[Quantification of the probability of milk contamination by Listeria monocytogenes during manufacture of hard cheese].

BACKGROUND

The present work is concerned with the probability of contamination by Listeria monocytogenes in the artisanal manufacture of Swiss Emmental hard cheese made from raw milk. The simulation model follows the evolution of the contaminant flora from raw milk at the farm to milk mixing, storage at the cheese factory and to the cheese manufacturing process.

METHODS

The simulations are based on models of predictive microbiology, namely the exponential growth model of bacteria including the lag-time, a cardinal growth model and a Log-linear model of thermal deactivation of bacteria.

RESULTS

The results of the actual simulation indicate that the contamination of milk at the farm is a rare event (P=0.0036), but the mixing of milk at the cheese factory leads to a higher probability of contamination of cheese milk (P=0.07). Elevated bacterial concentrations are mainly due to cases of mastitis involving Listeria monocytogenes. The decline in bacterial counts during cheese manufacture depends on the curing temperature (52-54 degrees C) and varies between 1.5 and 3.2 Log cfu/ml. Freshly manufactured Emmental-cheese made from contaminated raw milk is expected to have only 4.6 cfu of heat injured Listeria monocytogenes /kg cheese mass in the press.

CONCLUSION

Depending on listeria evolution, from the press to the product consumption, consumer exposure has been evaluated and might result in 1 to 10 Listeria monocytogenes per portion of cheese. The bacterial presence could be due to recontamination during packaging, distribution and cheese preparation by the consumer. Based on the presented data and estimations, it is concluded that the consumption of traditionally/artisanal manufactured Swiss Emmental hard cheese presents an extremely low, but existent risk, especially for people with a deficient or diminished immune system.