Examination of the behaviour of bacterial pathogens in raw milk

Among recent trends towards less processed foods is a growing consumer demand for raw (unpasteurised) dairy products. In spite of the public health risks and an increased awareness of pathogens in milk, proponents of raw milk consumption are of the view that pasteurisation kills beneficial microorganisms believed to have antimicrobial properties towards pathogens. This study thus aimed to examine the behaviour of bacterial pathogens in raw milk. Four pathogenic bacteria relevant to the dairy industry, Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica and Staphylococcus aureus, were investigated. Milk-spiking experiments were performed at different temperatures over 5 days in raw milk collected from different sources. It was observed that the introduced pathogens showed limited growth at a low storage temperature (refrigeration); however, increased growth rates were observed at higher temperatures of 8°C and 15°C. In no cases were the pathogens eliminated. These observations suggest that consumption of raw milk is potentially hazardous, especially if the milk is subjected to mild and moderate temperature abuse, since bacterial pathogens remain viable, and indeed multiply, in the presence of normal milk microbiota.