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Cheese Safety 101

Part 3: Where do they come from?

Art Hill and Keith Warriner
Department of Food Science
University of Guelph,
Guelph, ON, N1G 2W1

Raw milk provides a rich source of nutrients to support the growth of spoilage and pathogenic microbes. Previously, we have described the pathogenic bacteria associated with raw milk and in this edition the common sources of these potentially deadly bugs will be outlined. Milk can be contaminated from a variety of sources such as the animal, field or milk parlor environment (Figure 1).

If the teats of animals contact fecal material it is likely the milk will also become contaminated. Fecally contaminated equipment and water are additional sources of pathogens such as E. coli O157.

Although the above pathogens have especially adapted to the host some are equally at home outside the gastro-intestinal tract environment.

When the spores enter milk they can start germinating to form active cells that can grow especially if held at warm temperatures. Some endospore forming bacteria cause spoilage but others can produce deadly toxins (e.g. Cl. botulinum toxin).

Milk handlers can also pass on Staphylococcus aureus into raw milk.

Sources of bacteria in cheese production

Through evolution pathogenic bacteria have adapted to grow optimally within the hosts (i.e. animals and ourselves). Therefore, it is no surprise that the most virulent pathogens can be transferred between animals and also recovered at high levels in fecal material (i.e. transferred via the fecal-oral route). Between animal transmission (within herd and wild animals) is especially relevant for pathogens such as Brucillus, Q fever and Mycobacterium tuberculosis. Such pathogens can enter the blood system of the animals and end up in the raw milk.

Fecal material is a rich source of pathogens such as Escherichia coli O157, Salmonella and Campylobacter.

Of most concern is Listeria monocytogenes that can be found in rotting plant material and is endemic in many milking parlors.

The main success of L. monocytogenes is the ability to grow in cold temperatures where many other bacteria are inhibited. The threat from the pathogen shouldn't be taken lightly as there is a probability of 33% that the infection will result in the death of the infected person. L. monocytogenes can also cause miscarriages in pregnant women and animals.

Endospore forming bacteria (clostridia and Bacillus) can be commonly encountered in soil and water thereby introduced into raw milk via contaminated teats or contact surfaces.

Most people are aware the animals suffering from mastitis represent an important source of Staph. aureus. However, 40% of humans carry the pathogen on their skin, nose and throat so good hygienic practices are essential.

With knowledge of the sources of contamination it is possible to appreciate the importance of monitoring the health of animals, sanitation of equipment and parlor environment, in addition to worker hygiene. Such practices reduce the risk of introducing pathogens but the cheese marker has several approaches to suppress or inactivate pathogens that find their way into milk. That's the topic of Part 4 in this series. Stay tuned!