| Cooked meat products are named because of low heating temperature(normally 68~72°C)during their processes.They have become popular among consumers since their better flavor and chewing taste,higher preserved nutrients together with more integrate meat tissue structure compared with high-temperature meat products.Cooked meat industry is developing rapidly in China and is bound to be the future of meat product industry.However,low heating temperature during the meat process leads to the survival of heat-resisting bacillus bacteria and its fast proliferation and growth during product storage process.This will cause gas formation of the package,together with corruption phenomenon such as color fading,dehydration,oil leakage and sticky status.As a result,this study focused on the bacteria physiological changes of cooked meat products during storage process,the discovery of specific spoilage bacteria(SSOs)and the solution of the above-mentioned problems.A variety of bacteria species can cause the corruptions of meat and meat products.In order to reveal effective ways of prolonging the shelf life of meat products,the bacteria phase changes and SSOs compositions during storage process must be researched.On the other hand,tracing the source of key spoilage microorganisms and identifying the spoilage characteristics of SSOs are also essential for solving the problem.Firstly,the changes of sensory quality,physicochemical quality and microbial quantity in vacuum packaged bacon during 45 days under 0-4°C were studied.The results showed that the microorganism quantity was low at early storage phase but later increased rapidly with storage time.During storage process,total bacteria count(PCA 30°C),psychrophilic bacteria(PCA 4°C)and lactic acid bacteria(LAB)increased more evidently,compared with Staphylococci,Enterobacteriaceae,Pseudomonads,Brochothrix thermosphacta,moulds and yeasts.Sensory score,pH value and redness value a*decreased;total volatile basic nitrogen(TVB-N),L*,b*,putrescine(PUT),cadaverine(CAD)and tyramine(TYR)showed an upward trend;A_w value,salinity,nitrite,thiobarbituric acid reactive substance(TBARS)did not change significantly.Aldehydes in volatile components showed a downward trend while acids,alcohols and phenols increased much.The substances with higher correlation coefficient were ethanol,2-furanmethanol,hexanol,1-propanol,phenol and acetic acid.The microbial diversity and dynamic changes of vacuum packaged bacon at 0~4°C were analyzed by means of microbiological traditional culture coupled with modern molecular technology denaturing gradient gel electrophoresis(PCR-DGGE)and high throughput sequencing technology(HTS).The results showed that 26 kinds of spoilage microorganisms were identified by traditional culture,separation and 16S rDNA methods,of which lactic acid bacteria accounted for a relatively large proportion.Total 13 kinds of bacteria were identified by combining PCR-DGGE and 16S rDNA gene sequence analysis,most of them were lactic acid bacteria,and there were fewer kinds of bacteria in the initial storage stage and Leuconostoc were the most prevalent species at the late storage.In comparison,HTS offers a more advanced method and provides a comprehensive description of the complex interactions among the species.336 different bacterial genera were detected in total by HTS showing that the microbial composition changed greatly with the prolongation of storage time.The diversity of microbiota was abundant at the initial stage then experienced a continuous decrease.Leuconostoc was the dominant bacteria at the storage metaphase and in the late stage of the storage period,while Lactobacillus was an overwhelming dominant genus at the end of the storage.They were recognized as the SSOs and may be related to the spoilage of bacon.Brochothrix,Serratia,Fusobacterium,Rahnella and Lactococcus were occasional components at some storage points of individual batches and may also be considered as potential contributors to the spoilage.Six points(raw meat,after curing,after stewing,after smoking,after slicing and after packaging)were sampled along the bacon processing line to investigate the dynamic changes of microorganisms by traditional isolation culture and HTS methods.The purpose was to trace the origin of SSOs,to find out the source,and to provide theoretical basis for product process improvement and quality control eventually.The results showed that among the 33 species of spoilage microorganisms identified by the two above mentioned methods,a large number of microorganisms grew on the raw meat and after brining and curing,while most of them were sterilized after cooking.HTS results showed that 428different species of microorganisms were identified and there were different dominant microorganisms in different processing stages.It was obvious that storage-stage SSOs,Leuconostoc,Vibrio,Pseudomonads and Staphylococci were mainly from the brining and rolled stage.It is speculated that water,spices and accessories were responsible for this phenomenon,and contact with processing machinery may also cause contamination.Therefore,the main work of this stage is using corrupt microorganisms to find out the source.Five spoilage bacteria,including P2(Staphylococcus xylosus),P6(Leuconostoc mesenteroides),P9(Carnobacterium maltaromaticum),P16(Leuconostoc gelidum)and P20(Serratia liquefaciens)were isolated by traditional methods during storage.The 5species were reversely inoculated into irradiated vacuum-packed bacon to evaluated the decay ability by monitoring the physiochemical properties,while the bacterial phase change and spoiling ability were detected by HTS method.The results demonstrated that P6,P9and P20 were the dominant spoilage bacteria with strong growth and putrefaction ability producing more lipids,ketones and acids,which had great potential for spoilage.Thirty-nine kinds of natural preservatives were selected to study their inhibitory effects upon 4 species of spoilage bacteria.We selected 9 kinds of preservatives with better effects using the MIC.The results showed that polylysine(?-PL),cinnamaldehyde,mustard,cinnamaldehyde,oregano,thyme,straw fruit,cinnamon and clove had good antibacterial effects.The MIC of these 9 natural preservatives was further studied,which were divided into 3 groups for formulation optimization.The first group included?-PL,cinnamaldehyde,mustard and Nisin;the second group included cinnamaldehyde,oregano and thyme essential oils;the third group was straw fruit,cinnamon and clove extract group.It was found that M3(oregano+thyme)of essential oil group and m1(clove+straw fruit)of extract group had better antibacterial activity by orthogonal optimization.Then bacon added with the three groups respectively was applied in the experiment.Based on the number of microorganism,decline of PH value and TVB-N,we found that?-PL,cinnamic aldehyde and mustard were better for bacteriostatic efficacy,0.125 g/kg compound with essential oils(oregano+thyme)and 0.25 g/kg complex spice extracts(clove+straw fruit)had good antibacterial effect,which can delay the corruption and prolong the product shelf life.This study aimed to produce safe,healthy and long shelf life cooked meat products. |
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