Validation via challenge test of a dynamic growth-death model for the prediction of Listeria monocytogenes kinetics in Pecorino di Farindola cheese.

Pecorino di Farindola is a typical cheese produced in the area surrounding the village of Farindola, located in the Abruzzo Region (central Italy), unique among Italian cheese because only raw ewe milk and pig rennet are used for its production. In the literature it is well documented that raw milk is able to support the growth of pathogenic microorganisms such as Listeria monocytogenes. Predictive microbiology can be useful in order to predict growth-death kinetics of pathogenic bacteria, on the basis of known environmental conditions. Aim of this study was to compare predictions obtained from a model, originally designed to predict the kinetics of L. monocytogenes in the dynamic growth-death environment of drying fresh sausage, with the results of challenge tests performed during the ripening of Pecorino di Farindola produced from artificially contaminated raw ewe milk. A challenge test was carried out using ewe raw milk inoculated with L. monocytogenes, in order to produce Pecorino di Farindola cheese stored at 18 °C for 149 days of ripening. During the ripening period, pH and aw values decreased in all samples analysed; lactic acid bacteria become the prevailing microbial population, while for L. monocytogenes a period of stability (neither growth nor death) followed the initial situation. The growth inhibition and the following inactivation may mostly be due to competition with the autochthonous microbiota and to the reduction of water activity. Mathematical modelling was used in order to predict microbial kinetics in the dynamic ripening environment, joining growth and death patterns in a continuous way, and including the highly uncertain growth/no growth range separating the two regions. The effect of lactic acid bacteria on the growth of pathogens was also included. Predicted microbial kinetics were satisfactory, as confirmed by the absence of statistically significant difference between observed and predicted values (p > 0.05). The present study proved, via challenge tests, that a dynamic growth/death model, previously used for a meat product, can be fruitfully used in cheese characterized by active competitive microbiota and progressive drying during ripening.

Listeria monocytogenes in poultry: Detection and strain characterization along an integrated production chain in Italy.

In this study, thirteen batches of broiler chicken from an integrated Italian poultry company were investigated for the detection of Listeria monocytogenes. The prevalence was evaluated in faeces samples at farm level and after transport, caecal contents and carcass neck skin from 2 slaughterhouses (M1 and M2), for a total of 2080 samples, throughout a 27-month period. No positive results were recorded in faeces, while the overall prevalence of contamination in carcass neck skin was 26.7%. Then, 123 isolates out of 139 positive skin samples, with the prevalent serotypes 4b (76%) and 1/2b (94%) from slaughterhouses M1 and M2 respectively, were PFGE characterized, showing the presence of 18 different pulsotypes and 8 genetic clusters. The same pulsotypes were found in carcasses from different farms, but slaughtered in the same abattoir, highlighting the environmental origin of contamination. The persistence of the pathogen over long time seemed to be very likely, considering that undistinguishable pulsotypes were found in carcasses slaughtered in the same slaughterhouse after periods up to 18 months long. The implementation of cleaning and sanitation at slaughterhouse level could represent the main factor for the control of such pathogen in the poultry meat processing line.

Predicting the kinetics of Listeria monocytogenes and Yersinia enterocolitica under dynamic growth/death-inducing conditions, in Italian style fresh sausage.

Traditional Italian pork products can be consumed after variable drying periods, where the temporal decrease of water activity spans from optimal to inactivating values. This makes it necessary to A) consider the bias factor when applying culture-medium-based predictive models to sausage; B) apply the dynamic version (described by differential equations) of those models; C) combine growth and death models in a continuous way, including the highly uncertain growth/no growth range separating the two regions. This paper tests the applicability of published predictive models on the responses of Listeria monocytogenes and Yersinia enterocolitica to dynamic conditions in traditional Italian pork sausage, where the environment changes from growth-supporting to inhibitory conditions, so the growth and death models need to be combined. The effect of indigenous lactic acid bacteria was also taken into account in the predictions. Challenge tests were carried out using such sausages, inoculated separately with L. monocytogenes and Y. enterocolitica, stored for 480h at 8, 12, 18 and 20°C. The pH was fairly constant, while the water activity changed dynamically. The effects of the environment on the specific growth and death rate of the studied organisms were predicted using previously published predictive models and parameters. Microbial kinetics in many products with a long shelf-life and dynamic internal environment, could result in both growth and inactivation, making it difficult to estimate the bacterial concentration at the time of consumption by means of commonly available predictive software tools. Our prediction of the effect of the storage environment, where the water activity gradually decreases during a drying period, is designed to overcome these difficulties. The methodology can be used generally to predict and visualise bacterial kinetics under temporal variation of environments, which is vital when assessing the safety of many similar products.

Definition of a Standard Protocol to Determine the Growth Potential of Listeria Monotgenes and Yersinia Enterocolitica in Pork Sausage Produced in Abruzzo Region, Italy.

Pork meat products consumed raw or after a short period of fermentation can be considered at risk for food safety. Sausages (fresh sausage made from pork meat) are produced in several Italian regions, with variation in ingredients. In some Italian Regions, including Abruzzo, these products are frequently consumed raw or undercooked, after a variable period of fermentation. The European Community food regulation promotes the use of challenge tests to determine safety levels. This study is aimed to ensure safety of Abruzzo's sausages, compared with growth potential (δ) of Listeria monocytogenes and Yersinia enterocolitica, and also aims to define an experimental standard protocol document to carry out challenge tests. Guidelines classify ready-to-eat foods in categories that are able to support (δ>0.5 log10 ufc/g) and not support (δ≤0.5 log10 ufc/g) the growth of Listeria monocytogenes. The products were manufactured according to traditional recipes and were contaminated in laboratory. Results from the experiment yielded information useful to assess the ability of these products to support the growth of pathogenic microorganisms. The batches of sausages were stored at 8, 12, 18 and 20°C to get statistical evaluation. The results showed that, despite the conditioning of the storage temperature and the level of water activity, both organisms remain in the product in concentrations similar to those leading or being able to increase its charge. In particular, the period of greatest consumption of this product (7/8 days of preparation) corresponds to the period of greatest growth of pathogenic microorganisms studied, except for those stored at a temperature of 8°C, which are safer for the consumer.