Understanding microbial spoilage mechanisms of modified atmosphere packaged live mussels for quality improvement and shelf-life extension
Creators: name: Odeyemi, Olumide
Publisher: name: University Of Tasmania
Year: 2023

description: Post-harvest conditions have a significant effect on the quality and shelf-life of seafood particularly live seafood such as mussels and oysters. The short shelf-life is a limiting factor for supplying live mussels to consumers. Currently, Tasmanian live mussels are distributed in modified atmosphere packaged (MAP) pouches with elevated oxygen concentration giving a shelf-life of 7-10 days. The main objective of this study was to understand the biological and physical mechanisms that lead to reduced product quality and spoilage of MAP live mussels (Mytilus galloprovincialis) to enable systematic improvements to be made. In Chapter 2, the optimization of post-harvest storage conditions required for retaining freshness and quality of MAP live mussels was investigated. The impacts of storage temperature, initial headspace oxygen and duration of depuration on shelf life were determined in three phases. No commercial reference was used in phase 1 but was used in phases 2 and 3. The commercial packs were packed by and according to the company supplying the mussels used in this study. MAP live blue mussels were stored at the following temperatures (phase 1): 4vÄv¿C, 6.5vÄv¿C, 10vÄv¿C and 13vÄv¿C for 240 hrs, 120 hrs, 96 hrs and 78 hrs. Three concentrations of oxygen gas headspace composition: 100% O\\(_2\\) (M100), 80% O\\(_2\\) and 20% N\\(_2\\) (M80) and 60% O\\(_2\\) and 40% N\\(_2\\) (M60) were investigated. The results showed that storage of MAP live mussels at 4¬¨‚àûC with 80% initial oxygen in the headspace was most effective in terms of length of shelf-life. In phase 2, the headspace oxygen of the commercial packs reduced significantly from day 7 onwards. In phase 3, mussels were depurated for 4, 8 and 12 hr and packed at optimised headspace oxygen (80%) and stored at optimised temperature (4¬¨‚àûC). This showed that 8 hr depuration of harvest mussels was more practically effective (requires less time) when compared with other treatments. In Chapter 3, the microbial quality, freshness and volatile organic compounds (VOC) produced during storage of MAP live mussels at refrigerated temperature (4¬¨‚àûC) were evaluated using headspace solid phase micro-extraction gas chromatography mass spectrometry (HS-SPME GC/MS). Multivariate statistical analysis was used to identify VOC contributing to either freshness or spoilage. The shelf-life of commercial packs (CP) and undepurated samples was found to be 7 days based on microbial counts and mortality; while that of depurated samples was 10 days. Freshness volatiles such as hexanal, heptanal, and octanal decreased with storage while spoilage volatiles such as dimethyl trisulphide, decanal, phenol and octadienol increased with storage. Dimethyl disulphide was more dominant in CP and undepurated samples than in depurated mussels and could be used as spoilage indicator. In Chapter 4, 16S rRNA amplicon sequencing was used to describe spoilage microbial community succession in the mussel meat and pouch water of undepurated and depurated mussels of MAP live mussels stored at 4vÄv¿C. The microbial community diversity on days 7 and 10 were observed to be closely related. Spoilage microbiota became predominant mostly between day 7 and 10. Proteobacteria, Cyanobacteria and Firmicutes were the three major phyla observed in the mussel meat and pouch water of undepurated and depurated mussels. Only Proteobacteria were observed to be dominant in both mussel meat and pouch water of commercially packed mussels. Among these phyla, Cyanobacteria were more predominant on day 0 in mussel meat of undepurated and depurated mussels while Proteobacteria were predominant in commercially-depurated mussels. Proteobacteria were dominant across the storage days in both undepurated and commercially packed mussels. They were dominant from days 0 ‚Äö- 10 in depurated mussels with Firmicutes becoming predominant by day 15). Synechococcus was dominant on days 0 ‚Äö- 7 in the mussel meat of undepurated mussels and days 0 -10 in depurated mussels. Shewanella was dominant on day 0 in commercially packed mussels. Acidaminococcus became dominant on day 10 in undepurated and day 15 in depurated mussels. However, Psychrobacter was observed to be dominant in commercially-depurated mussels on day 7 and further shift to Acinetobacter occurred by day 15. In the pouch water, Acinetobacter was dominant throughout the storage days in undepurated mussels while Psychrobacter was predominant in both depurated and commercially-depurated mussels. Commonly, Acinetobacter and Psychrobacter preceded Shewanella and Acidaminococcus as spoilage bacteria in live mussels. In Chapter 5, the spoilage potential of specific spoilage bacteria in MAP live mussels was evaluated. A total of 46 H2S-producing bacteria were isolated. Twenty-eight isolates were obtained from pouch water and 18 from mussel meat. The isolate with the highest enzymatic activities (protease, lipase and DNase) and H2S production was identified by 16S rRNA analysis as Shewanella baltica. An axenic culture of the isolate was inoculated into sterile mussel broth and incubated at 4¬¨‚àûC for 10 days. Volatile metabolites produced during storage were evaluated with HS-SPME GC/MS. Dimethyl trisulphide, nonanal, decanal, phenol, pentyl furan 2 and octadienol were off-odour volatile metabolites produced by spoilage bacteria during storage of cooked mussels at 4¬¨‚àûC for 10 days. In Chapter 6, a Quality index method (QIM) tool that adapted the existing QIM concept and applied it to the assessment of MAP live mussels was developed in this study. The QIM tool consisted of 3 quality attributes with 7 parameters with 28 descriptors and up to 21 demerit points. A linear model was obtained that described the change in quality attributes (QI) with respect to storage days (QI = 1.261 x storage days - 1.6789, R¬¨‚â§ = 0.9546, n = 3). The total viable count (TVC), mortality and headspace CO\\(_2\\) all increased with storage days while headspace oxygen decreased. The shelf-life was estimated as 10 days based on acceptance rejection (due to objectionable smell) by the assessors as well as microbial and biochemical assessment. The QIM tool developed could be used to assess the freshness and acceptance of live mussels. In conclusion, inadequate duration of depuration, pouch water and low headspace oxygen in the gas atmosphere contribute to the spoilage of live mussels. Inadequate duration of depuration likely enhances succession of spoilage bacteria by leaving a higher residual microbial population, while pouch water provided the biological medium for extensive microbial growth. Microbial respiratory consumption of the reduced headspace oxygen may stress the mussels which may result in mortality and evolution of volatile compounds that led to objectionable smell in MAP live mussels.