Changes in meat properties, protein oxidation and free amino acids of oval pomfret during pickling and air drying

Oval pomfret (Trachinotus ovatus), commonly known as golden pomfret and yellow pomfret with tender flesh and no intermuscular spines, rich in protein, fat, polyunsaturated fatty acids and other nutrients, is a marine aquaculture economic fish species
.
In the southern region of China, people often make cured fish and air-dried fish through pickling and air drying processes, so as to improve the added value of raw materials, extend the shelf life of products, and improve product flavor
.
Protein is one of the main components of aquatic products, and oxidative hydrolysis during processing affects product quality
.
Peptides and free amino acids are produced by the degradation of proteins under the action of endogenous proteases and are precursors of flavor substances
.
Free amino acids can react with reducing sugars to form volatile odor substances, which play an important role
in the formation of product flavor.

Zhang Jinwei, Hu Xiao and Chen Shengjun* of the National Aquatic Products Processing Technology Research Center, the South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, the Key Laboratory of Aquatic Products Processing of the Ministry of Agriculture and Rural Affairs, and the National Aquatic Products Processing Technology Research and Development Center mainly analyzed the moisture content, salt content, texture characteristics, myofibrillar secondary structure, carbonyl content, sulfhydryl content, protein hydrolysis index, sodium lauryl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) pattern and free amino acids in the process of pickling and air drying of oval pomfreight.
The law of protein oxidation and free amino acid changes in oval pomfret meat during pickling and drying was preliminarily revealed, which provided a theoretical reference
for quality control during the processing of pickled aquatic products.

1.
Effects of pickling and drying on moisture content and salt content of oval pomfret trevally

It can be seen from Figure 1 that the salt content of oval pomfret trevally increased during the pickling and air drying processes, among which the salt content of oval pomfret trevally increased sharply after 2 days of pickling (P<0.
05), while the moisture content showed a downward trend during this process, and the difference in moisture content after 1, 3 and 4 days of drying was not significant (P>0.
05).

Pickling is the process of osmotic diffusion, dry curing is to evenly apply salt on the surface and inside of the fish, so that it forms a high osmotic pressure inside and outside, so that the salt content increases significantly and the moisture content decreases
.
During the air drying process, the salt content gradually increased, but the salt content of oval pomfret was not significantly different after drying for 2 d and 3 d (P>0.
05), which may be due to the rapid removal of water from the epidermis of oval pomfret and the slow migration of internal water to form a hard shell, so that the moisture content changed slowly and the salt content did not change significantly
.
In addition, after drying for 4 days, the salt content of oval pomfret reached the highest value, which was (3.
96±0.
13)%.

2.
Effect of pickling and drying on the texture characteristics of oval pomfret trevally

It can be seen from Table 1 that the elasticity of fish increases after marinating for 2 days, and decreases with the process of air
drying.
Usually when the salt content of fish meat is low, its elasticity will increase, and in the process of air drying, the salt content of fish meat increases significantly, the moisture content decreases, and the elasticity decreases with the decrease of moisture content, and at the same time, with the diffusion of internal moisture in the fish, the diameter of muscle fibers and its cross-sectional area decreases, and muscle fibers contract, resulting in a decrease
in the elasticity of fish meat.
It can be seen from Table 1 that during the process of curing and air drying, the adhesiveness of fish meat is on the rise
.
Studies have shown that fat content has a certain effect on adhesiveness, and during the curing and drying process, the moisture content of fish decreases and the relative content of fat increases, so it may lead to changes in the adhesiveness of
fish.
It can be seen from Table 1 that the chewiness of oval pomfret showed an overall upward trend.
Among them, after marinating for 2 days, the chewiness of fish increased significantly, but after 1 day of air drying, it decreased slightly but not significantly, and then showed a significant increase trend (P<0.
05).
<b14>

3.
Effect of pickled air drying on myofibrillar protein content in oval pomfret trevally

It can be seen from Figure 2 that the content of myofibrillar protein in oval pomfret showed a downward trend during the curing and drying process.
After 2 days of pickling, 1 day of drying, and 2 days of drying, the content of myofibrillar protein decreased significantly (P<0.
05), which may be caused by<b10> the dissolution or hydrolysis of myofibrillar protein during processing to produce polypeptides and free amino acids.

