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A case of sporadic ovine mastitis caused by Listeria monocytogenes and its effect on contamination of raw milk and raw-milk cheeses produced in the on-farm dairy

Schoder, Dagmar, Winter, Petra, Kareem, Abdoulla, Baumgartner, Walter, Wagner, Martin
Journal of dairy research 2003 v.70 no.4 pp. 395-401
Listeria monocytogenes, acidification, bulk milk, buttermilk, cheesemaking, coagulation, ewe milk, farmers, farms, flocks, manufacturing, mastitis, raw milk, sheep, soft cheeses, starter cultures, yogurt
We describe a case of listerial mastitis in a flock of 130 sheep. The animals were housed at a farm where the bulk raw ewe milk was processed to produce raw milk soft cheese. List. monocytogenes was shed from the right mammary complex. Shedding was observed over a period of 99 d. A mean level of 4·56×10(4) cfu (colony forming units) Listeria monocytogenes/ml was recovered from the raw milk originating from the infected udder. The numbers ranged from 9×10(1) to 2·95×10(5). The bulk milk was contaminated by approx. 5·7×10(3) cfu/ml. In the cheese product, 2·0×10(2) cfu List. monocytogenes/g were constantly detectable for a period of 7 d post manufacture. The starter culture used for coagulation had a pivotal influence on the behaviour of List. monocytogenes during cheesemaking. Using the same mesophilic buttermilk culture as used by the farmer allowed numbers of Listeria to increase 60-fold within 12 h owing to a delayed acidification of the bulk milk. Addition of a thermophilic yogurt culture reduced the numbers of Listeria within 8 h of incubation.