Quality-determination-period Mechanism And Delayed Effects On Fresh And Frozen Products Of Farmed Brown Flounder (Paralichthys Olivaceus)

Marine white meat fish is an important category of aquatic products,which plays an important role in both domestic consumption demand of fresh products and international trade demand of frozen products.In this study,Paralichthys olivaceus was selected to systematically study the regulation mechanism of freshness lock during quality determination period(QDP),so did the delayed effect on post-quality determination period(P-QDP)of fresh and frozen products.Research objectives:firstly,to change the quality status quo of farmed white meat fish for domestic consumption;secondly,to understand the quality concerns raised by the industry for export frozen products.(1)Raw material properties of farmed brown flounderFirstly,the basic properties of healthy living brown flounder were analyzed.The results showed that:(1)The muscle of Paralichthys olivaceus had the typical characteristics of high protein and low fat of white fish,with protein content of 24.01%and fat content of 0.74%;Amino acid composition of muscle:it is rich in 17 kinds of amino acids,with essential amino acids accounting for 44.9%of the total amino acids.(2)Brown flounder has a certain natural storage stability,under the condition of quick killing and cooling,the maximum rigidity reached 44 h,and the maximum rigidity lasted for 4 h.The content of ATP was 3.59?mol/g and the hardness was up to 16.05 N.(3)Stability of muscle structure before and after rigidity:in the early stage of rigidity,myofibrils were intact and dense,and clear striated muscle could be observed.At the maximum rigidity,some mitochondria atrophied,dense material in intercrest was lost,and the shape of membrane was not continuous.At the same time,gradual Z-line damage,endoplasmic reticulum swelling and sarcomere length shortening due to muscle contraction were observed.After decompression,endoplasmic reticulum swelling was more obvious,sarcomere length was restored,I band was almost unrecognizable,and Z-line density was reduced and partially discontinuous.Collagen fibers were clear and stable before and after rigidity.(2)Quality limits relating to preslaughter stress during QDP and its delayed effect on the P-QDP quality stability of fresh farmed brown flounderStarting from the natural properties of healthy raw materials,the quality limit category from the theoretical upper limit to the possible lower limit was explored,and the relationship between the intensity of slaughter stress and the quality of brown flounder was interpreted from the lethal method,especially the delayed effect on post-quality determination period cold storage.Different methods of death were used to obtain upper quality of spinal cord destruction(SCD)and lower quality of suffocation in air(SA),compared with commercial operation of(SCC).The results showed that:(1)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 resolved stiffness.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.(2)The lowest slaughter stress group retained the highest ATP,and the ATP content in muscles decreased from 3.13?mol/g to2.13?mol/g in SCD group,compared with the 1.99?mol/g in SCC group and 0?mol/g in SA group at postmortal 12 h,respectively.(3)It dropped to the limit p H in muscles was 48 h in the SCD group,48 h in the SCC group,and 12 h after death in the SA group.(4)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 the stress,the more serious congestion in muscle.(3)Delayed effects of the onset of rigor mortis during QDP on the P-QDP quality stability of frozen farmed brown flounderIn order to understand the phenomenon and mechanism of thawing rigidity of cultured white meat fish,the delayed effect of quick freezing on the stability of frozen quality of cultured brown flounder during the quality determination period was investigated.Two groups of frozen before-rigor(F_br)and frozen after-rigor(F_ar)were designed to investigate the quality changes during the frozen storage period at 180 days.The results showed that:(1)Freezing before rigor rigidity can inhibit the biochemical degradation of fish during freezing storage.ATP content of frozen F_br group decreased from 3.60?mol/g to 0.45?mol/g at 180 d after freezing,while that of frozen F_ar group was 0.Freezing before rigor mortis also effectively delayed glycolysis.p H in F_ar group decreased to the lowest 6.35 at 90 d,while p H in F_br group decreased from initial 7.15 to 6.27 at 180 d.(2)Freezing before rigidity can effectively maintain the frozen storage stability of muscle protein.The initial Ca²⁺-ATPase activity of F_br group was0.084?mol Pi/(mg·min)and 0.059?mol Pi/(mg·min)at 180 d.The initial Ca²⁺-ATPase activity of Far group was 0.067?mol Pi/(mg·min),and the Ca²⁺-ATPase activity of 180 d was0.034?mol Pi/(mg·min).(3)In a certain frozen storage period,thawing rigidity occurred before rigidity,but the muscle structure was stable all the time.In the F_br group,muscle thawing and contraction were observed in microstructures within six months of storage,and the muscle tissue was intact and the cell space was small within 180 days,compared with the F_ar group after decompression.However,the gap between muscle cells in F_argroup was larger.In addition,the drop loss and fracture strength of F_br group were higher than those of F_ar group,which corresponded with F_br group.