The Quality Chananges During Storage Of Mackerel And Specific Spoilage Bacteria

Mackerel（Pneumatophorus japonicus）is a kind of widely distributed,fast growth and high yield fish species.As it is rich in protein,fat and other nutrients,it has become the commercial importance fish loved by people in recent years.In this study,real-time quantitative PCR（Q-PCR）technology,sensory evaluation,microbial counts and general chemical analysis were used to detect the quality changes of mackerel during storage.And the growth models of histamine-forming bacteria were constructed through the detection of hdc gene by Q-PCR,which would help to optimize the mackerel storage conditions.Besides these,the metagenomics analysis was used to investigate the diversity of bacterial communities associated with mackerel under modified-atmosphere packaging and vacuum packaging conditions.The main conclusions are as follows:1.The seneory study showed that good quality life of mackerel fish stored at 4℃,15℃ and 25℃ were 144 h,24 h and 12 h,respectively,and the shelf life were 192 h,48 h and 20 h.At the ends of good quality periods and shelf lives under the different storage temperature,the mean TVB-N concentration were（22.97 ± 1.97）mg/100g and（49.38 ± 2.36）mg/100g,the average TMA values were（11.79 ± 1.94）mg/100g and（18.71 ± 2.62）mg/100g,the total number of colonies were（3.97 ± 0.53）lg（cfu/g）and（6.23 ± 0.52）lg（cfu/g）,the Pseudomonas bacteria were（3.57 ± 0.41）lg（cfu/g）and（6.01 ± 0.47）lg（cfu/g）,histamine producing bacteria hdc gene were（3.88 ± 0.55）lg copy number,（6.05 ±0.21）lg copy number,Shewanella tora gene were（3.56 ± 0.75）lg copy number,（5.78 ± 0.53）lg copy number.These data showed the results of Q-PCR,microbial count,TVB-N,and TMA values were consistent with sensory evaluation,and they could be used as an indicator to detect freshness of mackerel.2.When the copy numbers of hdc gene of histamine-forming bacteria and the accumulation of histamine were studied,they started by（3.15±0.21）log copies/L and（5.23±0.32）mg/100g in fresh fish,reached to maximum level of（5.38±0.23）log copies/L and（60.19±2.75）mg/100g at the end of shelf life under 4℃.These values reached to（4.73±0.33）log copies/L and（63.79±4.23）mg/100g under 15℃,and reached to（6.67±0.30）log copies/L and（106.60±5.31）mg/100g under 25℃.According to Chinese standard protocols GB/2733-2005（2005）guidelines,our data indicated the histamine level exceed the histamine limit of edible at the end of shelf life when storage at 25℃.3.According to the hdc gene quantified using Q-PCR,the growth prediction models of histamine producing bacteria were established.Under the storage conditions of 25℃ and 15℃,the results indicated the values of of Rss,Rse,AIC fitted by Gompertz model were minimum,and they were 0.022,0.103,-21.269 and 0.019,0.098,-21.760.At 4℃,the values of of Rss,Rse,AIC fitted by Logistic model were minimum,and they were 0.009,0.067 and-25.600.Thus,the optimal growth model of mackerel histamine producing bacteria was Gompertz model at 25℃ and 15℃,and Logistic model was appropriate for 4℃.4.Using the metagenomic approach,the diversity of bacterial communities and the dominant microorganisms associated with mackerel under modified-atmosphere packaging and vacuum packaging conditions were investigate.The dominant microorganisms of initial mackerel sample were Bacillus（2.2%）,Pelagibacteraceae（10.0%）,Halomonadaceae（8.5%）,Pseudomonas（2.8%）.Comparing with the initial mackerel sample,the microbial composition in modified-atmosphere packaging mackerel did not show a marked variation during storage,except the significant increase of Psychrobacter（12.0%）at the ends of good quality periods.Yet there was a substantial change in community composition of vacuum packaging samples.In which Shewanella（56.7%）was the dominant microorganisms at the ends of good quality periods,whereas Photobacterium（68.0%）was dominant at the ends of shelf lives.And these data indicated that the modified-atmosphere packaging（50%CO₂/50%N₂）was more effective than vacuum packaging,with regard to the freshness of mackerel.