Deterministic and stochastic behaviour of L. monocytogenes suspended cells or detached from stainless steel surfaces during cheese manufacture

Growth probability and kinetic models for Listeria monocytogenes in response to multiple hurdles occurring during cheese anufacturing are mainly focused on suspended L. monocytogenes cells. This study aimed to compared: (i) the growth/no growth interface of L. monocytogenes cells attached on stainless steel (SS) surfaces, or in suspension, within adjusted media and (ii) the behavior of planktonic and detached Listeria cells during manufacturing and ripening of two popular Greek cheeses: Feta and Graviera. A multi-strains composite of L. monocytogenes isolates from cheese, factory and farm in Greece and Ireland, were grown in TSBYE, MRD, Milk, Feta and Graviera cheese in the presence of SS coupons (2x5cm) for 3d at 20 °C, to obtain the following inocula: planktonic cells (P), and cells detached from the SS coupons (D). Detachment took place by the bead vortexing method. For growth/no growth evaluation P and D cells were inoculated in TSBYE, adjusted to 5 pH (6.8-4.8) by lactic acid and at 4 aw (0.945-0.995) by NaCl. For evaluation of L. monocytogenes kinetics in cheese, P and D cells were inoculated at three simulated stages of Feta and Graviera manufacture: in pasteurized milk, after cutting the curd and after the first ripening. The growth of D cells slightly delayed compared to P cells while it was more affected by aw than pH. On cheese, L. monocytogenes survived throughout the ripening at low levels. The differences in probability of growth of single cells for both inocula (P and D) were assessed by stochastic approaches. Furthermore, PFGE analysis resulted that 91 % of the cells of any tested condition belonged to the cheese factory isolate. The re- sults may address safety implications relevant to the potential of attached cells to proliferate, whereas data may contribute to filling data gaps on risk assessment of L. monocytogenes isolates from the dairy industry