4.
Effect of pickling and air drying on the carbonyl content of protein in ovate pomfret trevally

It can be seen from Figure 3 that with the progress of pickling and air drying, the carbonyl content increases significantly and the degree of protein oxidation increases.
The carbonyl content of the raw material of ovate pomfrally was 1.
37 nmol/mg, and the carbonyl content of protein increased significantly (P<0.
05) to 1.
94 nmol/mg after pickling for 2 days, and the carbonyl content of protein continued to increase during air drying, while the difference in protein carbonyl content was not significant (P>0.
05)
after drying for 1 d and 2 d.

5.
Effect of pickling air drying on sulfhydryl content of ovate pomfret

It can be seen from Figure 4 that the change trend of sulfhydryl content is opposite to that of carbonyl group as a whole, and with the progress of pickling air drying, the sulfhydryl content decreases significantly (P<0.
05), and the difference in sulfhydryl content is not significant (P>0.
05)
after drying for 4 days.
The reason for this phenomenon is that the protein structure of fish meat is destroyed by the curing and air drying process, resulting in the exposure of sulfhydryl groups buried inside the protein, resulting in disulfide bonds, and the total sulfhydryl content decreases
.

6.
Effect of pickling and air drying on the secondary structure of myofibrillar protein in oval pomfret trevally

It can be seen from Figure 5 that with the pickling and air drying, the content of α-helix in the secondary structure of myofibrillar protein decreases, while the content of random curl increases
.
Studies have shown that sodium chloride may have a destructive effect
on the secondary structure of myofibrillar protein.
It may be because the salt penetrates into the fish meat during the pickling and drying process, the myofibril protein structure is destroyed, and the carbonyl content increases, so that the α-helix content is reduced and the random curl content is increased
.

7.
SDS-PAGE analysis of myofibrillar protein in ovate pomfret trevarian

As shown in Figure 6, during the processing process, the SDS-PAGE band of myofibrillar protein of oval pomfret vifrillar underwent obvious changes, among which the main proteins were myosin heavy chain (MHC), actin, tropomyosin and so on
.
The degradation of protein molecules is mainly manifested as blurring, weakening and expansion of bands with higher molecular weight, and new bands or darkening
of band color in lower molecular weight areas.
From Figure 6, it can be seen that the MHC band and actin have obvious changes; Among them, MHC bands changed little after 2 days of pickling, which may be that myofibrillar proteins were crosslinked or aggregated during pickling.
The MHC band disappeared after air drying, indicating that the macromolecular weight protein was degraded
under the action of endogenous proteases.
Actin bands become thinner after pickling for 2 days, which may be caused by increased sodium chloride concentration, resulting in actin dissolution; However, after drying for 1 days, the actin band became thicker, which may be caused by the degradation of macromolecular mass proteins to form smaller molecular weight proteins during the air drying process
.

8.
Effects of pickling and drying on total nitrogen and non-protein nitrogen content of oval pomfrantifer

As shown in Figure 7, during the processing process, the total nitrogen content of oval pomfret decreased significantly after 2 days of pickling (P<0.
05), which may be caused by the dissolution of water-soluble proteins during the pickling process, which is consistent with Zhang Huili's research results, while the total nitrogen content showed a slow decrease trend during the air drying process<b10>.
In addition, the content of non-protein nitrogen decreased after pickling processing, which was caused by the dissolution of water-soluble nitrogenous substances; However, the content of non-protein nitrogen increased significantly during the air drying process, mainly due to
the hydrolysis of proteins into polypeptides and free amino acids under the action of endogenous proteases in fish.

It can be seen from Figure 8 that the proteolysis index changes little after 2 days of pickling and the difference is not significant (P>0.
05), while the proteolysis index increases significantly during the air drying process, which may be due to the increase of enzyme activity during air drying, the degradation of fish protein into polypeptides, free amino acids, etc.
, and the dissolution of nitrogen-containing substances is less, so that the proteolysis index increases
.

9.
Effect of pickling and drying on free amino acid content in oval pomfret trevally

As shown in Table 2, the total amount of free amino acids in the raw material was 380.
03 mg/100 g, of which the content of alanine, hydroxyproline, glycine and glutamic acid was high.
After drying for 4 days, the total amount of free amino acids was 1 596.
07 mg/100 g, of which the contents of alanine, glycine, glutamic acid, leucine and lysine were high, accounting for about 50.
67%
of the total free amino acid content.

Conclusion