Study of the bacterial flora and the growth of Listeria monocytogenes in raw milk butter

Listeria monocytogenes is a foodborne pathogen for which growth in food depends on the latter's physico-chemical characteristics and the presence of inhibitors such as lactic acid bacteria. To study the growth of Listeria monocytogenes in raw milk butter, challenge test was conducted. Two approaches of butter production were simulated. The first one used cream matured at 4°C while the second one used cream matured at 14°C. Samples of cream and butter were taken at several days for pH and microbiological analyses. Metagenetic analyses were also conducted on all samples. The two batches of butter showed different bacterial profile. The first batch made from refrigerated cream presented a big diversity and was characterized by an abundance of psychotropic bacteria. This batch showed an increase in the level of Listeria monocytogenes during butter storage. The growth potential of Listeria monocytogenes, defined as the difference between the median of the counts at the end and the median of the counts at the beginning of storage, was 1.81 log cfu/g. The second batch made from cream matured at 14°C was characterized by a dominance of Lactococcus. The latter was even more abundant in inoculated samples than in control samples. This batch presented a decrease of Listeria monocytogenes during butter storage with a growth potential of -1.72 log cfu/g. These data suggest that cream maturation is an important factor to the growth of Listeria monocytogenes and lactic acid bacteria especially Lactococcus.