Freezing Method Of Soft Shell Mud Crab(Scylla Paramamosain)and Its Quality Changes During Frozen Storage

To optimize the freezing method of soft shell crabs（Scylla paramamosain）,the quality changes of short-term frozen soft shell crabs which were treated respectively by refrigerator direct（CK7）,salt solution（SS）,liquid nitrogen（LN）freezing as well as long-term frozen soft shell crabs which were treated by LN freezing were studied in this paper.The p H value,texture characteristics,microbial index,biochemical characteristics,and small molecular metabolite profiles were investigated to unravel the effects of different freezing methods on the quality of soft shell crabs during 7 d-frozen storage and characterize the quality changes of soft shell crabs during 8 m-frozen storage.The main obtained results are as follows:1.The effects of three different freezing methods on the quality of soft shell crabs during 7 d-frozen storage were firstly studied.The p H value was significantly increased after one-week frozen storage without freezing treatment.However,LN freezing retarded the increasing of p H and SS freezing even caused lower p H than that in fresh crab muscle.LN freezing caused a higher texture property in hardness,spring and chewiness in the muscle of soft shell crabs than frozen control and SS freezing.LN freezing also caused lower values of TVC and TBARS,but higher protein concentrations in the muscle and hepatopancreas,as well as higher Ca²⁺-ATPase activity in the muscle than frozen control and SS freezing.Our metabolomic results further showed that highly accumulated dimethylamine in the frozen crab muscle was inhibited by SS and LN freezing and LN freezing resulted in a much closer metabolite composition to that of fresh control than SS freezing.These results suggest that freezing is necessary to prolong the frozen storage of soft shell crabs and LN freezing is more attractive in fresh-keeping than SS freezing.2.The quality changes of soft shell crabs frozen treated by LN freezing during long-term frozen storage were studied.The results showed that the quality of soft shell crabs decreased with the frozen storage time.This was manifested by the significantly decreased texture characteristics and Ca²⁺-ATPase activity in the muscle as well as the significantly decreased protein concentration,the significantly increased TVC and TBARS values in the muscle and hepatopancreas of soft shell crabs.Furthermore,a significant change was observed in the nutrient composition of soft shell crabs during storage at-20℃ even though the crabs had been treated LN freezing.The change was highlighted by the increased levels of muscle inosine,maltose,and glucose as well as hepatopancreas inosine,glutamine,and uridine.These nutrients may have minor negative or even positive effects on the quality and taste of soft shell crabs except inosine.Notably,no significantly accumulated DMA was observed in the soft shell crabs with LN freezing during 8 month-frozen storage.In addition,the bacterial community compositions of soft shell crabs at the phylum,family,and genus levels significantly changed during long-term frozen storage.Theα-diversity of muscle and hepatopancreas in soft shell crabs were not affected by frozen storage,but ANOSIM analysis showed that there were significant differences between the bacterial community compositions of each sample in muscle before and after frozen storage.The increase in the relative abundance of Enterobacteriaceae and Bacillus may indicate the shelf life of soft shell crabs during frozen storage.Moreover,Enterobacteriaceae had a significant positive correlation with inosine in muscle and Shewanellaceae had a significant negative correlation with inosine in hepatopancreas during frozen storage.The significantly decreased relative abundances of Rhodospirillaceae may induce the elevation of glucose and maltose in muscle.This study provides a new method for frozen storage of soft shell crabs and theoretical reference for understanding the quality changes of soft shell crabs during frozen storage.