Biodiversity, dynamics, and characteristics of Propionibacterium freudenreichii in Swiss Emmentaler PDO cheese

Propionibacteria are naturally present in raw milk at low levels, but little is known regarding the influence of these wild-type strains on cheese quality. To evaluate the prevalence of wild strains of propionibacteria in Emmental cheese, three cheeses were manufactured from the same raw milk using three commercial Propionibacterium freudenreichii starters (A, B, and C). A total of 165 isolates from the raw milk and 479, 436, and 476 isolates from cheeses A, B, and C, respectively, were typed by insertion sequence-restriction fragment length polymorphism or by multiple-locus variable number of tandem repeat analysis in order to determine the composition of the propionibacterial flora in the raw milk, in the curd, and in the cheeses after 2, 4, 6, and 8 months of ripening. Nine starter strains and 17 wild P. freudenreichii strains isolated from the curd and cheese were characterized with regard to their specific activity of aspartase and for their ability to grow at low temperature (11 °C). After 8 months of ripening, more than 80% of wild-type strains of P. freudenreichii were isolated from cheese A, whose starter strains had low aspartase activity. Although warm room storage (23 °C) stimulated the growth of various wild-type strains in all cheeses, strains of starters B and C, which had high aspartase activity or fast growth at 11 °C, remained dominant over the whole ripening period. In all cheeses, the growing wild-type strains exhibited high aspartase activity, indicating that this strain-specific property is a key factor in the control of propionic acid fermentation and in the prevention of late fermentation in Swiss-type cheeses.