## 3 Results and Discussion  
### Quality Attributes  
The changes of sensory and physicochemical qualities of HTS fillets were evaluated during storage. Organoleptic properties play great role in consumer acceptance and marketability of food products. Fresh fish is among the most perishable foods due to lipid content, microbiological changes, and external factors [15]. Thereby maintaining its organoleptic characteristics becomes very important. As showed in Figure 1a, sensory scores decreased significantly with storage time in CK1, however, C2 and C3 had relatively slower reduction. This fact indicted that bioactive edible coating combined with MAP had promising effects on the maintenance of sensory attributes. Figure 1: Changes in sensory scores (**a**), moisture content (**b**), fat content (**c**), and TVB-N value (**d**), K value (**e**), MDA (**f**) and activity of Ca\({}_{2}\)+-ATPase (**g**) of samples during refrigerated storage (CK1: distilled water + air packing; CK2: edible coating + air packing; C1: edible coating + MAP (60% CO\({}_{2}\)/40% N\({}_{2}\)); C2: edible coating + MAP (65% CO\({}_{2}\)/35% N\({}_{2}\)); C3: edible coating + MAP (70% CO\({}_{2}\)/30% N\({}_{2}\))). Figure 1b suggested that there was a gradual moisture content reduction in all samples during 24-day storage. Then the decrease rate of edible coating treated group combined with MAP was slower than that of CK1 and CK2. That may be the MAP provides an effective inbuilt water vapor barrier and prevents loss by evaporation [7]. The fat content with different treatments showed a similar trend with moisture content during storage (Figure 1c). Interestingly, there were no obvious differences between C2 and C3 during storage, indicating that 65% CO\({}_{2}\)/35% N\({}_{2}\) or 70% CO\({}_{2}\)/30% N\({}_{2}\) were suitable treatments of MAP combined with edible coating applied refrigerated HTS fillets. TVB-N value is usually considered as an important index to assess the quality and shelf life of aquatic products [16]. As shown in Figure 1d, the initial at day 0 was 7.14 mg/100g and there was an increase in all the samples during storage. The TVB-N value in CK1 reached an unaccepted level (31.32 mg/100 g) on the 12 th day and the samples with MAP incorporated with bioactive edible coating had lower TVB-N values compared with CK1 and CK2, especially 70% CO\({}_{2}\)/30% N\({}_{2}\) of MAP. The dominant reason was that the high concentration of CO\({}_{2}\) could retard bacterial action in the conversion of trimethylamine oxide (TMAO) into trimethylamine (TMA), which was one of the main substrates for the production of volatile bases [17]. The K value is defined as the ratio of inosine (HxR) plus hypoxanthine (Hx) to the total adenosine triphosphate (ATP) and related compounds (ADP, AMP, IMP, HxR, and Hx) in fish muscle extract, reflecting the degree of nucleotide degradation [18]. As shown in Figure 1e, the initial K value of HTS fillets was 11.56%. K values increased progressively (\(p<0.05\)) over time for different treated samples because of the degradation of ATP to derivative compounds [1]. Generally, a K value of 60% fish muscle was taken as the highest acceptable level; as proposed by the European Commission [19]. Briefly, the K value of CK1 reached 62% at day 12, while the CK2, C1, and C2 group exceeded this limitation at day 16, 20, and 24, respectively. The fact suggested that a high concentration of CO\({}_{2}\) in the MAP environment could effectively retard the increased rate of K value and extend the shelf life of HTS fillets. Reports showed that the transformation process from IMP to Hx can be affected by spoilage bacteria [20]. Therefore, a high concentration of CO\({}_{2}\) can suppress ATP breakdown via retarding the growth of microbial metabolism. The amount of malonaldehyde (MDA) is commonly used to measure the level of rancidity and is mainly related to the accumulation of secondary products via liquid oxidation [20].