3.1 Microbiological analysis
The rich nutrients and suitable environment of the fish created a perfect medium for the growth of bacteria, so the fish was prone to spoilage under the destruction of microorganisms (Wong, Loke, Chang, Ko, & Hsieh, 2020). The changes of TVC in mackerels treated with SI, SAEW-SI, AOB-SI, and SA-SI were shown in Fig. 1. The initial TVC in fresh mackerel was 2.14 log CFU/ g, which is one of indicatives for evaluating the good quality of fish (Fig. 1). The TVC of mackerels treated with SI increased significantly (P< 0.05) to 5.7 log CFU/g during 24 days of storage. Nevertheless, the TVC of mackerels treated with SAEW-SI increased slowly from 2.1 to 3.4 log CFU/g for the initial 12 days, and reached 4.9 log CFU/g on the 24th day. Moreover, the TVC in the samples of the AOB-SI and SA-SI treatment was 5.0 log CFU/g and 4.6 log CFU/g respectively on the 24th day. SA-SI treatment has the most powerful inhibitory effect on microbial growth compared with SI or SAEW, AOB treatment alone. The results showed that the combined action of SAEW and AOB exhibited a synergistic effect on the antibacterial effect. It was speculated that the flavonoids, phenolic acids, and lactones in AOB can enhance the permeability of the cell wall, so that the available chloride ions in SAEW and the biological macromolecules in AOB were released in the cell, which was malignant for bacterial growth. Thus, SA-SI treatment significantly inhibited the growth of microorganisms (P <0.05) compared to SAEW and AOB, alone.
3.2 Sensory evaluation
Sensory analysis could more intuitively judge the freshness of mackerels, including the color, texture, odor, and overall acceptability score (J. Cao, Wang, Ma, Bao, & Li, 2019). The sensory evaluation of samples in SI, SAEW- SI, AOB-SI, and SA-SI treatments were shown in Fig. 2. The score of 5 was considered an unacceptable point. Originally, the quality of mackerel was excellent with the sensory score of 9, and the sensory attributes of all samples displayed a continuously downward trend during the whole storage. However, the scores of samples stored in SAEW- SI and AOB-SI were higher than SI samples, especially when antioxidants of bamboo leaves were dissolved in slightly acidic electrolyzed water to make SA-SI, whose samples had the highest sensory evaluation. Thus, SA-SI treatment effectively improved the odor of mackerels, further reducing the loss of sensory score, followed by SAEW-SI and AOB-SI samples.
3.3 Carbonyl content and total SH
Oxidation induces changes in the structure and function of MP, including the formation of carbonyl groups. Therefore, the carbonyl content was one of the important indicators for judging the degree of MP oxidation. (Nyaisaba, Liu, Zhu, Fan, Sun, Hatab, et al., 2019a). The change in carbonyl content of mackerel was shown in Fig. 3A. The carbonyl contents of samples in SI, SAEW-SI, AOB-SI, and SA-SI treatments were significantly increased (P< 0.05) with the increase of storage time. The formation of carbonyl groups was affected by the oxidation of sensitive amino acid side chains. Furthermore, hydroxyl radicals could induce the formation of sensitive amino acid side chain carbonyl derivatives, which involves the formation of reaction intermediates (hydroxyl radical –OH) through the Fenton reaction near the side chains of susceptible amino acids (YanqingLi, ohuaKong, XiufangXia, Qian, & XinpingDiao, 2013). The lowest carbonyl content of SA-SI (6.2 nmoL/g) samples showed that the increase of carbonyl content was effectively delayed, because the protein oxidation was inhibited owing to the combined effect of antioxidants of bamboo leaves and slightly acidic electrolyzed water.
Total SH group included all sulfhydryl groups of protein molecules which both buried in protein and exposed to the surface (Z. Zhang, Yang, Tang, Chen, & You, 2015). As shown in Fig. 3B, the significant decrease (P< 0.05) in total SH contents of different groups were observed as the storage time increased. The sulfhydryl content of fresh sample was 221.36 μmoL/g protein which was decreased to 50.58, 82.37, 93.31, and 113.90 μmoL/mg protein when the samples were treated with SI, SAEW-SI, AOB-SI, and SA-SI respectively. The decrease in sulfhydryl content was mainly due to the degradation of myosin, which lead to changes in the spatial structure of the protein, causing the sulfhydryl groups buried in the protein molecules to be exposed and oxidized (L. Zhang, Q. Li, S. Jia, Z. Huang, & Y. Luo, 2018). The results showed that the total sulfhydryl groups of myofibrillar protein in mackerels treated with SA-SI were higher than the other three groups during the whole storage, mainly because the available chlorine in SAEW and flavonoids, phenolic acids, and lactones in AOB could effectively inhibit the denaturation of fish protein and sulfhydryl auto-oxidation, and reduce the degree of denaturation polymerization of protein.
