA (Table 1). After different heat treatments, the content of EAA
in protein of walnut kernel changed to a certain extent compared to the
untreated sample. The steam treated samples were significantly higher
than the untreated and roasted samples
(P < 0.05).
After steaming, the total amount of total EAA in protein increased
significantly (P < 0.05) with the increase of heating
time. Roasting treatment had no significant effect on the total EAA
content of walnut protein (P > 0.05). The results
could be explained by their different processing methods. The thermal
stability of protein was comprehended by the composition of
AA.17 The higher the content of hydrophobic AA in the
protein, the more stable the protein was, which suggested that the more
stable protein contained the higher total hydrophobic AA content. The
hydrophobic AA content of the steaming treated walnut kernel gradually
increased compared with the untreated, while the roasting treated walnut
kernel gradually decreased, which was due to a thermodynamically less
stable for the protein of walnut kernel. However, there was the small
difference for total hydrophilic AA among samples.
It is generally believed that the closer the Amino Acid Score (AAS) is
to 100, the closer the AA pattern in samples is to the recommended
value,18 which suggests that the nutritional value of
sample is higher. AAS of untreated and heated walnut kernels is observed
in Table 1. For walnut kernel samples before and after heat treatment,
the AAS of rest AA except lysine was closest to 100 or more. It was
illustrated that these amino acids could basically meet the recommended
requirements [18]. Lysine was the first limiting amino acid in
walnut kernel samples according to the AA score (Table 1) [18]. It
was worth noting that the lysine AAS by steaming treatment increased
from 39.78 to 47.56%, while roasting treatment decreased from 39.78 to
37.11%. This showed that the steaming treatment could improve the
nutritional value of protein in kernel. These results proved that the
AAS of walnut kernels by steaming processing significantly increased
compared to the untreated and roasting processing (P <
0.05). Although the AAS of histidine, valine, leucine, isoleucine,
phenylalanine and threonine had decreased after roasting treatment, it
was still close to the recommended value of WHO/FAO/UNU Expert
Consultation [18], and AAS of
methionine by roasting treatment
for 15 min increased from 71.76
to 77.65%. These data indicated that the two kinds of heat treatment
could improve the nutritional value of protein in walnut kernel. The
amino acid content of each component of protein in walnut kernel changed
after different heat treatment, but most EAA could still meet the
recommended requirements. Therefore, the walnut protein by different
thermal processing could be used as a high-quality protein ingredient in
food [29].
In addition, the different heat treatments also changed the amount of AA
composition of the protein in walnut kernel. The results showed that
the content of other AA contents
except for glutamic gradually increased with the extension of steaming
treatment time. It indicated that the steaming processing led to a
significant increase in major AA contents, which might be contributed to
the release of bound amino acids during steaming with water [30].
The increase in the amino acid levels after the steaming of walnut
kernel could inspire utilization in food formulations. It was worth
noting that the content of AA components changed little when the
roasting time was 15 min; when the roasting time was extended to 20 and
30 min, other AA contents except
for arginine reduced. This
phenomenon could interpret the higher concentration of hydrophobic AA in
steamed walnut kernel. Compared with the untreated and steaming
processing samples, the major AA content and total AA of samples in
roasting processing from 15 to 30 min gradually reduced. This might be
due to little water in roasting processing, which would lead to the
partial degradation or loss of protein or the result of Maillard
reaction between amino acids and sugars [31].