Licentiate thesis, 2012
Microbiological food safety of cheese produced in Swedish small-scale dairiesRosengren, Åsa
AbstractThe number of small-scale dairies in Sweden has increased during the past decade. Current agricultural policy encourages such small-scale production and various ways to help producers have been proposed. Combining traditional cheese making and good product safety is a challenge, since several human pathogens pose a hazard in the products. Staphylococcus aureus is an important foodborne pathogen that can be transmitted via milk to cheese. Some strains can produce staphylococcal enterotoxin (SET) during growth in foods and cause food poisoning. Environmental conditions during the initial phase of cheese making are close to optimal for growth of S. aureus. This study examined small-scale artisan cheese production and relevant pathogens, with the aim of formulating advice on microbiologically safe production. A survey of fresh and short-time ripened cheeses produced in Swedish small-scale dairies investigated the occurrence and levels of S. aureus, Listeria monocytogenes and Escherichia coli. Information about production practices was collected and S. aureus isolates were characterised with emphasis on enterotoxin genes, antibiotic resistance, biotyping and genetic variation. Staphylococcus aureus isolates from cheeses were investigated in broth cultures for their potential for growth and production of staphylococcal enterotoxin A (SEA) in typical cheese making conditions as regards temperature, pH, lactic acid/undissociated lactic acid and water activity. Detected levels of the three pathogens were reasonable in most cheese samples tested. Listeria monocytogenes and SET were not found, but E. coli and enterotoxigenic S. aureus were frequently found in raw milk cheeses, sometimes at high levels. The S. aureus isolates were mainly of animal biotype and two-thirds were enterotoxigenic. The isolates tested were able to grow and produce SEA in the presence of moderate concentrations of undissociated lactic acid. SEA was produced continuously during incubation and high concentrations were found long after S. aureus levels had peaked. In practice, further storage of cheese samples prior to SET analysis may be beneficial when high staphylococcal levels are detected. Increased awareness of the importance of hygiene barriers is needed. Possible improvements in process control include e.g. controlling raw milk quality, using active starter cultures and monitoring acidification.
KeywordsSurvey; Food safety; Cheese; Small-scale dairies; Lactic acid; Staphylococcus aureus; Growth rate; Lag time; Staphylococcal enterotoxin
Publisher: Department of Microbiology, Swedish University of Agricultural Sciences
Swedish University of Agricultural Sciences, Department of Microbiology
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