3. 4 MFI and TCA-soluble peptides
The integrity of myofibrillar and skeleton proteins could be reflected by MFI. The increase in MFI value might be related to the degradation of myonectin and concomitant actin located in the I band of myofibrillar protein sarcomere. This degradation caused myofibrils to break near the Z-line, and the myofibrils were broken into smaller fragments (McDonagh, Fernandez, & Oddy, 1999). It was found that the larger MFI values at the 24-d storage in all groups compared to that at day 0 (Fig. 3C). Moreover, the highest MFI values were observed of samples in control group during the whole storage. The MFI value of samples in the SA-SI treatment was significantly lower (P <0.05) than SAEW-SI, AOB-SI, and the control group at each storage time point. In addition, there was no significant difference in the MFI value between the SAEW-SI treatment and the AOB-SI group (P＞0.05). The lowest MFI value of the SA-SI samples indicated that the damage of myofibril protein was inhibited, which was mainly due to the combined effect of SAEW and AOB that delayed the lysis or dissolution of sarcoplasmic and myofibril protein.
The increase in TCA-soluble peptide content in the early stage might be caused by endogenous proteases, while the later increase was caused by protein degradation owing to the combined action of endogenous enzymes and microorganisms. (Rawdkuen, Jongjareonrak, Phatcharat, & Benjakul, 2010). The TCA-soluble peptide content of samples in each treatment group increased significantly with the increase of storage time (P <0.05) (Fig. 3D). In addition, the TCA-soluble peptide content of SA-SI samples was significantly lower (P <0.05) than those of SI, SAEW-SI, and AOB-SI treatment, indicating that SA-SI treatment could inhibit protein degradation.
3. 5 Surface hydrophobicity
Bromophenol blue could specifically bind to myofibrils, which was considered to be a simple and reliable method for measuring surface hydrophobicity (Chelh, Gatellier, & Santé-Lhoutellier, 2006). Hydroxyl radicals promoted changes in the conformation of myofibrils, so a large number of hydrophobic groups in oxidized myofibril proteins were exposed (Lz, Ping, Yz, Jl, Qian, Hui, et al., 2019). The degree of specific binding between bromophenol blue and myofibril of samples in the SI, SAEW-SI, AOB-SI, and SA-SI group increased significantly (P <0.05) (Fig.3E) with the continuation of storage time. It might be due to the gradual expansion of myofibrillar protein molecules in fish meat during storage, resulting in the changes in the relative positions of hydrophilic and hydrophobic groups. The exposed hydrophobic amino acid residues resulted in the increased hydrophobicity of all samples. In the SI group, the content reached 123.70 μg at day 24, which was higher than those of the other three groups. Moreover, the surface hydrophobicity of SA-SI samples was significantly lower (P<0.05) than those of SAEW-SI and AOB-SI, and that of SAEW-SI samples was higher than AOB-SI samples, indicating that the combination of SAEW and AOB significantly delayed protein oxidation, and AOB might played a principal role.
3. 6 SDS-PAGE
The oxidation of protein could cause changes in internal structure including the formation of covalent bonds and the degradation of proteins with heavy molecular weight into those with light molecular weight. Therefore, SDS-PAGE was recommended as a formidable evidence for detecting the degree of protein oxidation (Nyaisaba, et al., 2019a). It was found that the bands of myosin heavy chain (MHC, 220kDa), actin (43kDa) and tropomyosin (TM, 38kDa) were the deepest and thickest of all the bands (Fig.4A). The heavy molecule chain in the lane meant that there might be protein cross-linking and aggregation. Moreover, the appearance of light molecule chain indicated that the protein had been oxidized, which lead to the degradation of the heavy molecular chain protein (M. Cao, et al., 2018). The MHC bands of the samples in four groups became weaker with the prolonging of storage time, which proved that MHC was susceptible to oxidative damage and degradation. In addition, the band density of actin and TM in SI samples was the weakest on 24th day. However, the two bands of samples in SAEW-SI, AOB-SI, and SA-SI treatment were only slightly reduced, especially the bands of SA-SI samples were the deepest. All these findings revealed that the SA-SI treatment was an effective technology to resist the decomposition of MP in mackerel.
3. 7 Secondary structure of protein
FTIR could measure the wavelength and intensity of infrared radiation absorbed by samples, and it was a feasible method to determine the changes in the secondary structure of proteins (Zhou, Hu, Yu, Yagoub, Zhang, Ma, et al., 2016). The protein oxidation in the samples could be determined by analyzing the changes of α-helix, β-sheet, β-turn and random coil in the amide I band (1600-1700 cm−1) owing to the C=O stretching vibration (Li, Hu, Zhao, Wan, & Xia, 2020).
