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Quantitative microbial risk assessment on the risk of listeriosis posed by locally manufactured cheese products


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Miles, David Wayne 2001 , 'Quantitative microbial risk assessment on the risk of listeriosis posed by locally manufactured cheese products', PhD thesis, University of Tasmania.

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This thesis presents a proces.s-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)¹, 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

¹. Murphy, P. M., Rea, M. C. & Harrington, D. (1996). Development of a predictive model for growth of Listeria monocytogenes in a skim milk medium and validation studies in a range of dairy products. J. Appl. Bacterial. 80: 557-564.

Item Type: Thesis - PhD
Authors/Creators:Miles, David Wayne
Keywords: Food, Listeriosis, Foodborne diseases, Cheese industry, Cheese
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

No loan or copying until 14/6/03. Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

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