Effect Of Endogenous Enzyme Activity And Quality Change Of Northern Pike (Esox Lucius) In Low Temperature Storage

The northern pike(Esox lucius)is popular for its high nutritional value,low fat content and digestibility.However,due to the geographical limitations of northern pike growing conditions,it takes a long time to transport northern pike from the location of capture to market.A bad storage condition can easily cause the degradation of fish protein,structural changes,autolysis of endogenous enzymes,and deterioration of texture.Therefore,it can effectively improve the northern pike's good quality characteristics and economic benefits by improving the storage conditions.In this paper,we study the effects of physicochemical properties,cathepsin D activity,lysosomal membrane stability and mitochondrial permeability on the quality changes of northern pike under the conditions of refrigeration and partial freezing storage.The effects of changes in endogenous cathepsin B and B+L activity of different subcellular components under refrigeration and partial freezing storage on the quality of northern pike fillets were discussed.The effect of pullulan on the structure of myofibrillar protein in northern pike fillets under refrigeration storage was studied.We have discussed the influence of two storage conditions on fish quality and the effect of pullulan on myofibrillar protein of northern pike fillets under refrigeration storage condition theoretically.It can provide data support for the quality control of northern pike during low temperature storage.1.Changes of cathepsin D activity,lysosomal membrane stability,and mitochondrial permeability on the quality of northern pike fillets under refrigeration and partial freezing storageThe purpose of the experiment is to study the effects of endogenous cathepsin D activity and changes in the properties of two organelle membranes on the texture of fish fillets under two storage conditions.The back muscle of the northern pike was stored under refrigerated and partial freezing storage conditions,and changes in endogenous cathepsin D activity,lysosomal membrane stability,mitochondrial membrane permeability,and fish quality characteristics were measured.The results showed that the pH had the same trend during storage at 4? and-3?.The partial freezing group effectively slowed down the change rate of pH;the water holding capacity of fillets continued to decrease.The lysosomal membrane stability continued to decrease.Cathepsin D activity increased first and then decreased gradually,but the enzyme activity was always higher than that of partial freezing under refrigeration storage.Mitochondrial membrane permeability increased continuously.However,partial freezing would cause the generation of ice crystals and reduce the water holding capacity(WHC)of fish fillets,but It is beneficial to inhibit the adverse effects of endogenous enzymes on the degradation of fish fillets caused by the rupture of membranes of various subcellular organelles,and effectively prolong the shelf life of fish fillets.Refrigeration can better maintain the water holding capacity and chewiness of fish fillets only during short-term storage.2.The influence of endogenous cathepsin in different subcellular fractions on the quality deterioration of Northern pike(Esox lucius)fillets during refrigeration and partial freezing storageThe purpose of this study was to investigate the endogenous cathepsin activity in each subcellular fraction and the effect of this activity on myofibrillar protein and texture during refrigeration and partial freezing storage of northern pike(Esox lucius)fillets.The results showed that fillets stored under the refrigerated condition were more susceptible to oxidation than partial freezing.Endogenous cathepsin activity indicated that partial freezing destroys the integrity of lysosomes more effectively than refrigeration and inhibits the increase in cathepsin B and B+L in lysosomes.The activity of cathepsin B and B+L in lysosomes,mitochondria and myofibrils under the partial freezing conditions was always lower than that under refrigeration.Texture analysis showed that refrigeration had a negative impact on hardness and springiness.In conclusion,the cathepsin activity in each subcellular fraction was effectively inhibited and better textural characteristics were obtained with partial freezing than refrigeration.3.Effect of pullulan on the structure of myofibrillar protein in northern pike fillets under refrigeration storageIn this section,the effects of adding pullulan(1%,2%,and 3%)on the physicochemical and structural properties of northern pike fillets' myofibrillar protein under refrigeration storage conditions were studied.The results showed that low concentration of pullulan(1% and 2%)inhibited the increase of protein carbonyl content and the loss of total sulfhydryl content at the storage of first 6 days,but the inhibitory effect was lost at the later stage of storage.SDS-PAGE showed that the addition of pullulan can keep the protein from being degraded at first 6 days storage,but it has not obviously effect during the later period of storage.The existence of pullulan protects the good characteristics of fish fillets by slowing the degree of myofibrillar protein oxidation,protein secondary and tertiary structure and delayed protein degradation in the early storage period.In general,the addition of pullulan at the concentration of 1% can reduce the protein structure change and improve the quality of northern pike fillets during the shelf life(the first 6 d).