Listeria monocytogenes in Milk Products

peer-reviewedMilk and milk products are frequently identified as vectors for transmission of Listeria monocytogenes. Milk can be contaminated at farm level either by indirect external contamination from the farm environment or less frequently by direct contamination of the milk from infection in the animal. Pasteurisation of milk will kill L. monocytogenes, but post-pasteurisation contamination, consumption of unpasteurised milk and manufacture of unpasteurised milk products can lead to milk being the cause of outbreaks of listeriosis. Therefore, there is a concern that L. monocytogenes in milk could lead to a public health risk. To protect against this risk, there is a need for awareness surrounding the issues, hygienic practices to reduce the risk and adequate sampling and analysis to verify that the risk is controlled. This review will highlight the issues surrounding L. monocytogenes in milk and milk products, including possible control measures. It will therefore create awareness about L. monocytogenes, contributing to protection of public health

Modelling the fate of Listeria monocytogenes during manufacture and ripening of smeared cheese made with pasteurised or raw milk.

The dynamics of the physicochemical characteristics of foods help to determine the fate of pathogens throughout processing. The aim of this study was to assess the behaviour of Listeria monocytogenes during cheesesmaking and ripening and to model the growth observed under the dynamic conditions of the cheese. A laboratory scale cheese was made in 4 independent replicates from pasteurised or raw cow's milk, artificially contaminated with L. monocytogenes. No growth of L. monocytogenes occurred during raw milk cheese-making, whereas growth did occur in pasteurised milk. During ripening, growth occurred in raw milk cheese, but inactivation occurred in pasteurised milk cheese. The behaviour observed for L. monocytogenes was modelled using a logistic primary model coupled with a secondary cardinal model, taking into account the effect of physicochemical conditions (temperature, pH, water activity and lactate). A novel statistical approach was proposed to assess the optimal growth rate of a microorganism from experiments performed in dynamic conditions. This complex model had an acceptable quality of fit on the experimental data. The estimated optimum growth rates can be used to predict the fate of L. monocytogenes during cheese manufacture in raw or pasteurized milk in different physicochemical conditions. The data obtained contributes to a better understanding of the potential risk that L. monocytogenes presents to cheese producers (growth on the product, if it is contaminated) and consumers (the presence of high numbers) and constitutes a very useful set of data for the completion of chain-based modelling studies

Absence of Listeria monocytogenes growth during raw milk cheesemaking: a modelling approach

The presence of Listeria monocytogenes in certain foods and the risk that this poses to public health and food quality is still a problem. Currently, the field of food mi- crobiology focuses on obtaining data on the behaviour of microorganisms in food, but the responses obtained provide little insight into the relationship between physiological processes and growth or survival. This link can be made through mathematical models. In a simple form, a mathematical model is a simple mathe- matical description of a process. The application of mathematical models to food microbiology has been developed in recent years and now constitutes a new disci- pline named as Predictive Microbiology. However, most predictive models are based on laboratory experiments in microbiological media under static conditions. As such models tend to be inaccurate, we have undertaken our experiments in a food system under dynamic conditions. Cheese was made with raw and pasteurised milk deliberately contaminated with L. monocytogenes . Listeria was monitored through the manufacture and ripening period of the cheese. The results showed that L. monocytogenes did not grow during manufacture of raw milk cheese, but did grow during manufacture of pasteurised milk cheese. The data obtained for growth, survival and inactivation was modelled. The application of models that can explain the behaviour of Listeria in cheese and the further predictions that can be obtained from these models are useful for the improvement of ongoing re- search on biotraceability and for the better understanding of the general behaviour of these microorganisms under dynamic conditions, such as in dairy product