Study On Texture Softening And Protein Deterioration Change Of Puffer Fish(Takifugu Obscurus) During Refrigerated Storage

With the development of tetrodotoxin control technology and aquaculture technology,the safety and nutrition of puffer fish are recognized.In September,2016,the State Ministry of Agriculture and the State Food and Drug Administration jointly issued a document,that the processing business of farmed Takifugu rubripes products and farmed Takifugu obscurus products would be conditionally released.Puffer fish has long been popular among consumers because of delicious taste and it can be expected to have a broad market prospect.At present,puffer fish is mainly consumed as a delicacy in senior restaurants.There is also packaging products adopted 4oC refrigerated preservation after slaughtered,but the shelf life is only 4 days,thus extending the puffer fish shelf life and maintaining fish quality became an urgent problem to be solved.This paper use the farmed Takifugu obscurus as research object to study the texture degradation causes of puffer fish in refrigerated storage,and then the combination of mild heat treatment and cold storage have been further put forward,which had important guiding significance for prolonging the shelf life and provided some theoretical basis for the development of the puffer fish industry.First of all,iodoacetic acid and sodium azide solution were used to inhibit the activity of endogenous protease and microbes of soaked puffer fish respectively.The indicators including texture,total viable counts,endogenous protease activity and protein degradation were studied to research the role of endogenous protease and microbes in its texture deterioration.The result showed that the total viable counts and total psychrotrophic counts in the microbial inhibition group were below 4 logCFU/g and 3 logCFU/g respectively;the activity of endogenous protease was similar to that of the control group,and the content of ?-amino nitrogen and TVB-N was the lowest,indicating that protein degradation was effectively inhibited,but serious texture degradation still occurred during storage.In the enzyme-inhibition group,the 90% activity of cathepsin B,L and calpain was effectively inhibited,and the total viable counts and total psychrotrophic counts were similar to those of the control group.The content of TCA-soluble peptide was the lowest in the endogenous enzyme-inhibition group,indicating that the protein degradation products were effectively inhibited;meanwhile the texture deterioration of puffer fish was effectively alleviated.Therefore,although both endogenous proteases and microorganisms were involved in the texture degradation of refrigerated puffer fish,endogenous proteases were the main factors that are responsible for texture deterioration.The effects of heat treatment on the quality of puffer fish were studied by texture,total viable counts,endogenous protease activity and protein degradation.The results showed that the heat treatment could effectively sterilize and destroyed the enzyme.The total count of psychrotrophic bacteria in the direct refrigerated group was more than 6 log CFU/g at about 9th day.The initial contents of TCA-soluble peptide and TVB-N in the heat-refrigerated group were 0.6 ?mol Tyr/g and 21 mg/100 g,respectively,which were significantly higher than those in the direct refrigerated group,but there was no significant change throughout the whole storage period.The contents of TCA-soluble peptide and TVB-N in the direct refrigerated group significantly increased to 1.2 ?mol Tyr/g and 45 mg/100 g on the 15 th day,respectively.In addition,the TVB-N content reached the upper limit of spoilage of aquatic products on the 9th day,indicating that shelf life of refrigerated vacuum packaging puffer fish was about 9 days,and the combination of heat treatment and refrigeration had a longer shelf life,at least 15 days.From the observation of the microstructure,the gap between the muscle fibers in the direct refrigerated group was widening during the storage and the obvious fracture could be observed at the later stage of storage.The muscle fibers of the heat treatment-refrigerated group had a gap at 0 d,but there was no significant change during the following storage.The gap in heat treatment-refrigerated group was similar to that of the direct refrigerated group stored for 6~9 days.In addition,the mild heat treatment before refrigerated storage had a certain effect on the texture of puffer fish,but the quality is maintained better than the direct refrigerated group.To further compare the quality of the puffer fish fillets,the fresh,refrigerated,heat-refrigerated puffer fish fillets were heated,and the sensory evaluation,flavor composition and amino acid nutrition evaluation were used as indicators.The results showed that the sensory score of heat-refrigerated group was lower than that of refrigerated group at the beginning of storage,but the difference was not significant.The sensory score of refrigerated group decreased during storage,and it was significantly lower(P<0.05)than that of heat-refrigerated group on the 9th day.Compared with the fresh group,the relative content of the aldehydes increased and the alcohols decreased in heat-refrigerated and refrigerated group.With regard to the non-volatile flavor compounds,the content of free amino acids(Gly,131.32%)and nucleotides(AMP,446%)with fresh taste in the heat-refrigerated group was increased,and the bitterness amino acids(Phe,164.64%;Ile,338.16%;Leu,139.06%)were sharply increased.The amino acid composition and nutritional evaluation among three groups were similar.Therefore,heat treatment before refrigerated storage does not significantly affect the texture quality of puffer fish,but because of the degradation of protein and lipolysis during the heating process,the flavor of the fish is more abundant and the umami taste is markedly improved.