View Report Details

Risk Assessment of Listeria monocytogenes in UK retailed Cheese

Project Code: B12006

30/04/2006

Food Safety Assurance
Banks, J

This work highlights the complexity of cheesemaking and the variety of types of cheese that are available to the UK consumer. The study suggests that some, but not all categories of cheese are vulnerable to Listeria monocytogenes contamination, survival or growth.

In the categories of cheese that account for the majority of UK consumption by volume, the vulnerability from L. monocytogenes appears to be very low, and this is supported by the paucity of data on foodborne listeriosis associated with cheese. With the available evidence, information and predictive models, it is only possible to suggest a tentative risk ranking for cheese categories.

With the exception of hard cheeses, the use of raw milk appears to introduce an additional risk factor for L. monocytogenes, however, it should be noted that the overwhelming majority of UK consumption by volume is comprised of pasteurised cheese.

By the nature of the maturation, handling and packing processes for some hard and semi-hard cheese, it may be possible to occasionally detect very low numbers of L. monocytogenes on the rind, however this contamination without subsequent growth, may be of little public health consequence.

Most blue-veined cheese consumed in the UK is made from pasteurised milk, however the moderate maturation times, piercing of the rind and significant changes to the acidity level, would suggest that a potential L. monocytogenes risk may exist and it is possible to occasionally detect low numbers of L. monocytogenes in blue-veined cheese, especially on the rind, in both pasteurised and raw milk types.

Surface, film-ripened cheeses tend to be imported from manufacturers using large scale production, modern equipment and pasteurised milk. As the salting and washing of the cheese are potential risk factors, and as the acidity at the surface of these types of cheese may fall during maturation and rind development, control over the process is important. It is possible to occasionally detect low numbers of L. monocytogenes in film-ripened cheese, especially on the rind, in both pasteurised and raw milk types. Further attention on this category of cheese may be warranted.

A substantial proportion of UK-retailed soft mould-ripened cheese is imported from manufacturers using modern equipment in highly automated production with pasteurised or thermised milk. This category does, however, account for the largest proportion of raw milk cheese in the UK retail market. The nature of the maturation process, activities of the mould, and changes over time in controlling factors for L. monocytogenes suggest that some risk is present. If L. monocytogenes has been introduced in the milk, it is unclear if the fermentation process will eliminate the pathogen on a consistent and reliable basis. Conditions typical in ripening may introduce additional risk factors from cross contamination, however our observations in large-scale manufacturing plants suggest that these risks are minimised. Lastly, the physico-chemical changes during maturation and shelf life of soft mould-ripened cheese will theoretically permit growth of L. monocytogenes.

Fresh, soft cheese is a large and diverse category dominated by a few branded or retail *own label* cheeses made from pasteurised milk. These are typically manufactured using large scale production, modern equipment and very hygienic environments to assure a reasonable shelf life. Process and packaging of this category of cheese probably both play a part in minimising the threat from L. monocytogenes.

In most categories of cheese, the easily measurable controlling factors, such as water activity and pH appear to offer insufficient protection against survival or growth L. monocytogenes alone, or in combination. The inclusion of salt appears to have little significant effect on the fate of L. monocytogenes. For these reasons, the available predictive models tend to grossly over-estimate the likelihood of growth or capacity to survive. One potentially significant unknown is the influence on L. monocytogenes of the fermentative microbial associations and the microbial successions that arise during ripening. It can be anticipated that, for example the lactic starters will initially suppress L. monocytogenes and may further accelerate L. monocytogenes inactivation, however the data are complex, contradictory and beyond the scope of this work. Similarly the influence of ripening bacteria, yeasts or moulds on L. monocytogenes is largely unknown.

Most cheese types offer a dynamic and changing physico-chemical environment to the microorganisms present therein, making prediction of the fate of L. monocytogenes complex and uncertain. Minimising or eliminating the sources of L. monocytogenes from milk, equipment and the environment are important in an overall risk reduction strategy. Process understanding and control are critical factors in ensuring that the fermentation and maturation stages of cheese making minimise the risk from L. monocytogenes.

Main File

Some of the files on this site may be in a format that your computer can't read. However, you can download Readers and Viewers for the following document types below: