Effects of temperature and water activity on Escherichia coli in relation to beef carcasses

To understand and be able to predict the behaviour of pathogenic Escherichia coli in foods would greatly assist decision-making in the food industry in relation to handling practices, processes and formulation. The Australian red meat industry has a particular interest in the behaviour of E. coli on carcasses, on which temperature and water activity are the most important factors affecting bacterial growth. This subject is the topic of this thesis. Nine Shiga toxin-producing E. coli (STEC) strains, including serotypes of particular public health significance (0157:H7 and 0111) were obtained from two public hospitals. Predictive models are generated for STEC growth rates with respect to temperature and water activity and are not found to be significantly different to those for non-pathogenic E. coli. The performance of a model for non-pathogenic E. coli compares favourably with published models when compared to literature data. An investigation is also made of the growth of E. coli 0157:H7 in the range 44-45.5°C. Growth is found to be sufficient in this range to enable detection using traditional incubation temperatures of 44-45.5°C. The probability of growth of an STEC strain at the growth/no growth interface is modelled with varying combinations of temperature and water activity. Data fitted to a 'generalised non-linear regression model' gave an approximate concordance rate of 96.5%. The results suggest that even at the growth/no growth interface, bacterial growth is predictable. While earlier 'kinetic models' focused on growth under favourable environmental conditions, this data provides useful information about the probability of growth of a pathogenic microorganism at the boundary of growth and no growth. Carcass surface temperature is easily monitored but previously, water activity at the surface could not be measured. Two methods of water activity measurement are assessed: electrical conductivity and direct water activity measurement following excision of tissue using a skin-grafting scalpel. The latter method allows fast and accurate measurement of the water activity of carcass surface tissue. Monitoring water activity of carcass surfaces in two Australian abattoirs during chilling with concomitant collection of temperature data allows more accurate prediction of microbial growth and the ability to modify current chilling regimes to improve microbial status of beef carcasses.