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Modelling pseudomonad growth in milk and milk-based products


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Neumeyer, K 1995 , 'Modelling pseudomonad growth in milk and milk-based products', Research Master thesis, University of Tasmania.

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Predictive microbiology is a method by which the growth responses of
microorganisms of concern are modelled mathematically in respect of the major
factors affecting growth. A model for the growth of Pseudomonas (the main
organism of concern in refrigerated milk and some milk products) was developed
and validated.
Psychrotrophic psuedomonads were isolated from various modified and whole
milks, using both Victorian (obtained from CSIRO Dairy Research Laboratories)
and Tasmanian milks. Growth rates in artificial broth media were determined
using turbidimetric methods. By fitting a sigmoidal curve (using the Gompertz
function) to the data, generation times can be calculated and growth rates (by
turbidimetric or viable count methods) determined. All strains were grown in
artificial broth media at 10°C and growth rates compared. The fastest growing
strain (Pseudomonas putida 1442) was used to develop the model i.e., a worst
case approach was adopted. A square root model (Ratkowsky et al, 1982) was
developed for P.putida 1442 in artificial broth media by determining the
generation time at 0.5°C intervals from 0 to 50 °C. A similar procedure was used
to model the effect of water activity (using sodium chloride as the solute) on
Pseudomonas spp.
Generation times calculated by turbidimetric and viable counts were found to
differ. This difference was found to be constant with respect to temperature and
was incorporated into the modelling process so that all models expressed
generation times equivalent to those calculated by viable counts, the standard
method for enumerating microorganisms in food products.
A literature search comparing the notional minimum temperature (T min) for
growth of psychrotrophic pseudomonads found that the Tmin was the same
(265.4K ± 0.7) despite the source of the organism. This implies that only one
model for temperature dependence is needed for psychrotrophicPseudomonas in
dairy, meat and poultry products. The model was validated both in the laboratory and in industry using various milk
and milk based products. The validation process involved monitoring the growth
of pseudomonads at various temperatures and comparing the observed generation
times to those predicted using bias and accuracy factors (Ross, 1993). The
temperature model has been incorporated into prototype computer software and
trialled in the dairy industry. The validation process showed the psychrotrophic
pseudmonad model to accurately predict the growth of pseudomonads in the
products tested. In some instances in industry, further information in the form of
the duration of lag phase, is required to maximise the accuracy of the model. The
capacity to input this information was included in the prototype software.

Item Type: Thesis - Research Master
Authors/Creators:Neumeyer, K
Keywords: Pseudomonads
Copyright Holders: The Author
Copyright Information:

Copyright 1995 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:

Includes bibliographical references (leaves 142-164). Thesis (M.Sc.)--University of Tasmania, 1997

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