Preliminary Study On Relationship Between Nutrition And Fish Quality Of Farmed Large Yellow Croaker(Pseudosciaena Crocea)

Large yellow croaker (Pseudosciaena crocea) is a specific and famousmariculture fish species in China. Compared with the wild one, there are severequality problems in farmed fish, such as vague yellow body color, degenerated fishflavor and loose flesh quality. This study is to quantitatively compare the differencesin fish quality between the wild and the farmed large yellow croaker. The former isused as the standard. Furthermore, the present study is conducted to preliminarilyestablish the evaluation system about fish quality of this species, and screen the mainquality indicators covering the morphology, color, flesh quality and flavor. Meanwhile,this study analyzes the trends of the related quality indicators changing with thegrowth stage of large yellow croaker fed two kinds of diet. In addition, the effects ofdietary protein or lipid on the fish quality are also studied. The main results are asfollows:1. Comparative study on quality of wild and farmed large yellow croakerThe study is forced on the quality differences between wild fish (WF) andfarmed fish fed trash fish (TF) or fed feeds (FF)(body weight:500g). Results showthat there are significant differences in the morphology, color, flesh quality and flavorof the fish among three treatments (P<0.05). Compared with wild fish, farmed fishshowed the fattier body, whiter color, softer muscle, higher fishy odour and odourintensity and lower taste feeling. Moreover, farmed fish fed feeds can improve the fishquality in morphology, body color and flesh quality aspect in compared with fish fedtrash fish. These traits can be quantitatively evaluated by the followed indictors:(1) inmorphological aspect, condition factor of WF is significantly lower than that of TFand FF (P<0.05), and it in FF treatment is also significantly lower than in TF treatment (P<0.05).(2) In fish color, yellowness (b*) of skin and redness (a*) ofmuscle in WF are significantly higher than those in TF and FF (P<0.05). However,lightnees (L*) of skin and muscle in WF are significantly lower than those in TF andFF (P<0.05). In addition, yellowness of ventral skin in FF is higher than in TF(P<0.05).(3) In flesh quality, flesh hardness, alkaline-soluble hydroxyproline (a-sHyp), alkaline-insoluble Hyp (a-i Hyp) and total Hyp contents in WF and FF aresignificantly higher than those in TF (P<0.05). However, there is no significantdifference between WF and FF (P>0.05). WF had significantly higher pyridinolinecrosslink concentration (PYD) and moisture contents but lower fat and proteincontents in muscle than the other fish (P<0.05).(4) In fish odour aspect, WF has thelowest muscle fatty acid composition and content, which results in lower volatilearoma compounds species and total content in WF than those in TF and FF (P<0.05).Compared with TF and FF, WF has the highest muscle pH and trimethylamine oxide(TMAO) content but lowest trimethylamine (TMA) content. FF has significantlyhigher TMA and lower TMAO than TF.(5) In fish taste aspect, the total free aminoacid, inosinic acid (IMP) and mineral (such as Ca, Na and Mn) content are higher inWF than those in TF and FF (P<0.05).2. Effect of dietary type and fish size on the muscle quality of large yellowcroakerA2×3factorial experiment is conducted to track and compare the muscle qualitydifference of fish between fed trash fish and fed feeds when fish weight is100g (smallsize),300g (middle size) or500g (large size), respectively. From this, the study is toanalyze the trends or deposition law of the related quality indicators. Results showthat with fish size increasing, condition factor, hardness, springiness, chewiness anda-i Hyp, total Hyp, muscle protein and lipid, IMP and TMAO contents are increased(P<0.05); however, muscle pH, moisture, total volatile aroma compounds content andTMA content are decreased (P<0.05). These indicate that large size fish have fatterbody，firmer muscle and better taste. However, small size fish have higher odourintensity and fishy odour. The effect of two dietary types on the muscle quality of thisfish showed that fish fed feeds have longer body and firmer muscle; fish fed trash fish have higher total volatile aroma compounds content and TMAO content (P<0.05).3. Effect of dietary protein levels on the muscle quality of large yellow croakerThree protein level diets are used in the study, which are30%protein level (lowdietary protein treatment),42%protein level (growth-optimum dietary proteintreatment) and50%protein level (high dietary protein treatment). After63daysgrowth trial, the precent study is to compare the effect of dietary protein levels on themuscle quality of large yellow croaker. Results show that (1) fish fed50%protein diethas significantly higher condition factor than fish fed30%and42%protein diet.(2)High dietary protein level can significantly improve flesh hardness, muscle crudeprotein and lipid content, as well as water/salt solution protein and collagen content(P<0.05).(3) Muscle lipid content, TMA content, volatile aroma compounds speciesand total content are significantly increasd with increasing of dietary protein levels(P<0.05).(4) Total free amino acid and IMP content can be significantly improve bydietary protein level increasement (P<0.05). Hence, high dietary protein level canimprove the flesh quality of this fish, but also can increase fat deposition in muscleand cause high odour intensity and fishy odour. Low dietary protein level leads to lowdelicious taste of this fish for low taste compounds contents.4. Effect of dietary lipid levels on the muscle quality of large yellow croakerThree lipid level diets are used in the study, which are3%lipid level (low dietarylipid treatment),12%lipid level (growth-optimum dietary lipid treatment) and18%lipid level (high dietary lipid treatment). After60days growth trial, the present studyis to compare the effect of dietary lipid levels on the muscle quality of large yellowcroaker. Results show that (1) flesh hardness and water-holding capacity, as well asmuscle collagen and lipid content of large yellow croaker can be significantlyimproved by dietary lipid level increasement (P<0.05).(2) Muscle lipid content, TMAcontent, volatile aroma compounds species and total content are significantly increasdwith increasing of dietary lipid levels (P<0.05).(3) Fish fed12%lipid diet has thehighest muscle IMP content, which is significantly higher than fish fed3%lipid diet.However, Fish fed3%lipid diet has significantly higher TMAO content than fish fed18%lipid diet. Hence, low or high dietary lipid level can decrease flesh quality and delicious taste. The muscle lipid and fatty acid content are increased with dietary lipidlevels increasing. These lead to increasing of odour intensity and fishy odour.