## 3 Results and Discussions
### Chemical Results in HTS Fillets
After 12 days, the IMP concentrations decreased sharply in all the samples, however the IMP concentrations in the treated samples were significantly higher than that of control samples (\(p<0.05\)) during storage, which indicated that chitosan-based coating containing \(\varepsilon\)-PL and RA inhibited the further degradation of IMP due to controlling the growth of spoilage organism and preventing the secretion of microbial enzymes. The Hx concentration at day 0 was 0.18 mmol/kg, and it increased during storage because of IMP degradation (Figure 2d). The control samples showed significantly higher (\(p<0.05\)) Hx concentration than that of the treated samples except C1 during storage, which indicated that 0.05% (_w/v_) \(\varepsilon\)-PL addition might not effectively inhibit the secretion of autolytic enzymes. At the end of the storage, CK1, CK2, and C1 samples were characterized by similar values (around 3.98-4.67 mmol/kg); C2, C3, and C4 showed a significant lower value (\(p<0.05\)) (around 1.36-1.58 mmol/kg). As for \(K\) values, the rejection limit of \(K\) value is commonly regarded as 60%, as the fish muscle shall lose the possibility of edibility and acceptance once the \(K\) value surpasses the limit point [26]. As shown in Figure 2e, the \(K\) value was 11.21% in fresh HTS fillets and \(K\)-values of all samples increased during storage, however the usage of chitosan-based coating significantly delayed the \(K\)-values increase, which had an analogous trend with the TMA and TVB-N. CK1 and CK2 surpassed the maximum permissible level at day 12 and C2, C3, and C4 were still under the rejection levels of the \(K\)-value at day 18. Thiobarbituric acid test measured the malondialdehydedehyde (MDA) content that can be used to determine the rate of lipid oxidation [27]. As recorded in Figure 2f, the initial MDA value of HTS fillets was 0.83 nmol/mgprot and increased gradually in all samples during the first 12 days. Thereafter, there were slight decreases, with the highest value (3.77 nmol/mgprot) in CK1 group at day 12. The TBA value at day 0 for the HTS fillets was 0.45 mg MDA kg\({}^{-1}\). Subsequently, the TBA values of all samples showed upward trends during the first 12 days and then slightly decreased due to the reaction of MDA with aldehydes and ketones [28]. The highest TBA value was 3.77 mg MDA kg\({}^{-1}\) in CK1 samples, while the TBA values for C1, C2, C3, and C4 were 0.64, 0.65, 0.62, and 0.61 mg MDA kg\({}^{-1}\), respectively, which may be due to the chitosan-based coating barrier against oxygen penetration and RA acting as an antioxidant, resulting in delayed lipid oxidation [29]. FAAs are responsible for flavor development and can be precursors of aromatic components. The most abundant FAA in HTS fillets was arginine, followed by glycine and proline, which accounted for 36.33-54.41% of total FAAs (Table 2). As a flavor-detracting amino acid, lysine of CK1 increased from initial value of 14.6 mg/100 g to 51.21 mg/100 g at day 18, while the content in CK2, C1, C2, C3, and C4 was 41.99, 36.67, 22.88, 28.57, and 28.17 mg/100 g at the corresponding storage time, respectively. Similar change properties were found in histidine, which was basically caused by the oxidation process from trimethylamine oxide based on the growth of spoilage organism and was consistent with results of TMA-N value. Aspartic acid, proline acid, and cysteine showed upward trends at the early storage and, afterwards, gradually decreased. Alanine and glycine progressively decreased during storage, which was owing to the positive enhancing effects of desired tastes [30]. For threonine and glutamic acid, no matter what treatments were applied for HTS fillets, no significant differences were observed among treated samples. The decrease of special flavor-enhancing amino acids and accumulation of flavor-detracting amino acids could lead to the flavor deterioration, and the coating could effectively slow down the process and maintain the quality of HTS fillets.