Freshness Change And Factor Affecting IMP Degradation During Sturgeon Storage

The sturgeon has no intermuscular thorns and is rich in unsaturated fatty acids and proteins,which has a beneficial effect on human health and is one of the major freshwater aquaculture species in China.At present,sturgeon meat is mainly frozen storage.Since freezing causes problems such as juice loss,tenderness reduction and protein denaturation,it is necessary to further study the quality changes of non-frozen storage of sturgeon meat.In this paper,the changes of freshness of sturgeon during storage at 4? and ice storage at?1?were studied.The effects of spoilage microorganisms and endogenous enzymes on IMP of sturgeon were determined.Firstly,the effects of non-frozen storage(4? and ?1?)on the freshness of sturgeon meat and the effect of pre-bacteria treatment on the freshness quality of sturgeon meat were investigated,and whether there was a certain relationship between the spoilage bacteria and the degradation of sturgeon meat IMP.The surface of 75% ethanol sturgeon meat was used for bacteriostatic treatment to compare the freshness of untreated sturgeon meat and 75% ethanol sterilized sturgeon meat at 4?(refrigerated)and ?1?(ice temperature)during storage(total number of colonies,TVB-N value,K value,IMP content,pH value and TBARS value).After 75% ethanol treatment,the total number of salmon colonies decreased by about 2 log CFU/g.At 4?,the total number of colonies in the untreated group was always 1.5 to 3.5 log CFU/g higher than that in the 75% ethanol reduction group,and the difference between the two groups was 1.2 to 2.3 log CFU/g at ?1?.At two temperatures,the TVB-N value of the untreated group was always significantly higher than that of the 75% ethanol reduction group,but the differences in K value and IMP content between the two groups were small or even no difference.At 4?,the pH value of the 75% ethanol reduction group was lower than that ofthe untreated group,while at?1?,the pH value of the 75% ethanol reduction group was only lower than the untreated group at 4 to 14 days,and the four groups of salmon storage were stored.The final pH value was lower than the initial pH(6.53),and the TBARS value of the75% ethanol reduction group was always smaller than that of the untreated group.The above results indicated that the bacteriostatic treatment had significant effects on the TVB-N and TBARS values of sturgeon meat stored in chilled storage and ice temperature,but the effect on K value and IMP content was not obvious,suggesting the effect of exogenous spoilage bacteria on IMP degradation may be small.In order to verify the above speculation,the changes of microbial structure during chilled storage(4?)and ice temperature(?1?)of sturgeon meat were further investigated.The dominant spoilage bacteria were screened and their ability to degrade IMP was detected by liquid culture.The results showed that the dominant bacteria in the early stage of storage of sturgeon meat was Acinetobacter,and the dominant spoilage bacteria in the middle and late stages were Shewanella and Pseudomonas.The six dominant spoilage bacteria selected were A.Johnsonii and A.lwoffii in the Acinetobacter genus,S.baltica in the Shewanella genus,P.fragi,P.koreensis and P.antarctica in the Pseudomonas genus.The degradation of IMP(initial value 0.160 mg/mL)and the secretion of extracellular ALP,ACP and 5'-NT in LB medium during the culture of these six strains were analyzed.The results showed that the content of IMP in S.baltica,P.koreensis and P.antarctica decreased first and then increased.After 48 h,the IMP content was 0.208 mg/mL,0.181 mg/mL and 0.159 mg/mL,respectively.A.lwoffii and P.fragi had a slight consumption of IMP.After 48 h,the IMP content was 0.135mg/mL and 0.138 mg/mL,respectively.A.johnsonii showed a sustained consumption of IMP,and the IMP content decreased to 0.048 mg after 48 h.It was found that the activities of ALP,ACP and 5'-NT in the cultures of the six dominant spoilage bacteria were very low,and only5'-NT had a slight degradation effect on IMP.Therefore,it is concluded that the dominant spoilage bacteria represented by Shewanella and Pseudomonas have no direct contribution tothe degradation of IMP in the middle and late stages of storage of sturgeon meat.Finally,the effects of endogenous and spoilage bacteria on IMP degradation during non-frozen storage(4? and?1?)of sturgeon meat were further investigated by heating the endogenous enzyme and adding endogenous enzyme inhibitors in a water bath.The results showed that the spoilage bacteria grew faster during the storage period at 65? for 2 h,but the IMP did not degrade.It further suggested that the spoilage bacteria had no effect on the degradation of IMP in the sturgeon meat.The endogenous ACP activity of sturgeon meat was very high,but no effect on the degradation of IMP;ALP inhibitor provanadate and 5'-NT inhibitor nickel chloride have strong antibacterial effect,the total number of colonies in ALP inhibition group and 5'-NT inhibition group is very high.Moreover,the content of IMP in the two groups of sturgeon meat was only slightly decreased in the middle and late stages of storage,but both were much higher than the control group.It can be seen that endogenous ALP and 5'-NT are the main factors affecting the degradation of IMP in squid meat,while exogenous spoilage bacteria have almost no effect on IMP degradation.The above results will provide suggestions for the control technology of sturgeon meat storage quality,especially the freshness control of non-frozen sturgeon meat products.