Upper limit of quality and freshness control of the culture Paralichthys olivaceus

Post-harvest fish suffer slow and painful deaths by asphyxiation, crowding, or even crushing. Stress and stress-related quality changes of farmed fish is necessary to explore. Instead of live fish in poor condition at the market, providing freshness-locked products with sushi-grade near to upper-limited quality of farmed fish is more valuable. In order to establish a technical system for post-fishing regulation of freshness of factory farmed fish, the formation and change mechanism of sashimi grade quality of mariculture white meat fish were analyzed from the point of limit quality. The brown flounder (Paralichthys olivaceus) was taken as the research object. The limit of muscle quality was explored via different slaughter methods. That is, extreme treatment was used to explore the upper limit of fresh quality. The upper limit of quality was obtained by spinal cord destruction treatment (SCD), and the lower limit of quality was obtained by suffocation in air treatment (SA). The two limits of quality were compared with the spinal cord cut treatment (SCC), which is in wide commercial use. The three slaughter treatment groups were refrigerated at 2 ℃ for 120 h. During this period, rigidity, breaking strength, muscle shrinkage, muscle pH, ATP and its related compounds, whiteness, microstructure and surface color were analyzed. The results showed that the effects of slaughter stress on quality and stability were very important. The SA group with the strongest slaughter stress rapidly got into postmortem rigidity after 6-12 h and then the stiffness was relieved. The SCD group with the lowest slaughter stress reached the maximum postmortem rigor from 48 h to 72 h, and the rigor index increased slowly, showing the best stability of cold storage. The SCC group of conventional commercial disposal was between the two groups, and the quality was significantly better than the lower limit quality of SA group and inferior to the upper limit quality of SCD group. Similarly, the lowest slaughter stress group retained the highest ATP, and the ATP content in muscles decreased from 3.13 μmol/g to 2.13 μmol/g in SCD group, compared with the 1.99 μmol/g in SCC group and 0 μmol/g in SA group at postmortal 12 h after slaughter, respectively. The time to drop to the limit pH in muscles was 48 h in the SCD group, 48 h in the SCC group, and 12 h after death in the SA group. The results of observation and colorimetric analysis showed that the strongest slaughter stress SA group had the worst muscle appearance, showing the lowest L^* value, higher a^* value and lower b^* value. The microscopic structure of the muscle showed that the intercellular space was the largest and most unstable in the SA group. The effects of slaughter stress intensity can also be obviously observed on the surface of fish body. The greater of stress, the more serious of congestion in fins. In conclusion, spinal cord destruction disrupts the motor nervous system with minimum stress intensity, minimizes muscle movements caused by spinal reflexes, and minimizes slaughter stress in living P. olivaceus , which is the key factor to meet the limit quality. This study provides a reference for controlling slaughter stress to obtain the best quality.