ReviewA review of the microbiological hazards of dairy products made from raw milk
Introduction
Consumers' attitudes show a trend towards increased consumption of foods that are not or only minimally processed such as raw milk and dairy products made from raw milk. Cheeses made from raw cow, sheep and goat milk are the most frequently consumed type of dairy products made from raw milk; however, other types of dairy products made from raw milk are also consumed such as mozzarella made from raw buffalo milk, as well as raw milk butter made from cow milk. The consumption of raw milk can hold a risk for the consumer, due to the possible presence of human pathogenic microorganisms. The risks and benefits of the consumption of raw cow milk were described in a review by Claeys et al. (2013) and the nutritional and health benefits of the consumption of raw milk from animal species other than cows were described in a review by Claeys et al. (2014). The microbiological hazards of raw milk from animal species other than cows were described in a review by Verraes et al. (2014). Also EFSA evaluated the public health risk from the consumption of raw drinking milk (EFSA, 2015).
In the present review, a collation is made of available information in the scientific literature concerning the microbiological hazards of dairy products made from raw milk, in particular cheese, butter, cream and buttermilk. Dairy products made from raw milk from cows, sheep and goats are taken into account. Dairy products made from raw milk from other animal species are less relevant in Europe, with the exception of mozzarella made from raw buffalo milk. Only zoonotic human pathogenic microorganisms and pathogens originating from the environment have been taken into consideration. Pathogenic agents originating from human contamination due to human illness, e.g., Salmonella Typhi, Shigella spp. and noroviruses are not covered in this review.
Section snippets
Occurrence of human pathogenic microorganisms in raw milk and dairy products made from raw milk
In general, pathogenic microorganisms can contaminate raw milk in two ways. The first way is an endogenous contamination where the milk is contaminated by a direct transfer from the blood (systemic infection) to the milk or via an infection in the udder called mastitis. The second way is an exogenous contamination, where the milk is contaminated during or after milking by the faeces, the exterior of the udder and teats, the skin, the environment, etc.
Based on recent review documents (Claeys
Reported human cases and outbreaks due to the consumption of dairy products made from raw milk
The development of a disease after consumption of contaminated dairy products made from raw milk depends on several factors such as the pathogenicity of the strain, the number of ingested microorganisms, the physiological state of the microorganism, the health condition of the consumer at the moment of ingestion, etc. Persons belonging to the YOPI group (young, old, pregnant, immunodeficient) have a higher risk of infection for certain pathogens such as L. monocytogenes and healthy persons can
Growth and survival of pathogenic microorganisms during production and storage/ripening of dairy products made from raw milk
The circumstances of production and storage of dairy products made from raw milk determine the behaviour (i.e., growth, survival or inactivation) of the microorganisms that are potentially present in the raw milk. The pathogens can grow, survive or be inactivated. Some of these important influencing factors are summarised as follows for raw milk cheeses. The growth of the fermenting bacteria reduces the growth possibilities of the pathogens by, on the one hand, competition and, on the other
Identification of main microbiological hazards in dairy products made from raw milk
The information on the possible presence of human pathogenic microorganisms in raw milk, frequencies of occurrence of human pathogenic microorganisms in dairy products made from raw milk in Europe, outbreaks resulting from consumption of dairy products made from raw milk in Europe, the United States and Canada and the worst case scenario of the behaviour in dairy products made from raw milk is summarised in Table 4.
The main microbiological hazards linked to raw milk cheese are L. monocytogenes,
Discussion
The contamination levels of several pathogens such as L. monocytogenes and S. aureus usually encountered in raw milk are mostly lower than the level they have to reach to cause disease. It is assumed (based on the authors' expert opinion) that numbers of L. monocytogenes in raw cow milk are usually lower or at the level of ca. 1 log cfu mL−1. In the case of a subclinical mastitis of the milk producing animal, high initial numbers of L. monocytogenes or S. aureus could be encountered in the raw
Conclusion
Overall, the available information on some pathogens or dairy products made from raw milk is limited, but if everything is taken into consideration the microbiological hazards and dairy products made from raw milk of main concern are linked to raw milk cheeses and L. monocytogenes, pathogenic VTEC, enterotoxin-producing S. aureus, Salmonella and C. jejuni/coli. Next, L. monocytogenes, VTEC and S. aureus have been identified as microbiological hazards in raw milk butter and cream, albeit to a
Acknowledgements
The authors wish to thank the Scientific Committee of the Belgian Federal Agency for the Safety of the Food Chain (FASFC) for their scientific support.
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