Fresh-Keeping Processing Technology Of Grouper By Liquid Quick-Freezing And Its Effect On Meat Quality

Grouper is a high-quality seawater fish with delicious taste and rich nutrition,high content of unsaturated fatty acids and mineral elements.In recent years,with the rapid development of grouper breeding and aquaculture technology,grouper aquaculture production has increased year by year,the annual output has exceeded 100,000tons.Influenced by the consumption consciousness of"raw seafood"in China,grouper mainly sells fresh seafood,and some of them are frozen.However,the traditional freezing processing is not conducive to the maintenance of good quality of grouper.Because of the high price of grouper,the fresh-keeping and intensive processing technology of grouper lags behind,which seriously restricts the development of grouper industry.Therefore,in order to develop the circulation and marketing mode of grouper and meet different market demands.In this paper,grouper as the research object,on the basis of previous scholars'research,we further screened and optimized the refrigerant used in liquid quick-freezing aquatic products,and optimized the cooling conditions of grouper before quick-freezing,the effects of quick-frozen liquid treatment on the quality and shelf life of grouper during normal temperature logistics were studied,and the quality changes of liquid quick-frozen grouper under different frozen conditions were discussed,which provided a theoretical basis for freshness preservation and frozen storage of grouper.The main research contents are as follows:1.A formulation of cryogenic quick-frozen liquid for aquatic products was developed:The freezing point of ethanol,fructooligosaccharide,citric acid and calcium chloride at different mass fractions was analyzed by single factor test.On this basis,taking freezing point and viscosity as response values,the dosage of refrigerant in quick-freezing liquid was optimized by Box-Behnken response surface method.The optimum formula of quick-frozen liquid is obtained as follows,19.9%ethanol,9.5%fructooligosaccharide,3%citric acid,5%calcium chloride and 10%propylene glycol.The freezing point and viscosity of the quick-frozen liquid prepared by this formula can reach-63.50?and 4.64 mPa·s.2.The effects of different lethal methods on the quality of grouper during ice storage were discussed in order to provide reasonable pretreatment methods for improving the quality of grouper meat during ice storage.The grouper was treated with five different lethal methods?I:put live fish in crushed ice for 20 min;II:knock on the head to death;III:live grouper lethal after temperature gradient of 18?,14?,10?,6??20 min after each temperature treatment?;IV:live grouper died after temperature gradient of 18?,12?,6??20 min per temperature treatment?;V:live grouper lethal through temperature gradient 18?,6??each temperature treatment 20 Min??,and the grouper was frozen in liquid and then stored in ice?0??.The effects of sensory evaluation,texture characteristics,pH,lactic acid content,sulfhydryl content and total bacterial colonies on the quality of fish during ice storage were determined and analyzed.The results show that the effects of five lethal treatments on the changes of lactic acid content and pH of fish meat during cold storage were as follows:group III>group IV>group II>group I>group V.The order of influence on the change of sulfhydryl group and the total number of colony is:group III>group IV>group V>group II>group I.According to the results of comprehensive quality evaluation index,the lethal mode of group III is the best,the stress reaction of grouper death is small,the lactic acid content and pH value are relatively stable,which is conducive to the quality maintenance of the storage process,and is more effective than the other four groups.The texture characteristics and the content of sulfhydryl groups are reduced,the microbial activity is inhibited,and the fish-like shelf life is prolonged.3.The effects of quick-frozen liquid treatment on the quality and shelf life of grouper during direct logistics at room temperature were studied.The results showed that fresh grouper treated with quick-frozen liquid can not only maintain the quality of fresh fish meat,but also significantly prolong the shelf life of normal temperature circulation.The logistics time can reach 80 hours.Under this time,its TVB-N reaches26.88 mg/100g,K value 47.965%,colony number 5.88lg?CFU/g?.The muscle fiber structure of fresh grouper is intact,close to fresh fish,and its sensory evaluation is good.It is 10 hours longer than that of static air freezing group,and is better than that of non-freezing group.The frozen control group was prolonged for 30 hours.4.The quality changes of grouper treated with cryoprotectant during storage under different freezing conditions were discussed.The quality changes of grouper during storage were studied by comparing the freezing rate and water loss rate,pH value,texture characteristics,TVB-N,K-value,tissue structure and Ca²⁺-ATPase activity.The results showed that the water loss rate,TVB-N and K value of fish treated by three different methods increased,the pH value decreased first and then increased,the structure of myofibril changed,the thawing rate,hardness,cohesion,elasticity and chewiness of grouper decreased,and the activity of Ca²⁺-ATPase decreased.The freezing effect of liquid quick-freezing at-30?is better than that of liquid quick-freezing at-18?,which is better than that of air-freezing at-18?.During storage,the liquid-frozen grouper had slower protein denaturation,less protein loss,more uniform ice crystals,less damage to tissue structure,better water holding capacity and elasticity than the control group,and better quality;The quality of grouper frozen at-30?is better than that frozen at-18?.