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Predictive model development and lag phase characterisation for applications in the meat industry


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Mellefont, Lyndal(Lyndal Anne) 2000 , 'Predictive model development and lag phase characterisation for applications in the meat industry', PhD thesis, University of Tasmania.

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Foods of bovine origin have been implicated as the principle vehicle in
disease outbreaks of pathogenic Escherichia coli, hence the increasing
interest in understanding the behaviour of this pathogen on carcasses
during processing and handling. In the absence of rapid and noninvasive
methods to determine the microbiological safety of meat
carcasses, predictive modelling has been suggested as a strategy to estimate
the consequences of processing and handling procedures on the fate and
numbers of microorganisms. This thesis reports two contributions in
realising the potential of predictive modelling for use in the food
industry: development and extensive validation of a growth model for
E. coli and advances in characterisation of bacterial lag phase duration.
A modified square-root model for predicting E. coli growth was developed
and its reliability assessed against a variety of data. The bias and accuracy
factors formed the basis of the model evaluations and provided an
objective summary of model performance. The model performed well
when compared to data generated in liquid growth media, ground beef
and for data collated from the literature. The model performed as well as,
or better than other published models for E. co/i growth. In particular, the
model predicted growth in meat and meat products better than other
models. Evaluation of predictive models in industry is considered the most
rigorous test of model performance. The deliberate introduction of
potential pathogens in abattoirs is precluded in Australia, thus the E. co/i
growth model could not be validated by trials on carcasses during normal
commercial processing. Instead, a predictive growth model for a
surrogate organism, Klebsiella oxytoca, was developed and evaluated
against data for the growth of K. oxytoca on carcasses during normal
chilling operations. Those studies suggest that predictive modelling can
be used to predict the average changes in numbers of bacteria on a carcass resulting from temperature and water activity changes caused by air
chilling processes.
Lag times have long been considered an uncontrollable variable in food
microbiology. Studies were undertaken to describe the effects of
environment and physiological history of the cell on lag times. Abrupt
temperature, pH and osmotic shifts of cultures were found to induce lag
phases in a variety of foodborne bacteria, highlighting the prospect of
inducing lags by manipulating the rate and extent of change of
environmental conditions. Variability in bacterial lag times was reduced
by using the concept of relative lag times or "generation time
equivalents", i.e. the ratio of lag time to generation time (RLT). The
physiological history of the cell, including growth phase and habituation,
affect the magnitude of the RLT response. In general, environmental
downshifts induce larger RLTs than equivalent upshifts. These
observations support the hypothesis that lag time can be understood in
terms of the amount of work to be done to adjust to new environmental
conditions and the rate at which that work is done. The results in this
thesis demonstrate that careful interpretation of RLT responses under
very stressful environmental conditions is required due to potential
changes in growth curve shapes. Additionally a normal physiological
range for water activity is proposed.
Characterisation of bacterial lag times using RLT simplifies their
inclusion in growth predictions, thus increasing the utility of predictive
models. Results in this thesis support those of Ross (1999) who observed
a common pattern of distribution of relative lag times for a wide range of
species across a wide range of conditions in the range of 4 to 6 generation
time equivalents.

Item Type: Thesis - PhD
Authors/Creators:Mellefont, Lyndal(Lyndal Anne)
Keywords: Escherichia coli, Food spoilage, Microbial growth, Food, Meat industry and trade
Copyright Holders: The Author
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Copyright 2000 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
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Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

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