Cheese producers get advice on Listeria

Brine baths, background flora, disinfection and drying can all help to get rid of Listeria, but the bacterium is very hard to kill off.

A great deal of the variety now found in Norwegian cheese is thanks to small, local producers. Now, they are able to find out more about what conditions can reduce the risk of Listeria in soft cheeses. The project has been led by the food research institute Nofima.

Environment and equipment tests take priority over product tests
Analysis of samples from production premises and products shows that the incidence of Listeria is low among small-scale cheese producers. At the same time, more than tests will be needed to confirm these findings and to reduce the risk even further.

"In the fight against Listeria, it is more effective to monitor the production environment than to check the product itself. Regular analyses of samples from premises and the production environment are needed, but this is expensive and time consuming," says Bjørn Schirmer, a research scientist at Nofima.

It is important to choose the right places to take samples. Equipment that is in contact with the soft cheese and niches that are particularly exposed, such as drains and damp crevices, should be prioritised. For the next step, it is important to choose the method that gives the best possibility of finding Listeria.

Difficult to get rid of Listeria
Conditions in the rooms where cheese is matured, with high humidity and low temperature, mean that Listeria can survive for an almost unlimited period. It is therefore important to avoid Listeria getting into the premises in the first place. This can be done most effectively by division into zones. It is also important to keep the premises as dry as possible.

Disinfection and drying are two important stress factors in food production, but Listeria is able to survive them both – albeit in reduced numbers. They can survive for several weeks, only to grow again when they get access to nutrition and there is high humidity or poor drainage. This nutrition may be in the form of milk spills, for example.

"It is important to minimise the bacteria’s access to nutrients, as well as to ensure that there are regular routines for washing and disinfection. Also, take samples just before production starts, because samples taken right after washing and disinfection will not necessarily show whether there is Listeria present," says Schirmer.

The brine must be concentrated
The researchers have made a number of trials to investigate whether the brine and microbiological flora found in cheese production environments can inhibit the growth of Listeria. They found that Listeria can survive for more than six months in brine. In the Listeriosis outbreak of 2007, large quantities of Listeria were found in the brine. Different strains of Listeria behave differently in brine, and it is the most pathogenic that are the best survivors.

It is important to ensure that the brine is concentrated, and stirring well is also important. Because Listeria can survive in brine for an extended period, it is also important to take samples of the brine..

A collaboration between Nofima, the National Veterinary Institute, the Norwegian Institute of Public Health , the Norwegian Food Safety Authority, Norwegian Farmhouse Cheese, Hanen – Norwegian Rural Tourism and Food from the Farm , Sogn School of Organic Agriculture and Horticulture, Sogn & Fjordane County Council, Innovation Norway, Norwegian small-scale cheese producers, ASNES and Actilaît.

Financed by the Foundation for Research Levy on Agricultural Products (FFL) and funding from the Agricultural Agreement.

Food safety and quality  

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