Understanding and minimising microbiological spoilage of chilled vacuum-packed lamb

Beef and lamb produced in Australia typically have shelf-lives of 26 and 12 weeks respectively when vacuum-packed (VP) and stored at temperatures of -1.5 to 0°C. The faster rate of quality loss of VP lamb is thought to be mainly due to its intrinsically higher pH (~ 0.4 pH units cf. beef) from large amounts of adipose tissue compared to beef. Meat with a higher pH increases bacterial growth rate, including the growth of spoilage bacteria. The inherently shorter shelf-life of lamb compared to beef meat can present a challenge in long distance export supply chains in which value loss may occur due to quality being compromised from unforeseen delays and temperature fluctuations. Therefore, the primary objectives of the research reported in this thesis are to provide insights into microbial spoilage mechanisms of chilled VP lamb, and to investigate potential interventions to extend the shelf-life. This was initially approached by investigating whether representative bacteria of meat spoilage communities independently play important roles in spoilage of chilled VP lamb. Subsequent studies evaluated the potential for glucose surface treatments to extend the shelf-life of VP lamb products. The latter experiments were expanded upon by investigating the effects of glucose treatments on the meat microbial community and the associated meat volatilome. There have only been a limited number of studies that have investigated the spoilage mechanisms of chilled VP lamb, as the focus has been on beef due to the larger size of that export market. However, the biochemistry of beef and lamb is different and by extension so are the spoilage processes. Therefore, an initial study was undertaken to establish whether particular organisms independently play important roles in the rate of spoilage of chilled VP lamb. The spoilage potential of 13 representative bacterial strains derived from the spoilage community of VP lamb was investigated through a series of shelf-life trials. Each isolate was individually inoculated onto sterile and non-sterile (i.e., containing natural microbiota) VP lamb meat. Meat quality was assessed over time by measuring sensorial qualities, bacterial growth and pH. Among all test organisms, Clostridium spp. had the highest spoilage potential and independently played a major role in spoilage. Specifically, Clostridium estertheticum caused premature ‘blown pack’ spoilage; however, the onset of this spoilage was delayed in the presence of the meat microbial community. Clostridium putrefaciens and Clostridium algidicarnis also caused premature spoilage of VP lamb both independently and in the presence of the meat microbial community. Of the facultative anaerobes tested Carnobacterium divergens and Serratia spp. were capable of spoiling meat at a faster rate when present in a community. These results suggest that these species have high potential to form metabolic by-products that lead to off-odours indicative of spoilage. The communityrelated responses observed also suggests that spoilage is implicitly affected by community member interactions including competition and exchange of metabolites. Overall, these studies highlighted that Clostridium spp. are important for causing quality loss of chilled VP lamb. Such knowledge helps to inform approaches that can enable shelf-life extension. Due to the relatively short shelf-life of lamb and the associated challenges it presents to exporters, there is an impetus to develop cost-effective interventions to extend the shelf-life of Australian lamb products. Therefore, a component of this research was to evaluate the efficacy of glucose (0.5%, 1%, 2.5%, 5% and 10% w/v) as a surface treatment for shelf-life extension of VP bone-in and boneless lamb. This was investigated through a series of trials, in which the dynamics of the sensorial qualities, bacterial growth, pH, residual glucose and lactic acid were measured. Added glucose extended the shelf-life of bone-in and boneless lamb, by 8% to 76% (2 to 9 days) relative to the control, with the 5% treatment being the most effective in both cases. Glucose treatments reduced meat pH, potentially affecting the microbial community composition as well as the rate and type of accumulation of spoilage metabolic by-products. The results indicate that the application of glucose has the potential to be developed for shelf-life extension of VP lamb. Subsequent studies were undertaken to investigate the effects of glucose surface treatment to influence the microbial community and associated volatilome of VP lamb. To achieve this, changes in the microbial community composition (16S rRNA gene amplicon sequencing) and volatilome (solid phase micro-extraction method coupled with gas chromatography-mass spectrometry) of bone-in and boneless lamb shoulder were assessed as dictated by storage time, glucose treatments (0.5%, 1%, 2.5%, 5% and 10% w/v), and odour scores. Glucose treatments had a significant effect on the microbial community composition on both meat cuts. The most effective glucose treatments for bone-in (1% and 5%) and boneless (5%) lamb led to significantly lower numbers of various Vagococcus amplicon sequence variants, and all tested glucose concentrations on bone-in lamb had significantly lower numbers of S. proteamaculans. Additional glucose is likely to alter the microbial community composition by shifting substrate utilisation and the accumulation of acidic end-products, having an overall impact on the rate and type of spoilage metabolic by-products. However, no difference was detected in the composition of organic volatile compounds between glucose treatments for either bone-in or boneless lamb. There was a significant increase in a range of sulphur, ester and carboxylic acid compounds later in storage, indicative of microbial and possibly postmortem metabolism that contributes to a decline in the odour quality of the meat. These findings provide an insight into the microbiological mechanisms underpinning shelf-life extension achieved by glucose addition; however, further investigation is required to determine how it influences the manifestation of spoilage metabolites. Overall, the findings of this PhD thesis provide fundamental knowledge of the microbiological spoilage mechanisms of chilled VP lamb produced in Australia and highlights the importance of Clostridium spp. in spoilage rate. These findings can help to inform opportunities for shelflife interventions targeting organisms with ‘high’ spoilage potential. The results also indicate that the application of glucose has the potential to be developed for shelf-life extension of VP lamb