Quantitative microbial risk assessment on the risk of listeriosis posed by locally manufactured cheese products
Creators: name: Miles, DW
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis presents a process-specific strategy for the direct interpretation of the cheesemaking process in terms of microbial growth, utilising elements of risk assessment and predictive microbiology. Locally manufactured fresh cheeses (Ricotta and Mascarpone) and a surface-ripened cheese (Brie) are identified as short shelf life products, susceptible to microbial contamination. A hazard analysis is used to determine that Listeria monocytogenes poses the main danger from the consumption of these cheeses. Several previously published predictive models for describing the behaviour of L. monocytogenes are evaluated by two methods: - comparison of predictions with observed growth through a series of challenge tests, and comparison with published scientific data. The results of the evaluation process demonstrate that the model of Murphy et al. (1996), a model specifically designed by the authors for use in dairy products, is the most suitable for predictions of L. monocytogenes growth in these cheeses. For each cheese product, the manufacturing process is defined in terms of the primary parameters controlling growth of L. monocytogenes (temperature, pH and salt concentration), and a microbiological profile of the process used to highlight potential contamination sources. A stochastic modelling approach, utilising commercial @RISK simulation software, is used to account for process variability and a distribution is defined for each parameter. A simplified modelling approach is initially used to generate a series of outcomes for each process step, rather than a single point estimation of growth. Correlations are established which quantify the degree to which each parameter influences the growth of L. monocytogenes, thus allowing the objective determination of high risk factors and highlighting critical areas where control must be exerted to assure food safety, an ideal implicit in the implementation of HACCP-based food safety systems. Potential growth of L. monocytogenes is estimated from post-heat treatment of the milk through to the end of the shelf life of the cheese. Results from Brie manufacture demonstrate that pH development during the initial production phase is crucial in limiting potential L. monocytogenes growth, until brining of the cheese takes place, when the concentration of salt introduced into the product becomes the most significant factor. The inhibition due to the increased salt concentration remains significant through the shelf life of the product. Storage temperature becomes the most significant limiting factor as the cheese matures and the pH rises after wrapping. The Ricotta manufacturing process contains no significant limiting factors and growth is limited only by the rate of cooling once the curd is scooped. The development of a lowered pH in Mascarpone manufacture has a significant effect on L. monocytogenes growth, as does the cooling rate. A secondary, more rigorous modelling process is also presented, incorporating less well defined parameters such as rates of contamination, lag phase duration and exposure assessment to present a full risk assessment of the number of listeriosis cases that may result from the consumption of these cheeses. The results demonstrate that all three cheeses provide favourable environments for the growth of L. monocytogenes, and that control measures such as good manufacturing practices must be in place to ensure that contamination with this organism is a very rare event. The outcomes presented demonstrate that should a contamination event occur L. monocyto genes has the potential to grow during the manufacturing process and storage of the product leading to levels that may cause illness in susceptible consumers by the end of shelf life.