Study On The Quality Of Large Yellow Croaker During Superchilling Storage And The Effect Of Epigallocatechin Gallate On The Quality Of Large Yellow Croaker During Superchilling Storage

Large yellow croaker（Larimichthys crocea）is an excellent marine fish with delicate flesh.It is regarded as one of China’s most important,economic,and commercial fishes.Fresh large yellow croaker flesh is susceptible to spoilage due to its high nutrition and water content.Superchilling is a storage technique by lowering the product temperature1-2℃below its initial freezing point to preserve safety and quality.However,the temperature of superchilling is in the zone of maximum ice crystal generation,and fluctuations in temperature can cause ice crystals to develop in an unfavorable direction,thus causing irreversible damage to the quality of marine fish.Therefore,the effect of temperature fluctuations on the quality of large yellow croaker during superchilling storage is yet to be evaluated.Furthermore,since microbial spoilage is still the main factor affecting the shelf life of superchilling,the addition of coatings containing EGCG,a natural preservative,to maintain the quality of large yellow croaker during superchilling storage becomes a possibility.The fencing technique delays the spoilage deterioration of large yellow croaker and extends its shelf life.The details of the study and the results are as follows.（1）The effects of different temperature fluctuation ranges（-3±1℃and-3±0.2℃）on the quality of large yellow croaker during superchilling storage were investigated.The results showed that the lower temperature fluctuation（SLF）group could effectively reduce the microbial population of large yellow croaker during superchilling storage.In addition,the SLF group could maintain higher water-holding capacity（WHC）,fixed water content and lower cooking loss.The results of total volatile basic-nitrogen（TVB-N）and K values showed that the SLF group samples could maintain higher freshness.However,larger temperature fluctuations（SHF）increased the deterioration rate of myofibrillar proteins and the oxidation levels of proteins and lipids.SEM results showed that the SHF group gradually broadened the water loss pathways of the samples,thus reducing the edibility by accelerating the meat deterioration.The above results suggest that SLF can effectively maintain the quality of large yellow croaker during superchilling storage,but cannot extend the shelf life measured by microbial limit.（2）To evaluate the antimicrobial impacts and biofilm influences on EGCG against Shewanella putrefaciens ATCC 8071.The minimum inhibitory concentration（MIC）of EGCG on S.putrefaciens was 160μg/m L.The growth curve exhibited that EGCG had a good antimicrobial activity.EGCG caused damages to the bacterial cell wall and membrane based the intracellular component leakage and cell viability analysis.The damage to the membrane integrity by EGCG confirmed by SEM and transmission electron microscopy（TEM）.SEM shows deformation of shape,TEM shows cell membrane and wall damage,and the leakage of cytoplasmic material.The treatment with EGCG at 0.25×and 0.5×MIC resulted in decreased motility and elevated levels of oxidative stress,leading to an increase in biofilm formation.These results demonstrated that EGCG may be used as a natural preservative to reduce S.putrefaciens in fish during storage.（3）The effect of gum tragacanth（0.5%w/v GT）and sodium alginate（1.5%w/v SA）active coatings incorporated with EGCG（0.16%,0.32%and 0.64%,w/v,respectively）on the quality of large yellow croaker fillets were evaluated during superchilling storage for 42 days.The results revealed that the GT-SA-EGCG active coatings delayed the microbial spoilage via reducing the numbers of total viable counts（TVC）,Pseudomonas spp.and psychrophiles bacteria,retarded the degradation of physical properties of WHC,cooking loss,color,odor and texture.Low-field nuclear magnetic resonance（LF-NMR）revealed that EGCG treatments might improve the association of water molecules with protein for fixed water.The GT-SA-EGCG coatings could also maintain the fillets’freshness by keeping lower TVB-N,K value and histidine content,inhibited the degradation of myofibrillar protein during superchilling storage.These results suggested that the GT-SA-EGCG active coatings could be used as a preservative to maintain the quality of large yellow croaker.（4）The active coatings supplemented with EGCG（0.16%,0.32%,and 0.64%,respectively）combined with superchilling storage were used to reduce hydrogen peroxide（H₂O₂）content,and inhibit lipid and protein oxidations of large yellow croaker during 42days of superchilling storage.EGCG coatings delayed lipid and protein oxidations by inhibiting the generation of H₂O₂,malondialdehyde（MDA）and carbonyl groups,and maintaining a higher Ca²⁺-ATPase activity and sulfhydryl content.We also found that EGCG treatments maintained myofibrillar organized secondary structure by keeping higherα-helix content,and also stabilized tertiary structure during superchilling storage.In addition,SDS-PAGE and SEM images both showed that these treatments could delay the myofibrillar degradation of fresh fish.Overall,we conclude that the active coatings containing EGCG treatments protect the lipid and protein of large yellow croaker during superchilling storage.（5）This research was to investigate the synergistic interactions of GT-SA active coatings incorporated with EGCG and lysozyme（LYS）on the quality of large yellow croaker during superchilling storage.Results showed that GT-SA active coatings containing EGCG（0.32%w/v）and LYS（0.32%w/v）reduced the TVC,psychrophilic bacteria and Pseudomonas spp.by about 1.55 log CFU/g,0.49 log CFU/g and 1.64 log CFU/g compared to the control at day 35.The GT-SA active coatings containing EGCG and LYS were effective in lowering the formations of off-odor compounds such as TVB-N,MDA and off-favor amino acid（histidine）.The solid phase microextraction-gas chromatography-mass spectrometer（SPME-GC/MS）was applied to characterize and quantify volatiles compounds of large yellow croaker samples during superchilling storage and the relative content of fishy flavor compounds（including 1-octen-3-ol and acetoin）was significantly reduced in the active coatings treated samples.Furthermore,the GT-SA active coatings containing EGCG and LYS treatments was found to be more effective in retarding the migration of water based on LF-MRI results,and in maintaining the organoleptic quality of large yellow croaker superchilling storage according to sensory evaluation results.The results showed that GT-SA active coatings containing EGCG and LYS were effective to be used as a fish preservative to improve the quality of large yellow croaker superchilling storage.