To evaluate and quantify all the important factors affecting pathogen growth and include them into mathematical models that will be able to describe the effect of environmental conditions, as well as the ecological complexity of specific meat products the methodology will include:

In vitro studies : Mono-cultures of pathogens (Listeria monocytogenes, Salmonella and Escherichia coli) or mixed-cultures of pathogens with meat spoilage bacteria e.g pseudomonads, Brochothrix thermosphacta, lactic acid bacteria and Enterobacteriaceae will be studied using different growth model systems (liquid laboratory media, gel cassettes and sterile meat blocks. By comparing the growth kinetics of each pathogen with or without the presence of a single or a group of different spoilage bacteria the effects of microbial interaction will be determined and quantitatively evaluated. Furthermore, in order to evaluate the source and type of interactions (production of metabolites, substrate utilization) physicochemical changes during storage will be also determined. The results from different model systems will provide information about the effect of structure on pathogens growth.

The effect of growth history on pathogens kinetics, mainly lag phase and growth/no growth interface will be determined by using cultures of the selected pathogens, pre-incubated at different conditions in terms of medium composition, temperature, aw, pH and growth state (stationary and exponential). A combination of the results with data on the origin of contamination during meat manufacture (derived from WP2) will be used in the model development process in order to mimic the processes of contamination in industry.

In situ studies : Natural contaminated meat products will be inoculated with the selected pathogens and stored under aerobic, MAP or VP conditions at different temperatures (from 0 to 15 oC). The behaviour of pathogens on products with different structure and composition (beef, pork and lamb cuts, ground meat with different fat concentration, different ready to cook products) will be determined and compared with the results obtained from in vitro studies. In all in situ studies sensory analysis will be performed and shelf life of the different products will be correlated with growth of the responsible for spoilage bacteria (pseudomonads for aerobic storage and lactic acid bacteria or Br. thermosphacta for MAP and VP conditions).

The data from in situ studies will be used for the validation of existing models or if necessary the development of new models for the effect of temperature on the growth of dominating meat pathogens on aerobically stored, MA packed and vacuum packed meat products. For this, empirical, kinetic or probabilistic models will be applied in order to model the temperature dependence of pathogens growth rate, lag phase and growth/no growth interface. In the process of model development the information derived from in vitro studies will be used for the optimization of the existing models and the development of new models that take into account any possible effect of structure, composition, microbial competition and physiological stage of the pathogens.

Meat products inoculated with Listeria monocytogenes, Salmonella and Escherichia coli will be stored at non-isothermal conditions using different fluctuating temperature scenarios that simulate the meat chill chain based on the temperature survey data derived from WP2. The effect of mild and abrupt temperature changes on the lag phase, exponential growth and growth/no growth interface of the selected pathogens will be evaluated and the data will be compared with the prediction of the models developed in Sub-workpackage 1.1. Where necessary, the appropriate adjustments will be made in order to increase as much as possible the accuracy of the models.