The α-helix content decreased with the extension of storage time. The content of α-helix in SI group was 25% at day 24, which was the smallest value among all treated groups. On the contrary, the content of β-sheets gradually increased with the extension of storage time (Fig.4B). The decrease of the α-helical and the increase of the β-sheet might be related to the decrease and breakage of intramolecular hydrogen bonds which induced by the exposure of the hydrophobic region of the protein (Min, Cla, Yz, Jp, Sh, Lh, et al., 2021). Additionally, the folding structure of the peptide chain caused by the oxidation of sulfhydryl groups and the formation of disulfide bonds would also promote the increase of β-sheets (Sun, Zhou, Sun, & Zhao, 2013). And there was no significant difference in the content of β-turn in different treated groups. Furthermore, the appearance of disorder in the protein was related to the increased surface hydrophobicity. The random coil content of four groups increased compared to fresh sample (FS) at day 0, but the increased random coil was partly inhibited by SA-SI treatment. The results showed that SA-SI treatment delayed the unfolding, depolymerization and rearrangement of protein molecules, thereby the protein structure was effectively protected.
3. 8 Tertiary structure of protein
When the fluorescence inside the protein was transferred to the surface, fluorescence quenching was induced, and the fluorescence intensity would decrease at this time (Dong, Li, Tian, Zhang, Ren, & Quek, 2019). Tryptophan was the primary contributor of intrinsic fluorescence. The fluorescence intensity was determined by the changes in its position (M. Cao, et al., 2018). It was obtained that the highest fluorescence intensity at 335 nm appeared in fresh samples (FS, Fig. 4C and 4D). Besides, the fluorescence intensity in SI samples was always at the lowest value during the entire storage, and dropped sharply at the end of storage. However, only a small decrease trend of fluorescence intensity was observed in SA-SI samples, followed by AOB-SI and SAEW-SI samples. The above results indicated that SA-SI treatment could delay the oxidation and denaturation of protein and prevent the structure of the protein from being severely damaged during storage. The results were consistent with other protein indicators, such as secondary structure and hyrophobicity, indicating that SA-SI treatment could effectively delay protein oxidation.
3.9 Atomic force microscope (AFM)
Changes of myofibrillar structure of four treatments were reflected by AFM measurements in Fig. 5. The myofibrils of FS had completed and linear characteristic structure. It was demonstrated that there were some myofibrils with a highly ordered structure in the MP aqueous suspension of FS (X. Chen, Xu, & Zhou, 2016). The myofibrils of highly ordered structure were completely destroyed at the end of storage, showing dispersed particles with reduced particle size. The myofibrils of the SI samples turned into linear filaments, and some large irregular aggregates appeared in the SAEW-SI and AOB-SI samples. Although the SA-SI sample had protein aggregation, the relatively ordered myofibril structure of samples were maintained, which was the closest to the fresh sample in all treatments. The degradation of MP may be closely related to bacterial activity and endogenous enzyme. Furthermore, endogenous enzymes such as myofibril-bound serine protease (MBSP), cathepsin and Caspase-3 may exert a proteolytic effect on myofibrils, leading to the fiber breakage and detachment (Feng, Bansal, & Yang, 2016). The results suggested that the SA-SI treatment effectively resisted the deterioration of MP, which might be encouraged by the fact that the being of available chlorine in SAEW and flavonoids, phenolic acids and lactones in AOB inhibited the activity of endogenous enzyme.
3.9 Correlation analysis
Correlation analysis was a method of studying the degree of correlation between two or more variable factors. It was meaningful to discover the correlation between protein and quality characteristics (Bu, Han, Tan, Zhu, Li, & Li, 2022). Consequently, the research focused on the analysis of the correlation between protein oxidation and quality characteristics in order to choose a feasible method such as SA-SI which could inhibit protein oxidation and maintain the freshness of fish. As shown in Fig. 6, strong significant correlations with TVC levels were obtained for carbonyl (r =0.92), total SH (r =−0.97), MFI (r=0.84), TCA-soluble peptides (r=0.87), surface hydrophobicity (r=0.88, all P<0.01). In addition, Overall acceptability score was significantly correlated with TVC levels (r=−0.96, P<0.01), indicating that the overall acceptability had a strong relationship with TVC and protein oxidation. Therefore, SA-SI treatment could inhibit the growth of microorganisms and protect the biochemical properties of protein, so the quality of mackerel was maintained.